apappa lagos geotechnical design

NOTRE DAME UNIVERSITY – LOUAIZE

DEPARTMENT OF CIVIL AND ENVIRONMENTAL

ENGINEERING

APAPPA LAGOS GEOTECHNICAL DESIGN

An Engineering Design II Report

SUBMITTED TO THE DEPARTMENT OF CIVIL AND ENVIRONMENTAL

ENGINEERING

In partial fulfillment of the requirements for the

Degree of

Bachelor of Engineering

By

Karim Taher

Zouk Mosbeh, Lebanon

2015

i

APAPPA LAGOS PROJECT GEOTECHNICAL DESIGN

An Engineering Design II

APPROVED FOR THE

DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING

BY

Mr. Michel Bouchedid, MCE, PE, MBA

Advisor

Dr. Naji Khoury, PE

Committee Member

Dr. Sophia Ghanimeh

Committee Member

Dr. Jacques Harb, PE

Chair of CEE Department

ii

SENIOR DESIGN REPORT CHECKLIST

Learning objective State how/where in the project the CLO is met

CLO 1 Implement an engineering project design

1.1. Identify a need/define a problem Design the foundation and shoring system for

a building to be built on soft soils with high

water table

1.2. State objectives Provide multiple foundation design

alternatives for the building and recommend

the most economical one

1.3. Collect information From existing geotechnical report

1.4. Identify constraints Poor soil conditions at the surface and high

water table

1.5. Identify adopted codes, standards, or

rules of practice

American Concrete Institute (ACI-318) and

British Standard Institute (BSI/1989)

1.6. Analyze the problem using acquired

engineering knowledge

Problem analysis includes raft foundation at

multiple levels, deep foundation, and shoring

system

1.7. Synthesize and propose a solution Recommended solution includes a 9 meter

excavation with a shoring system

CLO 2 Develop skills needed to function within a design team

3. Team work statement Project completed by one person

CLO 3 Produce a technical design report, a technical presentation and engineering

drawings

4.1. Technical report (hard and soft copies) Completed

4.2. Oral presentation (soft copy) Completed

4.3. Technical engineering drawings Completed

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ACKNOWLEDGMENTS

I would like to express my sincere gratitude to my advisor Mr. Michel Bouchedid, P.E. and Mr.

Nabil Houssayni for their guidance and continuous support to complete this study and prepare

this report. Besides my advisor and my friend, I would like to thank Rasha Joumaa and my

family for their moral support throughout the course of my engineering degree and my life in

general.

iv

ABSTRACT

One of the most important objectives of engineering is optimization. In this project, several

design alternatives will be considered in order to choose the most optimal design. The design

evaluation for the different alternatives will include a cost analysis in addition to an evaluation of

the practicality of the design and its constructability.

The project consists of a 12 story building that will be constructed in Lagos, Nigeria. One of the

main challenges facing the owner is the poor bearing capacity of the soil at the surface and the

high water table.

A local geotechnical company who prepared a geotechnical report for this project recommended

that the building be supported on drilled piles extending into harder soils. The drilled piles option

provides sufficient bearing support for the building. However, this alternative is uneconomical.

Another alternative consists of excavating 9 meters below ground surface to build underground

basements, thus building the foundation on harder soils. The 9 meters excavation alternative

requires the construction of a temporary shoring system along with a dewatering system to avoid

damaging the surrounding environment including other buildings and roads. The cost of the

alternative with basements is more economical because the basements can be sold. Also, the

shoring system is less costly than the pile foundation as discussed in the report.

As a summary, this report describes the problem that a developer in Nigeria has with the soil

conditions at his project site, provides the soil parameters to be used in the design, and discusses

different alternatives for foundation design and recommends the most economical alternative for

the developer.

v

CONTENTS

SENIOR DESIGN REPORT CHECKLIST .......................................................................................................... 3

ACKNOWLEDGMENTS ........................................................................................................................................ 4

ABSTRACT ............................................................................................................................................................... 5

A. INTRODUCTION ............................................................................................................................................ 1

B. GOAL AND METHODOLOGY ..................................................................................................................... 3

B.1. OBJECTIVES ............................................................................................................................................... 3

B.2. ASSUMPTIONS ....................................................................................................................................... 3

B.3. CODES, STANDARDS AND RULES OF PRACTICE ......................................................................... 4

B.4. PURPOSE OF THIS PROJECT ................................................................................................................ 4

B.5. SCOPE OF THE PROJECT ....................................................................................................................... 4

B.6. LOCATION OF THE PROJECT .............................................................................................................. 5

B.7. SUMMARY OF THE GEOTECHNICAL REPORT .............................................................................. 5

C. RESULTS AND ANALYSIS ....................................................................................................................... 10

C.1. SOIL PROPERTIES CALCULATION ................................................................................................. 10

C.2. CHECK FOR BEARING CAPACITY ................................................................................................... 15

C.3 SHORING SYSTEM DESIGN................................................................................................................. 19

C.4. DEWATERING SYSTEM ...................................................................................................................... 42

C.5. PILE FOUNDATION DESIGN ............................................................................................................. 43

C.6. COST ANALYSIS ..................................................................................................................................... 62

D. CONCLUSION AND RECONMMENDATIONS.................................................................................... 64

D.1. CONCLUSION .......................................................................................................................................... 64

D.2. LIMITATIONS ......................................................................................................................................... 64

D.3. CONSTRUCTION RECOMMENDATIONS ...................................................................................... 65

E. REFERENCES............................................................................................................................................... 66

F. APPENDIX .................................................................................................................................................... 68

vii

List of Figures Figure 1. Map of Lagos (Extracted from Google Earth, August 2015) .............................................. 5

Figure 2. Completed Boreholes Layout (Extracted from the Geotechnical Report by Basol Associates Ltd., 2014) ......................................................................................................................................... 6

Figure 3. Distribution of the Measured SPT Blow Counts with Depth .......................................... 11

Figure 4. Triaxial Compression Test Mohr Circle Diagram (Extracted from Basol Associates Ltd., 2014) ............................................................................................................................................................. 13

Figure 5. Load Eccentricities along x and y Directions ........................................................................ 17

Figure 6. Checking Column Pressure on Soil ........................................................................................... 18

Figure 7. Diaphragm Wall Supported by Ground Anchors (Extracted from the Internet for Illustration) .......................................................................................................................................................... 19

Figure 8. Ground Anchor Detailing ............................................................................................................. 20

Figure 9. Anchor Free Length According to the Assumed Failure Surface .................................. 27

Figure 10. Drawing the Model on PLAXIS................................................................................................. 27

Figure 11. PLAXIS Mesh Generation ........................................................................................................... 28

Figure 12. Water Table Definition ............................................................................................................... 28

Figure 13. Water Pressure Shadings .......................................................................................................... 29

Figure 14. Layer 3 Defined as a Dry Cluster ............................................................................................ 29

Figure 15. Initial Stress Definition ............................................................................................................... 29

Figure 16. Load Definition .............................................................................................................................. 30

Figure 17. Plate Definition .............................................................................................................................. 30

Figure 18. Excavating First 3 Meters .......................................................................................................... 31

Figure 19. Activating First Row of Anchors ............................................................................................. 31

Figure 20. Excavating Second 3 Meters ..................................................................................................... 32

Figure 21. Activating Second Row of Anchors ........................................................................................ 32

Figure 22. Excavating Last 3 Meters ........................................................................................................... 33

Figure 23. Phi/C Reduction ............................................................................................................................ 33

Figure 24. Run Calculation ............................................................................................................................. 34

Figure 25. Total Displacement of the Shoring System ......................................................................... 34

Figure 26. Total Horizontal Displacement of the Shoring System .................................................. 35

Figure 27. Total Vertical Displacement of the Shoring System ........................................................ 35

Figure 28. Plate Horizontal Displacement ................................................................................................ 36

Figure 29. Plate Vertical Displacement...................................................................................................... 37

Figure 30. Axial Forces on the Plate............................................................................................................ 37

Figure 31. Shear Forces on the Plate .......................................................................................................... 38

Figure 32. Bending Moment on the Plate ................................................................................................. 38

Figure 33. Anchor 1 Horizontal Displacement ....................................................................................... 39

Figure 34. Anchor 2 Horizontal Displacement ....................................................................................... 39

Figure 35. Reinforced Concrete Beam Stress in the Ultimate State (Deep Excavation, Theory and Practice) ........................................................................................................................................................ 41

Figure 36. Open Sumps Method for Dewatering .................................................................................... 43

Figure 37. Ultimate Load-Carrying of Pile ................................................................................................ 45

Figure 38. Variation of Nq* with L/D (Extracted from Das 2007b, after Coyle and Costello) ................................................................................................................................................................................... 48

vii

Figure 39. Unit Frictional Resistance in Sand (Extracted from Das, 2007b) .............................. 50

Figure 40. Variation of K with L/D (Extracted from Das 2007b, after Coyle and Costello, 1981) ....................................................................................................................................................................... 51

Figure 41. Estimation of Frictional Resistance using the λ Method ............................................... 55

Figure 42. Pile Reinforcement 45 cm Diameter ..................................................................................... 58



Figure 45. Pile Distribution under Raft Foundation ............................................................................. 61

Figure 46. Architectural Design .................................................................................................................... 70

Figure 47. Building Section ............................................................................................................................ 71

Figure 48. Parking Lots .................................................................................................................................... 72

Figure 49. Borehole No. 1 ............................................................................................................................... 73

Figure 50. Borehole No. 1 (continued) ...................................................................................................... 74

Figure 51. Borehole No. 2 ............................................................................................................................... 75

Figure 52. Borehole No. 2 (continued) ...................................................................................................... 76

viii

List of Tables

Table 1. Subsurface Profile Summary ........................................................................................................... 7

Table 2. Option A: 8 m Long Piles (Extracted from Basol Associates Ltd., 2014) ........................ 8

Table 3. Option B: 32 m Long Piles (Extracted from Basol Associates Ltd., 2014) ..................... 9

Table 4. SPT N-Values for Boreholes 1 and 2 .......................................................................................... 10

Table 5. Variation of NH, NB, NS and NR (Das, 2007b) ........................................................................... 12

Table 6. PLAXIS 2D Soil Data Input (Layer 1) ......................................................................................... 22



Table 9. Diaphragm Wall Properties .......................................................................................................... 23

Table 10. Anchor Free Length Properties................................................................................................. 24

Table 11. Anchor Grouted Length Properties ......................................................................................... 24

Table 12. Factor of Safety for Single Anchor (CICHE, 1998) ............................................................. 25

Table 13. Ultimate Frictional Strength of an Anchorage Body (Extracted from Deep Excavation) ........................................................................................................................................................... 26

Table 14. PLAXIS Outcomes Vs Acceptable Values ............................................................................... 40

Table 15. Interpolated Value of Nq* Based on Meyerhof’s Theory ................................................ 46

Table 16. Summary of Point Bearing Pile Load Capacity at 8 m Depth ....................................... 49

Table 17. Summary of Point Bearing Pile Load Capacity at 32 m Depth ...................................... 49

Table 18. Recommended Average Values for K (Das, 2007a) .......................................................... 51

Table 19. Summary of the Skin Friction Resistance at 8 m ................................................................ 53

Table 20. Total Foundation Pile Capacity at 8 m ................................................................................... 54

Table 21. Variation of λ with Pile Embedment Length (Extracted From Deep Foundation, Theory and Practice) ........................................................................................................................................ 55

Table 22. Variation of α (Interpolated Values Based on Terzaghi, Peck and Mesri, 1996) ... 57

Table 23. Summary of the Skin Friction Resistance at 32 m ............................................................. 57

Table 24. Total Foundation Pile Capacity at 32 m ................................................................................. 58

Table 25. Foundation Piles Reinforcement .............................................................................................. 58

Table 26. Cost Analysis for the Shoring System Alternative ............................................................. 62

Table 27. Cost Analysis for the Pile Foundations Alternative........................................................... 63

Table 28. Typical Floor Column Loads Exported from Etabs ........................................................... 68

Table 29. Maximum Pu for all Columns ..................................................................................................... 69

1

A. INTRODUCTION

I was introduced to this project, which is to be built in Lagos Nigeria, by a friend who was

working on the geotechnical design of the project’s deep foundations. The structure consists of a

12 story residential building to be built on poor soil conditions with a high water table. The local

geotechnical engineer in Lagos had recommended that the structure be supported on deep

foundation starting from ground surface with no underground basements. Since I had a strong

interest in completing my Senior II project with emphasis on geotechnical engineering, I thought

that this project would be a good candidate. After showing the project’s geotechnical report to

Mr. Bouchedid, he agreed to be my advisor on this project. However, he wanted me to check if

we can optimize the design in any way and make it more economical to the developer. Therefore,

the project was divided into three main alternatives:

1. Check the alternative of using raft foundations at different levels

2. Check the alternative of using drilled pile foundations if no basements are to be used

3. Design the shoring system for the raft foundation within a deep excavation alternative

The design is based on codes and regulations requested by the developer which will be further

described in Section B.3. The deep foundation alternative includes supporting the building on

drilled piles 8 or 32 m long. The raft foundation alternative consists of completing a 9 meter

excavation to build the foundation on harder soil. In this alternative, a shoring system will be

required.

The soil properties used in this report were obtained from the geotechnical report provided by a

local geotechnical engineer. The soil parameters that are not included in the geotechnical report

were estimated using the existing borehole logs which include SPT values. Once the soil

parameters are estimated, the bearing capacity for a raft foundation is checked at three levels

2

including 1.75 m, 4 m, and 9 m below ground surface. As is shown in subsequent sections of this

report, the allowable bearing capacity at 1.75 m and 4 m below ground surface is smaller than

the load applied by the building on the raft foundation. However, the allowable bearing capacity

at 9 m depth was found to be greater than the loads applied by the structure on the raft

foundation. The shoring system for the 9 m deep excavation consists of a diaphragm wall and

ground anchors.

The computer modeling is completed using the software PLAXIS 2D, which is a finite element

package intended for the two dimensional analysis of deformation and stability in geotechnical

engineering.

Since the local geotechnical engineer had recommended a deep foundation system and provided

pile capacity in the geotechnical report, an independent check was completed on pile capacities

using different methods to confirm the information provided in the report.

For both proposed alternatives, a cost analysis will be prepared to help the developer choose the

best alternative for his project. As is shown in the cost analysis section, the raft foundation at a

depth of 9 meters below ground surface using a diaphragm wall as a shoring system is more

economical than using a deep foundation system starting from ground surface with no

basements.

3

B. GOAL AND METHODOLOGY

One of the most important goals of this project is to provide the developer multiple foundation

options for his project in order to help him choose the most economical one, which gives him the

highest return on his investment. In this project, the recommended solution to the soft soils

problem in the upper soil layer consists of adding two underground basements that can be sold

after the project is completed.

B.1. OBJECTIVES

The objectives of this project are as follows:

Check the data provided in the existing geotechnical report.

Estimate the soil properties needed for the design but not included in the existing

geotechnical report.

Check the alternative of using raft foundations at different levels.

Check the alternative of using drilled pile foundations if no basements are to be used.

Design a shoring system for the deep excavation alternative.

Complete a cost analysis for both alternatives.

Recommend the best alternative for execution.

B.2. ASSUMPTIONS

All constraints and assumption are stated below:

The soil parameters of the top 1.75 m of surficial soil which consists of fill and top soil were

not included in the geotechnical report.

For the calculation of the SPT N60 value, the hammer type is not mentioned in the

geotechnical report, therefore it is assumed to be a Donut hammer.

4

The allowable total settlement of a raft foundation resting on sand for a residential building

was assumed to be 50 mm as per ACI-318 recommendation.

B.3. CODES, STANDARDS AND RULES OF PRACTICE

The existing geotechnical report was completed according to the British Standard Institute (BSI)

to estimate the soil parameters. Therefore, the BSI will be used for analyzing the shoring system

and the drilled pile design.

The American Concrete Institute (ACI) code was used for the structural reinforcement for the

Diaphragm wall and for the tendons used for the ground anchors.

B.4. PURPOSE OF THIS PROJECT

The purpose of this report is to design the raft foundation, shoring system, and deep foundations

of a residential building in Lagos, Nigeria based on geotechnical information provided by a local

geotechnical firm in Lagos. The design that will be completed as part of this project will be

compared with the design recommendations provided in the geotechnical report to evaluate if the

project can be completed more efficiently while maintaining safety standards required by the

design codes.

B.5. SCOPE OF THE PROJECT

This report includes the design of a shoring system, a dewatering system, and the foundations of

a residential building consisting of 12 stories and 2 basements, located in Lagos, Nigeria.

As part of this design the required type, size, and length of drilled shafts will be determined

along with their reinforcements for the deep foundations. In addition, the shoring system, which

consists of a diaphragm wall and anchors, will be designed.

The possibility of a shallow foundation system consisting of spread footings or mat foundation

will be evaluated.

5

B.6. LOCATION OF THE PROJECT

The project is located in Lagos, Nigeria, in the downtown area, at the location shown in a red

circle in Figure 1. The city of Lagos is the main city of the south-western part of Nigeria. Some

rivers, like Badagry Creek, flow parallel to the coast for some distance before exiting through the

sand bars to the sea. The two major urban islands of Lagos in the Lagos Lagoon are Lagos Island

and Victoria Island. These islands are separated from the mainland by the main channel draining

the lagoon into the Atlantic Ocean, which forms Lagos Harbour. The islands are separated from

each other by creeks of varying sizes and are connected to Lagos Island by bridges. The smaller

sections of some creeks have been sand filled and built over.

Figure 1. Map of Lagos (Extracted from Google Earth, August 2015)

B.7. SUMMARY OF THE GEOTECHNICAL REPORT

The soil report submitted by a local geotechnical firm in Lagos (Basol Associates Limited)

includes some of the important data needed for the design. Assumptions are made when needed

using codes and manuals, if the required information is not included in the report. The project

elevation is 15 meters above sea level.

http://en.wikipedia.org/wiki/Badagry

http://en.wikipedia.org/wiki/Victoria_Island_(Nigeria)

6

Boreholes:

The subsurface investigations involved exploratory borings using a Pilcon Wayfarer Shell and

Auger cable percussion drilling rig. Boring logs summarizing the results of the exploration are

included in the Appendix. In addition to the borings, Dutch cone penetrometer Testing (DCPT)

was performed using a 10 ton rig. A total of two borings and four DCPTs were completed at the

project site as shown in Figure 3. Laboratory testing of samples recovered from the boreholes

was completed.

Figure 2. Completed Boreholes Layout (Extracted from the Geotechnical Report by Basol Associates Ltd.,

2014)

The shell and auger cable percussion boring was drilled to about 40 m in depth. Each of the four

DCPTs was pushed to a depth of about 6.2 m below the existing ground surface where it

7

encountered refusal; this is because of the inability of the DCPT machine to penetrate through

gravel and concrete rubble present in the upper layer.

During the drilling program, samples were recovered at regular intervals of 0.75 m, while

standard penetration tests (SPT) were carried out at alternative intervals of 1.5 m. The subsurface

profile is summarized in Table1.

Table 1. Subsurface Profile Summary

Depth Range (m) Description of Sub-soil Encountered

0 to 1.5 Loose, wet, dark grey, silty SAND with fine to coarse gravel, concrete rubble and

plant roots (top soil/old fill)

1.5 to 13 Medium dense, becoming dense, dark/yellowish brown/grey silty SAND with fine

to coarse gravel, wet.

13 to 26 Firm to stiff, yellowish brown/grey, mottled silty, Sand CLAY with fine gravel, dry.

26 to 40 Medium dense/dense, dark grey/grey silty SAND with fine to coarse gravel, wet

Groundwater Conditions

Groundwater was encountered within the drilled boreholes and in the DCPT tests at about 1 m

below the existing ground surface. It should be noted that the investigation was carried out

during the dry season. Thus, the water level would be higher and the site would potentially be

prone to flooding at the peak of the wet season in view of the geology and topography of the area

and site.

Bearing Capacity and Settlements

Based on the in-situ and laboratory test results on samples obtained from the borings, a general

safe (allowable) bearing capacity value of 105 kN/m2 was recommended in the report for rigid,

8

well reinforced square or circular footings placed at a minimum depth of 1.75 meters below the

existing ground level, with a factor of safety of 3.

Shallow Foundations Recommendations

Soil parameters were obtained from available boring records, in-situ field data, and laboratory

test results obtained from the subsoil samples recovered during drilling and from results of the

DCPT results. The proposed building imposes approximately 180 kN/m2 of load which is higher

than the allowable bearing capacity at ground surface. Therefore, a raft foundation at ground

surface is not recommended for the proposed building.

However, shallow foundations in the form of spread footings placed at depths up to 1 m could be

adopted for ancillary structures made up of gates, generator houses, upon filling or compacting

the existing ground to densify the loose sand.

Deep Foundations Recommendations

Since ground improvement techniques to improve the allowable bearing capacity at ground

surface is not an option due to its high cost, deep foundation design in the form of drilled piles

may be the best alternative if no basements are to be built. Pile foundations would minimize

settlement of the proposed development and allow construction to commence immediately.

The following pile working loads were included in the geotechnical report as a guide based on

data obtained from the borings and DCPT results for bored cast-in-place piles:

Table 2. Option A: 8 m Long Piles (Extracted from Basol Associates Ltd., 2014)

Pile Type Pile Length (m)

Safe Working Load (kN)

Factor of Safety

45 cm Bored cast-in-place pile 8 560 3 60 cm Bored cast-in-place pile 8 1,010 3 80 cm Bored cast-in-place pile 8 1,865 3

9

Settlement of the proposed building on piles with the above quoted Safe Working Loads (SWL)

for Option A is expected to be minimal since the proposed piles will terminate within a medium

dense to dense sand, knowing that the first five meters of the second layer are medium dense,

then for the remaining part of the second layer, the sand will become dense.

Table 3. Option B: 32 m Long Piles (Extracted from Basol Associates Ltd., 2014)

Pile Type Pile Length (m)

Safe Working Load (kN)

Factor of Safety

45 cm Bored cast-in-place pile 32 1,085 3 60 cm Bored cast-in-place pile 32 1,710 3 80 cm Bored cast-in-place pile 32 2,725 3

Settlement of the proposed building on piles with the above quoted (SWL) for Option B is

expected to be minimal since the proposed piles will terminate within dense soils. These values

will be independently verified using different methods summarized in subsequent sections.

10

C. RESULTS AND ANALYSIS

C.1. SOIL PROPERTIES CALCULATION

All soil properties were determined according to ASTM, using the Standard Penetration Test that

was completed in accordance with B.S 1377:1975, Test 19. A split barrel thick-walled sampler

“split spoon” of about 35 mm internal diameter is driven 450 mm into the soil by repeated blows

from a trip hammer weighing 65 kg and free falling 760 mm. Note that the ground surface at the

project site is considered to be flat since no information was provided regarding the grade of the

site.

Table 4. SPT N-Values for Boreholes 1 and 2

BH1 BH2

Depth

(m) SPT N-Value

Depth

(m) SPT N-Value

Layer 1 0 0 0 0

-1.5 10 -1.5 12

Layer 2

-2.5 16 -2.5 15

-4 22 -4 20

-6 24 -5.5 22

-7 27 -7.5 25

-8.5 30 -8.5 28

-10 30 -10 30

-11.5 38 -12 33

-13 12 -13 12

Layer 3

-14.5 10 -14.5 12

-16 11 -17.5 11

-17.5 12 -22 12

-19 12 -23.5 14

-20.5 12 -25 18

-22 12 -26.5 22

-25 10 -28 24

-26 15 -30 24

Layer 4

-28 20 -31.5 25

-29.5 22 -32.5 25

-31 25 -34 27

-32.5 25 -36 33

-35 27 -37.5 35

-36 30

-37.5 33

-38.5 35

11

Figure 3. Distribution of the Measured SPT Blow Counts with Depth

SPT N60 Correction

The standard of practice is to express the SPT N-values to an average energy ration of 60%

(N60). Correcting the field data for the SPT N-value is as follows:

N60 = N×(NH)×(NB)×(NS)×(NR)

60 (Eq.1)

N60 = SPT N-value to an average energy ratio of 60%

N = Field SPT N-value

NH = Hammer efficiency

NB = Borehole diameter correction factor

NS = Sampler correction factor

NR = Rod length correction factor

12

Table 5. Variation of NH, NB, NS and NR (Das, 2007b)

Variation of NH

Variation of NH

Country Hammer

Type Hammer Release

NH

(%)

Japan Donut Free Fall 78

Donut Rope and Pulley 67

U.S Safety Rope and Pulley 60


Argentina Donut Rope and Pulley 45

China Donut Free Fall 60


Variation of NB

Variation of NB

Diameter

(mm) NB

60-120 1

150 1.05

200 1.15

Variation of NS

Variation of Ns

Variable Ns

Standard Sampler 1

With Liner for Dense Sand and Clay 0.8

With Liner for Loose Sand 0.9

Variation of NR

Variation of NR

Rod Length

(mm) NR

>10 1

60-100 0.95

400-600 0.85

0-400 0.75

Example calculation Layer 1 (0 to 1.5m): Disregarded because it consists of old fill

Layer 2 (1.5 to 13m): Average N-Value = 24 N60 = 24×45×1.05×1×0.75

60 = 14.175

Layer 3 (13 to 26m): Average N-Value = 14.437 N60 = 8.52

Layer 4 (26 to 40m): Average N-Value = 27.84 N60 = 16.443

13

Friction Angle

The angle of internal friction (friction angle) is a measure of the ability of a unit of rock or soil to

withstand a shear stress. It is the angle (), measured between the normal force (N) and resultant

force (R), that is attained when failure just occurs in response to a shearing stress (S).

Peck, Hanson and Thornburn (1974) give a correlation between SPT N60 value and the friction

angle which can be approximated as follows (Wolff, 1989):

’ = 27.1 + 0.3N60 – 0.00054(N60)2 (Eq.2)

Example calculation

Layer 1 (0 to 1.5 m): Disregarded because it consists of old fill

Layer 2 (1.5 to 13 m): ’ = 27.1 + 0.3x14.175–0.00054 x (14.175)2 = 31.34o

Layer 3 (13 to 26 m): Based on Triaxial test in geotechnical report, ’ = 5o

Layer 4 (26 to 40 m): ’ = 27.1 + 0.3x16.443– 0.00054 x (16.443)2 = 31.73o

Figure 4. Triaxial Compression Test Mohr Circle Diagram (Extracted from Basol Associates Ltd., 2014)

14

Modulus of Elasticity (Es)

Young's modulus (ES) describes tensile elasticity, or the tendency of an object to deform along

an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress

to tensile strain. It is often referred to simply as the elastic modulus.

The modulus of elasticity is important in estimating the elastic settlement of foundations. The

first order of estimation was given by Kulhawy and Mayne (1990) as follows:

𝐸𝑠

𝑃𝑎= ∝ 𝑁60 (Eq.3)

Pa: Atmospheric pressure = 100 kN/m2

α: constant

For our project, an approximated value using the classification of soil will be used as follows:

Layer 1 (0 to 1.5 m) Es = 12000 kN/m2

Layer 2 (1.5 to 13 m) Es = 45000 kN/m2

Layer 3 (13 to 26 m) Es = 40000 kN/m2

Layer 4 (26 to 40 m) Es = 45000 kN/m2

Cohesion

Cohesion is the component of shear strength of a rock or soil that is independent of internal

particle friction. In soils, true cohesion is caused by electrostatic forces in stiff overconsolidated

clays (which may be lost through weathering), it was estimated using the triaxial test and

reported in the geotechnical report as follows:

Layer 1 (0 to 1.5 m): not considered because it consist of old fill

Layer 2 (1.5 to 13 m): Assumed Cu = 0 kN/m2 for silty Sand (no Triaxial test data

available)

Layer 3 (13 to 26 m): Cu = 68 kN/m2

15

Layer 4 (26 to 40 m): Assumed Cu = 0 kN/m2 because silty Sand (no Triaxial test data

available)

C.2. CHECK FOR BEARING CAPACITY

Initially, a bearing capacity check of the soil in the first layer at a depth of 1.75 m is determined

to check whether the soil can handle the column loads.

Bearing Capacity under Mat Foundation

The net allowable bearing capacity for mats constructed over granular deposits can be adequately

determined from the standard penetration resistance number using the following approximated

equation:

𝑞 𝑛𝑒𝑡, 𝑎𝑙𝑙 = 𝑁60

0.08× 𝐹𝑑 ×

𝑆𝑒

25 (Eq.4)

Where

N60 = Standard Resistance Number

Fd = 1 + 0.33𝐷𝑓

𝐵 must be ≤ 1.33

Se = Settlement (mm) assumed to be 50 mm for sand

B = Width of the Mat (m)

Option 1: Bearing Capacity at 1.75 m Depth

N60 (z=1.75m) = 10.5 ==> Fd = 1 + 0.33×1.75

23.5= 1.0245 < 1.33

==> 𝑞 𝑛𝑒𝑡, 𝑎𝑙𝑙 = 10.5

0.08× 1.0245 ×

50

25 = 268.93 kN/m2

Following the conventional rigid method of mat foundation design procedure:

q = 𝑄

𝐴±

𝑀𝑦𝑋

𝐼𝑦±

𝑀𝑥𝑌

𝐼𝑥 (Eq. 5)

Where

A = area of the mat (m2)

16

Ix = moment of inertia about the x-axis

Iy = moment of inertia about the y-axis

Ex = Load eccentricity in the x direction = x’ -B/2

Ey = Load eccentricity in the y direction = y’ -L/2

Mx = Qey moment of the column loads about the x-axis

My = Qex moment of the column loads about the y-axis

Q = Total Column Loads

A = 23.5× 36.25 = 851.875 m2

Ix = 1

12BL3 =

1

12(23.5) (36.25)3= 93284.7 m4

Iy = 1

12LB3 =

1

12(36.25) (23.5)3= 39203.9 m4

Q = 6x2009 + 3392x4 + 4129x4 + 1828x5 + 6366x7 + 6572x4 + 6235x2 + 4598x3 + 4340x1 +

5762x1 + 4197x4 + 11800x2

Q = 198876 kN

17

Figure 5. Load Eccentricities along x and y Directions

ex= x’ −𝐵

2

x’ = 17.3507 m

ex = 17.35 –23.5

2 = 5.6 m

ey= y’ −𝐿

2

Y’ = 11.593 m

ey= 11.593 –35.25

2 = −6.03 m

q = 233.45 ± 28.4x ± 12.85y

Column C7 at the left edge will apply the highest point pressure on the soil as follows:

18

Figure 6. Checking Column Pressure on Soil

q = 233.45 + 28.4×11.5 + 12.85×15.52 = 759.482 kN/m2 > qnet,all not acceptable

Therefore a mat foundation at 1.75 m depth is not adequate.

Option 2: Bearing Capacity at 4 m Depth

After getting inadequate results from the preceding option, the bearing capacity is checked at 4

meters depth which is equivalent to adding a basement level to the structure.

N60 (z=4m) = 21 ==> Fd = 1 + 0.33×4

23.5= 1.056 < 1.33

𝑞 𝑛𝑒𝑡, 𝑎𝑙𝑙 = 21

0.08× 1.056 ×

50

25 = 554.48 kN/m2 < 759.482

This option is also not adequate; therefore the depth is increased by another 4 m which is

equivalent to adding another basement level to the structure.

Option 3: Bearing Capacity at 9 m Depth

N60 (z=9m) = 28 ==> Fd = 1 + 0.33×8

23.5= 1.112 < 1.33

𝑞 𝑛𝑒𝑡, 𝑎𝑙𝑙 = 28

0.08× 1.112 ×

50

25 = 778.63 kN/m2 > 759.48 OK

19

Therefore, increasing the depth of the raft to 9 meters below ground surface will be adequate for

the soil bearing capacity to handle all column loads under the mat foundation. However, by

adding two basement levels to the structure, a shoring system should be constructed to maintain

stability of the excavation.

C.3 SHORING SYSTEM DESIGN

This section includes the design of the shoring system which consists of a diaphragm wall

supported by anchors. This system will provide slope stability for the excavation. The wall and

anchors must interact and work together in order to resist earth pressure loads and surcharges

developing during and after construction. In addition, they should restrict deformations to

acceptable values. As the wall deflects toward the excavation under lateral loading, the anchor

stretches and initiates the load transfer to the fixed zone. The fixity imposed on the anchorage by

the soil restrains further wall deflection.

Figure 7. Diaphragm Wall Supported by Ground Anchors (Extracted from the Internet for Illustration)

20

Ground Anchors

A ground anchor normally consists of:

A high tensile steel cable or bar, called the tendon, one end of which is held securely in

the soil by a mass of cement grout.

The other end of the tendon is anchored against a bearing plate on the structural unit to be

supported

In general we can consider that an anchor consists of two parts:

The fixed anchor length: the grouted length of tendon, through which force is transmitted

to the surrounding soil.

The free anchor length: the length of tendon between the fixed anchor and the bearing

plate

Figure 8. Ground Anchor Detailing

21

Design Parameters and Process

The proposed solution was to increase the depth of excavation to provide adequate bearing

capacity under the mat foundation. Assuming the height of the proposed two basements is 3.5

meters each, with a 25 cm slab thickness, the excavation should extend from ground surface to 9

meters depth. This assumes that the mat foundation thickness is about 1.5 m.

After checking several sections and embedment depths, a 16 meters long diaphragm wall,

including 7 m embedment depth below the bottom of the excavation, and 40 cm thickness is

recommended for this project. A diaphragm wall is recommended instead of drilled secant piles

due to the high water table of the site and a full saturation of the second layer (silty sand), in

addition to the presence of concrete construction rubble in the top layer.

As was mentioned earlier, the top soil layer consists of fully saturated silty sand underlain by dry

sandy clay. Therefore it is recommended that test pits be dug inside the footprint of the

excavation to dewater the site once the D-wall is constructed. This D-wall will prevent the water

to seep into the site from outside while the water level inside the excavation will be lowered

using water pumps inside the test pits.

PLAXIS 2D Modeling

A two-dimensional finite element program PLAXIS 2D has been used to model a D-wall

supporting an excavation.

In a plane strain model all stresses are calculated along the three axes (x,y,z) but deformations

and strains are calculated in the 2D (x,z) plane.

22

The following soil parameters are used in the design

Table 6. PLAXIS 2D Soil Data Input (Layer 1)

Identification Fill

Material model Mohr-Coulomb

Material type Drained

General properties unsat(kN/m3) 14

sat(kN/m3) 14

Permeability Kx(m/day) 1

Ky(m/day) 1

Stiffness

Elasticity modulus Eref

(kN/m2) 12000

Poisson’s Ratio

0.3

strength

Cref

(kN/m2) 1

(o)

28

Interface (Adhesion

Coefficient) Rinter 0.67


Identification Silty Sand




sat(kN/m3) 17


Ky(m/day) 1

Stiffness


(kN/m2) 45000

Poisson’s Ratio

0.3

strength

Cref

(kN/m2) 1

(o)

30.27

Interface (Adhesion


23


Identification Sandy Clay




sat(kN/m3) 17


Ky(m/day) 1

Stiffness


(kN/m2) 40000

Poisson’s Ratio

0.3

Strength

Cref

(KN/m2) 68

(o)

5

Interface (Adhesion


Once the soil layer parameters are entered, the plate parameters, which in our case is the D-wall,

are entered as follows:

Table 9. Diaphragm Wall Properties

EA

(kN/m) 8x106

EI

(kNm2/m) 1.067x105

Thickness

(m) 0.4

Weight

(kN/m/m) 10

Poisson’s Ratio 0.18

Calculation

EA = 8x106 kN/m

EI = 1.067x105 kNm2/m

Weight = 0.4x25 = 10 kN/m/m

Poisson’s ratio of concrete = 0.18

24

Then the node to node anchor which is the free length of the anchor is defined as follows:

Table 10. Anchor Free Length Properties

EA

(kN/m) 135000

Lspacing

(m) 2

Material Type Elastic

Calculation

As specified in the BSI/1989 code the minimum horizontal spacing between two anchors should

be less than 3 m and greater than 1 m. In our case, the deflection was acceptable with a

horizontal spacing of 2 m.

The number of tendons used in our design is 5 strands having a diameter of 12.9 mm and a

modulus of elasticity 29,000 (steel 1860 type).

EA = 27000x5 = 135000 kN/m

Finally the geogrid which is the anchor grouted length should be defined as follows:

Table 11. Anchor Grouted Length Properties

EA

(kN/m) 2.65x104

Material Type Elastic

Calculation:

Egeogrid = 1.5x107𝜋0.152

4= 2.65x104 kN/m

Where the diameter of the grouted length is 15 cm.

The ultimate anchorage force Tu, for a friction type anchor can be calculated by the following

equation:

Tu = πDbLaτult

(Eq. 6)

25

Where

Tu = Ultimate anchorage force = 500 kN/m

Db = Diameter of the fixed section = 15 cm

La = Length of the fixed section (grouted length)

τult = Average ultimate shear resistance strength per unit area between the fixed section and the

soil.

The factor of safety is chosen using the following table:

Table 12. Factor of Safety for Single Anchor (CICHE, 1998)

Classification Tensile force

of tendon

Anchoring

Force

Bond Force

Of Tendon

temporary anchors whose working

period is not longer than 6 months

and which

do not affect public safety when

failing

1.4 2 2

temporary anchors whose

working period is not longer than 2

years and which do not affect

public safety when failing

1.6 2.5 2.5

Permanent or temporary anchors

which

are highly risky in rusting or which

affect public safety seriously due to

failure

2 3 3

La-first, row = 500×2.5

𝜋×0.15×176.5 = 15 m

La-second, row = 500×2.5

𝜋×0.15×196.5 = 13.5 m

26

Using the following table the factor of safety will be chosen:

Table 13. Ultimate Frictional Strength of an Anchorage Body (Extracted from Deep Excavation)

Type Of soil τult

(kg/cm2)

Rock

Hard Rock 15-25

Soft Rock 10--15

Weathered Rock 6--10

Mudstone 6--12

Gravel

N=10 1--2

N=20 1.7--2.5

N=30 2.5--3.5

N=40 3.5--4.5

N=50 4.5--7

Sand

N=10 1--1.4

N=20 1.8--2.2

N=30 2.3--2.7

N=40 2.9--3.5

N=50 3--4

τult (Sand Layer) = 176.5 kN/m2

τult (clay Layer) = 196.5 kN/m2

By drawing the assumed failure surface at 𝜋

4 +

𝜙

2 = 60o, the free length of the anchor can be

determined. Two meters should be added to the free length as a safety for this assumption as

shown in Figure 9 below.

27

Figure 9. Anchor Free Length According to the Assumed Failure Surface

After entering all the input data, we start by drawing the model on PLAXIS as shown in Figure

10 below.

Figure 10. Drawing the Model on PLAXIS

As previously mentioned, PLAXIS 2D is a finite element software that works by dividing the

soil layers into small portions to calculate the stress at each node. The model was taken as a 15

node element in a plain strain model.

28

Figure 11. PLAXIS Mesh Generation

After generating meshes, the model should be updated with initial conditions such as water

pressure, water table and stresses.

Figure 12. Water Table Definition

29

Figure 13. Water Pressure Shadings

Knowing that the third layer is stiff clay, a dry cluster is defined, to reduce the upheaval pressure.

Figure 14. Layer 3 Defined as a Dry Cluster

Figure 15. Initial Stress Definition

30

Afterwards, the construction phases are defined; these phases are a projection of the real

execution process.

Phase 1, Defining Surrounding Loads: a setback of 3 meters exists between the adjacent parcel

and the excavation, therefore a 5 kN/m2 surcharge was used. A small building exists in the

adjacent property; therefore a 20 kN/m2 surcharge was used for the building.

Figure 16. Load Definition

Phase 2, Plate Definition: The plate consists of the 16 m long and 40 cm thick diaphragm wall,

therefore this phase includes drilling and pouring the diaphragm wall.

Figure 17. Plate Definition

31

Phase 3, Excavation Stage 1: This phase includes the excavation of the first 3 m and updating the

water table to that level.

Figure 18. Excavating First 3 Meters

Phase 4, Defining Row 1 of Anchors: This phase includes defining and activating the first row of

anchors, which includes a 100 kN/m force.

Figure 19. Activating First Row of Anchors

32

Phase 5 Excavation Stage 2: This phase includes excavating the second 3 m and updating the

water table to that level.

Figure 20. Excavating Second 3 Meters

Phase 6, Defining Row 2 of Anchors: This phase includes defining and activating the second

row of anchors, which includes a 100 kN/m force.

Figure 21. Activating Second Row of Anchors

33

Phase 7, Excavation Stage 3: This phase includes excavating the last 3 m and lowering the

water table to 11 meters below ground surface which is 2 meters below the bottom of the

excavation.

Figure 22. Excavating Last 3 Meters

Phase 8, Phi/C Reduction: This phase includes running the program to calculate the factors of

safety to ensure a global factor of safety greater or equal than 1.25.

Figure 23. Phi/C Reduction

34

Once all the phases are defined, we run the program as shown in Figure 24 below:

Figure 24. Run Calculation

After verifying that all stages are safe for construction, an output of the model will be generated

as shown in the following figures:

Figure 25. Total Displacement of the Shoring System

35

Figure 26. Total Horizontal Displacement of the Shoring System

Figure 27. Total Vertical Displacement of the Shoring System

36

Figure 28. Plate Horizontal Displacement

37

Figure 29. Plate Vertical Displacement

Figure 30. Axial Forces on the Plate

38

Figure 31. Shear Forces on the Plate

Figure 32. Bending Moment on the Plate

39

Figure 33. Anchor 1 Horizontal Displacement

Figure 34. Anchor 2 Horizontal Displacement

40

Table 14. PLAXIS Outcomes Vs Acceptable Values

Analyzed Values Acceptable Values

Total Horizontal Displacement 6.692 cm < 10 cm ==> O.K

Total Vertical Displacement 5.935 cm < 10 cm ==> O.K

Plate Horizontal Displacement 6.692 cm < 10 cm ==> O.K

Plate Vertical Displacement 722.28x10-8 cm < 10 cm ==> O.K

Axial Force on the Plate 300.6 kN/m < 700 kN/m ==> O.K

Shear Forces on the Plate 151.9 kN/m < 350 kN/m ==> O.K

Bending Moments on the Plate 221.44 kNm/m < 714 kNm/m ==> O.K

Anchor Total Displacement 4 cm < 8 cm ==> O.K

The acceptable values defined in Table 14 may differ from one project to another. This usually

depends on several factors including the owner, the consultant, the type of construction, and the

type of codes used.

Diaphragm Wall Design

The design of the diaphragm wall includes the wall thickness and reinforcements. The thickness

of the D-wall usually depends on the stress analysis, the deformation analysis, and the concrete

reinforcement.

The reinforcement design follows the load and resistance factor design (LRFD). The main items

of design include the vertical and horizontal reinforcements as well as the shear reinforcement.

Based on the bending moment and shear envelop obtained from the PLAXIS 2D analysis of the

plate and according to the ACI code the following section will give a detailed illustration and

calculation of the reinforcement for the diaphragm wall.

For Bending:

Mu = 𝐿𝑓×𝑀

𝛼 (Eq. 7)

Mn = 𝑀𝑢

𝜙 (Eq. 8)

41

For Shear:

Vu = 𝐿𝑓×𝑉

𝛼 (Eq. 9)

Vn = 𝑉𝑢

𝜙 (Eq. 10)

Where

Mu = Bending moment for design

Mn = Nominal bending moment

Vu = Shear for design

Vn = Nominal shear

M = Bending moment obtained from PLAXIS 2D analysis

V = Shear obtained from PLAXIS 2D analysis

Lf = Load resistance factor = 1.6 according to ACI (2008)

ϕ = Strength reduction factor = 0.9 for bending moment and 0.75 for shear

α = Short term magnified factor for allowable stress = 1

Vertical Reinforcements

Figure 35. Reinforced Concrete Beam Stress in the Ultimate State (Deep Excavation, Theory and Practice)

42

As shown in Figure 35, the nominal resistance bending moment of concrete is

MR = 1

𝜙[𝑃𝑚𝑎𝑥 × 𝑓𝑦(1 − 0.59

𝑃𝑚𝑎𝑥×𝑓𝑦

𝑓′𝑐)] 𝑏𝑑2 (Eq. 11)

Where

D = distance from extreme fiber to the centroid of the steel layer

Pmax = 0.75Pb

f’c = Compressive strength of concrete

fy = Steel yield strength

Pb = Reinforcements ratio = 0.85𝑓′𝑐

𝑓𝑦 β1 (

6120

6120+𝑓𝑦) , β1 = 0.85 for f’c = 25 MPa

MR= 1

0.9[0.0365 × 420(1 − 0.59

0.0365×420

25)] 1 × 0.272 = 792.486 kN-m

Mu = 1.6×221.44 = 354.304 < ϕMR= 713.3 kN-m, therefore only tension reinforcement should be

designed for.

C.4. DEWATERING SYSTEM

The dewatering system is proposed to be done using the open sumps method. This method

consists of collecting the ground water seeping into an excavation from pits typically excavated

near the perimeter as shown in Figure 36.

The open sump method is the most common and economical method of dewatering when

applicable.

43

Figure 36. Open Sumps Method for Dewatering

C.5. PILE FOUNDATION DESIGN

Piles are structural members of timber, concrete, and/or steel that are used to transmit surface

loads to lower levels in the soil mass. This transfer may be by vertical distribution of the load

along the pile shaft or a direct application of the load to a lower stratum through the pile point. A

vertical distribution of the load is made using a friction or floating pile and a direct load

application is made by a point pile. This distinction is purely one of convenience since all piles

carry load as a combination of side resistance and point bearing except when the pile penetrates

an extremely soft soil to a solid base. Piles are commonly used for the following purposes:

To carry the superstructure loads into or through a soil stratum. Both vertical and lateral

loads may be involved

To resist uplift, or overturning forces, such as for basement mats below the water table or

to support tower legs subjected to overturning from lateral loads such as wind

To compact loose, cohesionless deposits through a combination of pile volume

displacement and driving vibrations. These piles may be pulled out of the ground later

44

To control settlements when spread footings or a mat is on a marginal soil or is underlain

by a highly compressible stratum

To stiffen the soil beneath machine foundations to control both amplitudes of vibration

and the natural frequency of the system

As an additional safety factor beneath bridge abutments and/or piers, particularly if scour

is a potential problem

In offshore construction to transmit loads above the water surface through the water and

into the underlying soil. This case is one in which partially embedded piling is subjected

to vertical (and buckling) as well as lateral loads

Cast-in-Place Piles

A cast-in-place pile is formed by drilling a hole in the ground and filling it with concrete. The

hole may be drilled, or formed by driving a shell or casing into the ground.

The casing may be driven using a mandrel, after which withdrawal of the mandrel empties the

casing. The casing may also be driven with a driving tip on the point, providing a shell that is

ready for filling with concrete immediately. The casing may also be driven open-end, where the

soil entrapped inside the casing can be jetted out after the driving is completed. Various methods

with slightly different end results are available and patented.

Estimation of Pile Load Capacity at 8 m Depth

The ultimate load-carrying capacity Qu of a pile is given by the following equation:

Qu = Qp + Qs (Eq. 12)

Where

Qp = Load-carrying capacity of the pile point

Qs = Frictional resistance or skin friction derived from the soil-pile interface.

45

Figure 37. Ultimate Load-Carrying of Pile

Point Bearing Capacity

There are many methods to estimate the point bearing capacity of a pile. In this report, it will be

discussed and calculated using the methods of Meyerhof, Vesic and Coyle-Costello. The average

of these three methods will be used for design.

After calculating the total point bearing capacity, the factor of safety should be used to obtain the

total allowable load per each pile, or

Qall = 𝑄𝑢

𝐹𝑆 (Eq. 13)

Typically for residential buildings, F.S. = 3

Meyerhof’s Method

In general, the point load capacity in sand increases with depth of embedment. However, beyond

the critical embedment ratio, (LB/D)cr, the value of Qp remains constant.

For the case of piles in sand, where c = 0, the following equation applies:

Qp = (Ap)(q’)(Nq*) (Eq. 14)

46

Where

Ap = Area of the pile

q = Effective vertical stress at the level of the pile tip = ’L

Nq* = Bearing capacity factor

The pile capacity will be investigated for the following diameters: 45 cm, 60 cm, and 80 cm.

= 30o from table 15, Nq* = 56.7

Table 15. Interpolated Value of Nq* Based on Meyerhof’s Theory

Soil Friction Angle Nq*

20 12.4

21 13.8

22 15.5

23 17.9

24 21.4

25 26

26 29.5

27 34

28 39.7

29 46.5

30 56.7

31 68.2

32 81

33 96

34 115

35 143

36 168

37 194

38 231

39 276

40 346

Qp (D = 45 cm) = 0.159x14x8x56.7 = 1226 kN Qu = 1226

3= 336 kN

Qp (D = 60 cm) = 0.282x14x8x56.7 = 2178 kN Qu = 2178

3= 598 kN

Qp (D = 80 cm) = 0.5x14x8x56.7 = 3873 kN Qu = 3873

3= 1063 kN

47

Vesic’s Method

Vesic (1977) proposed a method for estimating the pile point bearing capacity based on the

theory of expansion of cavities. According to this theory and the basis of effective stress

parameters, the following expression shall be used:

Qp = (Ap)( o’)(NϬ*) (Eq. 15)

Where

o’= Mean effective normal ground stress at the level of pile point = 1+2𝐾0

3xq

Ko = Earth pressure coefficient at rest = 1 −sin

NϬ* = Bearing capacity factor

o’ = [1+2( 1 − sin Ѻ)

3]xq’ = [

1+2( 1 − sin 30)

3]x14x8 = 74.66 kN/m2

(Es)(Pa)(m) = 100 x 450 (medium dense soil) = 45000 kN/m2

Poisson’s ratio (s) = 0.3

The rigidity index Ir = 𝐸𝑠

2(1+𝑢𝑠)𝑞𝑡𝑎𝑛Ѻ =

45000

2(1 + 0.3)14×8×𝑡𝑎𝑛30 = 267.65

The modified rigidity index = 267.65

1+220.425×0.011475 = 75.82

NϬ* = 45

Qp (D = 45 cm) = (0.159)( 74.66)(45) = 650 kNQu = 650

3= 177 kN

Qp (D = 60 cm) = (0.282)( 74.66)(45) = 1150 kNQu = 1150

3= 316 kN

Qp (D = 80 cm) = (0.5)( 74.66)(45) = 2040 kNQu = 2040

3= 560 kN

Coyle Costello Method

Coyle and Costello (1981) analyzed 24 large scale field load test of driven piles in sand and these

results gave the following equation:

48

Qp = (Ap)(q)(Nq*) (Eq. 16)

Where

Ap = Area of the pile

q = Effective vertical stress at the level of the pile tip = L

Nq* = Bearing capacity factor

Figure 38. Variation of Nq* with L/D (Extracted from Das 2007b, after Coyle and Costello)

Qp (D = 45 cm) = (0.159)( 14)(18)(30) = 1460 kN Qu = 1460

3= 402 kN

Qp (D = 60 cm) = (0.282)( 14)(18)(28) = 2423 kN Qu = 2423

3= 665 kN

Qp (D = 80 cm)= (0.5)( 14)(18)(25) = 3845 kN Qu = 3845

3= 1056 kN

49

Table 16. Summary of Point Bearing Pile Load Capacity at 8 m Depth

Pile Diameter

(cm)

Meyerhof

(kN)

Vesic

(kN)

Coyle Costello

(kN)

Factor of Safety

Average

(kN)

45 336 177 402 3 305

60 598 316 665 3 527

80 1063 560 1056 3 893

Table 17. Summary of Point Bearing Pile Load Capacity at 32 m Depth

Pile Diameter

(cm)

Meyerhof

(kN)

Vesic

(kN)

Coyle Costello

(kN)

Factor of Safety

Average

(kN)

45 1635 215 435 3 760

60 2900 383 665 3 1316

80 5140 680 998 3 2225

Frictional Skin Resistance

The frictional skin resistance of a pile can be calculated using the following equation:

Qs = ∑ 𝑃∆𝐿𝑓 (Eq. 17)

Where

P= Perimeter of the pile

∆L= Incremental pile length

f = Unit frictional resistance at any depth

The unit frictional resistance “f”, is hard to estimate. There are many ways to do so. In this report

several methods will be evaluated. Several important factors must be kept in mind:

The nature of the pile, knowing that the process of calculation for driven piles differs

from drilled piles. The vibration caused during pile driving helps densify the soil around

the pile thus increasing the friction angle of the sand

50

It has been shown that the nature of variation of “f” in the field is approximately as

shown in Figure 39 below. It increases with depth more or less linearly to a depth L’ and

remains constant thereafter. The magnitude of the critical depth L’ may be 15 to 20 times

the pile diameter.

Figure 39. Unit Frictional Resistance in Sand (Extracted from Das, 2007b)

L’ will be estimated as 15 diameter of the pile, thus L’ = 15D

At similar depths bored piles will have a lower skin friction compared with driven piles

For Z = 0 to L’

f = Ko’tan

Where

K = effective earth pressure

51

o’ = Effective vertical stress

= Soil pile friction

For Z = L’ to L use f = L’

Table 18. Recommended Average Values for K (Das, 2007a)

Bored Pile Ko = 1 –sinϕ

Low-Displacement Pile Ko = 1 -sinϕ to Ko = 1.4(1 - sinϕ)

High-Displacement Pile Ko = 1 -sinϕ to Ko = 1.8(1 - sinϕ)

The values of should be in the range of 0.5ϕ to 0.8ϕ

Coyle and Costello Method

Qs = (Ko’tan)pL

The earth pressure coefficient K will be deducted from the figure below:

Figure 40. Variation of K with L/D (Extracted from Das 2007b, after Coyle and Costello, 1981)

52

For L = 8 m and D = 45 cm K = 0.9

For L = 8 m and D = 60 cm K = 0.8

For L = 8 m and D = 60 cm K = 0.78

o’ = [1+2( 1 − sin Ѻ)

3]xq = [

1+2( 1 − sin 30)

3]x17x8 = 90.67 kN/m2

= 0.8= 0.8×30 = 24

P (D = 45 cm) = 1.41

Qs = (0.9×90.67×tan24)×1.41×8 = 409 kN

For F.S. = 3 use Qs (D = 45 cm) = 136 kN

P (D = 60 cm) = 1.88

Qs = (0.8×90.67×tan24)×1.88×8 = 485 kN


P (D = 80 cm) = 2.51

Qs = (0.78×90.67×tan24)×2.51×8 = 632 kN


Meyerhof Method

Meyerhof (1976) indicated that “f” for driven piles may be estimated using the standard

penetration number N60 as follows:

f = 0.02PaN60 for high-displacement piles (Eq. 18)

f = 0.01PaN60 for low-displacement piles (Eq. 19)

Where

Pa = Atmospheric pressure = 100 kN/m2

53

At 8 m depth, N60 = 14.175

f = 0.01×100×14.175 = 14.175

P (D = 45 cm) = 1.41; F.S. = 3

Qs (D = 45 cm) = 54 kN

P (D = 60 cm) = 1.88; F.S. = 3

Qs (D = 60 cm) = 71 kN

P (D = 80 cm) = 2.51; F.S. = 3

Qs (D = 80 cm) = 97 kN

Briaud’s Method

Briaud et al (1985) proposed another correlation for unit skin resistance using the standard

penetration resistance as follows:

f = 0.224Pa(N60)0.29 (Eq. 20)

At 8 m depth, (N60)avg = 14.175

f = 0.224×100×14.1750.29 = 48.32

P (D = 45 cm) = 1.41; F.S. = 3

Qs (D=45cm) = 182 kN

P (D = 60 cm) = 1.88; F.S. = 3

Qs (D = 60 cm) =243 kN

P (D = 80 cm) = 2.51; F.S. = 3

Qs (D = 80 cm) = 324 kN

Table 19. Summary of the Skin Friction Resistance at 8 m

Diameter

(cm)

Coyle and Costello

(kN)

Meyerhof

(kN)

Briaud

(kN)

Average

(kN)

45 54 210 182 149

60 71 280 243 198

80 97 380 324 267

54

Table 20. Total Foundation Pile Capacity at 8 m

Pile Diameter

(cm)

Pile Length

(m)

Safe Working Load

(kN)

Factor of

Safety

45 8 454 3

60 8 725 3

80 8 1160 3

Frictional Skin Resistance for 32 m Depth Piles

Three main methods for obtaining and estimating the unit frictional resistance of a pile in the

clay are as follows:

λ Method

Vijayvergiya and Focht (1972) proposed the lambda method based on the assumption that the

displacement of soil caused by a pile results in a passive pressure at any depth and the average

unit skin resistance is:

favg = λ(o’ + 2Cu) (Eq. 21)

Where

o’= Mean effective vertical stress for the entire embedment length

Cu= Shear strength

1’ = [1+2( 1 − sin Ѻ)

3]xq = [

1+2( 1 − sin 30)

3]x14x13 = 121.33 kN/m2

2’ = [1+2( 1 − sin Ѻ)

3]xq = [

1+2( 1 − sin 5)

3]x14x13 = 171.41 kN/m2

’ = [1+2( 1 − sin Ѻ)

3]xq = [

1+2( 1 − sin 30)

3]x14x6 = 56 kN/m2

From table 21, λ (at 13 m to26 m) = 0.1472

55

favg = 𝐴1+𝐴2+𝐴3

32 = 127.84 KN/m2

Qs (D = 45 cm) = 0.1472[ 127.84+2×68]×13×1.41

3+ 171 (sand) = 408 kN

Qs (D = 60 cm) = 0.1472[ 127.84+2×68]×13×1.88

3+ 222 (sand) = 538 kN

Qs (D = 80 cm) = 0.1472[ 127.84+2×68]×13×2.51

3+ 331 (sand) = 752 kN

Table 21. Variation of λ with Pile Embedment Length (Extracted From Deep Foundation, Theory and

Practice)

Embedment Length (m)

0 0.5

5 0.336

10 0.245

15 0.2

20 0.173

25 0.15

30 0.136

35 0.132

40 0.127

50 0.118

60 0.113

70 0.11

80 0.11

90 0.11

Figure 41. Estimation of Frictional Resistance using the λ Method

56

β Method

When piles are driven into a saturated clay layer of soil, the pore water pressure in the soil

around the pile increases. This excess pore water pressure in normally consolidated clays may

increase up to four to six times cu. However, the unit frictional resistance of the pile can be

determined on the basis of the effective stress parameters of the clay, thus the following equation

can be used:

f = βo’ (Eq.22)

o’= Vertical effective stress

β = Ktanϕ’

ϕ’ = Friction angle of the clay

K = Earth pressure coefficient = 1 - sinϕ’

Thus f = (1 - sinϕ’)(tanϕ’)o’ = 16.61

Qs (D = 45cm) = 16.61×1.41×13

3 + 171 (sand) = 273 kN

Qs (D = 60cm) = 16.61×2.88×13

3 + 222 (sand) = 430 kN

Qs (D = 80cm) = 16.61×2.51×13

3 + 331 (sand) = 512 kN

α Method

According to the α method, the unit skin resistance in clayey soils can be represented by the

following equation:

f = αCu

Where α is the empirical adhesion factor. The approximate value of α is shown in Table 22

57

Table 22. Variation of α (Interpolated Values Based on Terzaghi, Peck and Mesri, 1996)

𝑪𝒖

𝑷𝒂 α

≤0.1 1

0.2 0.92

0.3 0.82

0.4 0.74

0.6 0.62

0.8 0.54

1 0.48

1.2 0.42

1.4 0.4

1.6 0.38

1.8 0.36

2 0.35

2.4 0.34

2.8 0.34

Pa is the atmospheric pressure = 100 kN/m2

For Cu = 68 kN/m2, thus α = 0.588

Qs (D = 45cm)= 0.588×1.41×68×13

3 + 171 (sand) = 415 kN

Qs (D = 60 cm) = 0.588×1.88×68×13

3 + 222 (sand) = 548 kN

Qs (D = 80 cm) = 0.588×2.51×68×13

3 + 331 (sand) = 766 kN

Table 23. Summary of the Skin Friction Resistance at 32 m

Diameter

(cm)

α Method

(kN)

β Method

(kN)

λ Method

(kN)

Average

(kN)

45 415 273 408 366

60 548 430 538 506

80 766 512 752 677

58

Table 24. Total Foundation Pile Capacity at 32 m

Pile Diameter

(cm)

Pile Length

(m)

Safe Working Load

(kN)

Factor of

Safety

45 32 1326 3

60 32 1822 3

80 32 2902 3

Pile Foundation Structural Reinforcement

The following table summarizes the reinforcement for each of the drilled pile sections according

to the American concrete institute (ACI-318) also shown in the figures below.

Table 25. Foundation Piles Reinforcement

Column Diameter 45 cm 60 cm 80 cm

Area of Steel 3600 mm2 6000 mm2 8400 mm2

Reinforcement 12 at 20 mm 12 at 25 mm 12 at 30 mm

Figure 42. Pile Reinforcement 45 cm Diameter

59



Group Action in Piled Foundation

The supporting capacity of a group of vertically loaded piles can, in many situations, be

considerably less than the sum of the capacities of the individual piles comprising the group.

In all cases, the elastic and consolidation settlements of the group are greater than those of a

single pile carrying the same working load as that on each pile within the group. This is because

the zone of soil or rock which is stressed by the entire group extends to a much greater width and

depth than the zone beneath the single pile. Even when a pile group is bearing on rock the elastic

60

deformation of the body of rock within the stressed zone can be quite appreciable if the piles are

loaded to their maximum safe capacity.

Group action in piled foundations has resulted in many recorded cases of failure or excessive

settlement, even though loading tests made on a single pile have indicated satisfactory

performance.

The allowable loading on pile groups is sometimes determined by the so-called efficiency

formulae, in which the efficiency of the group is defined as the ratio of the average load per pile

when failure of the complete group occurs, to the load at failure of a single comparable pile.

61

Pile Distribution under Raft Foundation

When designing a raft or spread footings over piles, if a column needs more than one pile to

carry the structural loads, it is recommended that the piles be equally spaced such that their

combined center of gravity coincides with the center of gravity of the column they are

supporting. When piles groups are used, a minimum center to center spacing of three times the

pile diameters used. In order to minimize the amount of drilled piles to be completed on the

project, 32 m long piles are recommended for the deep foundations alternative. The following

figure shows the plan of the piles relative to the column location.

Figure 45. Pile Distribution under Raft Foundation

62

C.6. COST ANALYSIS

Tables 26 and 27 below summarize the cost of the deep foundation and shoring system

alternatives. These values are estimated based on the construction market in Lagos, Nigeria.

Based on this cost analysis, it is clear that the deep foundation alternative recommended by the

local geotechnical engineer in Lagos is less economical than the shoring system alternative.

However, the schedule for completing the deep foundation alternative will most likely be

significantly faster than the 9 meter deep raft with a shoring system and dewatering alternative.

Table 26. Cost Analysis for the Shoring System Alternative

Item Description Qty Unit

Rate

Amount

U.S.$

Diaphragm

Wall

Constructing D-wall (30 MPA),

Drilling and

installation of reinforcement as

specified in the design

(16 m Depth)

1792 m2 $300/m2 537,600

Grouted

Tieback

Anchors

Drill and complete with all related

accessories and

material.

The anchors will penetrate the

D-Wall. No need for whaler beams.

1st row of anchors

Spacing =2 m

No. of strands = 5

Angle of inclination = 20o

Pull out force = 50 ton

56 anchors

26 m long $100/lm 145,600

2nd row of anchors

Spacing = 2 m

No. of strands = 5

Angle of inclination = 20o

Pull out force = 50 ton

56 anchors

23 m long $100/lm 128,800

Excavation Excavation and disposal of soil 6801 $8/m3 54,405

Dewatering dewatering system 1 $10,000 10,000

Basements Basement area that can be sold is

about 755 m2 2 $800/m2 - 604,000

Total $ 271,805

mailto:56@26m

mailto:56@26m

63

Table 27. Cost Analysis for the Pile Foundations Alternative

Item Description Qty Unit Rate Amount

U.S.$

Deep

Piles

Pile construction (30 MPA)

complete with drilling, concrete,

reinforcement and all related work

as specified and as directed by the

engineer

60 cm Diameter 32 m long 22 $7933.6/pile

174,539.2 Drilling 32 m $230/m

Concrete 1.12 m3 $180/m3

Reinforcements 1.776 ton $750/Ton

80 cm Diameter at 32 m long 75 $11832.8/pile

887,460

Drilling 32 m $230/m

Concrete 16.08 m3 $180/m3

Reinforcements 2.1312 ton $750/Ton

Total $ 1,061,999.2

64

D. CONCLUSION AND RECONMMENDATIONS

D.1. CONCLUSION

The alternative with 2 basements and the D-wall shoring system is more economical than the

deep foundation alternative. However, it should be noted that the execution for each alternative is

quite different. The shoring system is more time consuming due to the excavation and

dewatering phases. However, if the schedule is not very tight, there is a larger profit to make in

selling the two basements in addition to the cheaper construction approach.

As a conclusion, this report demonstrates that a small engineering exercise can provide insight

for the owner to come up with the most economical solution for his project.

D.2. LIMITATIONS

Some of the limitations that should be noted are as follows:

This analysis is based on limited soil exploration. It is possible that during construction

different soil conditions may be observed. In that case, the current design should be

revised and modified accordingly.

Appropriate machinery should be used by the shoring contractor during construction to

obtain a satisfactory diaphragm wall system.

65

D.3. CONSTRUCTION RECOMMENDATIONS

Some recommendations concerning the execution of the shoring system are as follows:

The tieback anchor drilling machine is preferred to be of the type no 3 Bauer C6/C8.This

excavators has a high torque and can be fitted with many types of excavation heads.

The diaphragm wall excavation machine is preferred to be either SoilMech or

Casagrande. These excavators have appropriate calibration instruments to prevent high

wall inclination.

A tremmy pipe should be used for the diaphragm wall during the concrete pouring

process to avoid segregation of concrete.

It is recommended to use bentonite slurry in order to stabilize side walls during

excavation.

During the pouring of the diaphragm wall, the sand might undergo permeation due to the

concrete. As a result, when the excavation process begins, there should be an excavator

(helicopter) in order to break additional concrete without damaging the diaphragm wall.

Since the water will remain present behind the diaphragm wall, there might be a high

probability of water leakage from the holes of the anchors. This unwanted water seepage

can be collected by a network of small water pipes to take the water to a sump pump and

dispose of it.

66

E. REFERENCES

Basol Associates Limited (2014). Report on Subsoil Site Investigations of the Proposed

Development for Jubali Group, Apapa, Lagos, December 2014.

Bowles, J. (1977). Foundation analysis and design (2d ed.). New York: McGraw-Hill.

British standard Bs 8110-1: 1997 incorporating amendments Nos.1 and 2

Budhu, M. (2011). Soil Mechanics and Foundations (3rd ed.). Hoboken: Wiley

Textbooks.

Building code requirements for structural concrete: (ACI 318-99) ; and commentary

(ACI 318R-99). (1999). Farmington Hills, Mich.: American Concrete Institute.

Coduto, D. (2001). Foundation design: Principles and practices (2nd ed.). Upper Saddle

River, N.J.: Prentice Hall.

Construction Guides, Thomas Telford Pub.Co., London.

Das, B. (2002). Soil mechanics laboratory manual (6th ed.). New York: Oxford

University Press.

Das, B. (2007a). Principles of geotechnical engineering (7thEd.). Stamford, Washington:

Cengage Learning.

Das, B. (2007b). Principles of foundation engineering (6thEd.). Boston: PWS-Kent Pub.

Department of the army, U.S. army corps of engineers, Washington, Dc 20314-1000,

Bearing capacity of soils.


Design of pile foundation.

67


Settlement analysis.

Geotechnical engineering circular no. 4, ground anchors and anchor system, U.S

department of transportation, office of bridge technology 400 seventh street, SW

Washington, DC 20590, June 1999

Look, B. (2007). Handbook of geotechnical investigation and design tables. London:

Taylor & Francis.

Mansur, C.I. and Kaufman, R.I. (1962) Dewatering, in Foundation Engineering

Ed.byG.A. Leonards pp.241-350, McGraw-Hill Book Co.

Ou, Chang. (2006). Deep excavation: Theory and practice. London: Taylor &

Francis/Balkema.

Powers, J.P. (1992), Construction Dewatering, 492p., 2nd ed. John Wiley and Sons Inc.

Quinion, D.W. and Quinion, G.R.(1987), Control of Groundwater, ICE Works

Somerville, S.H.(1986), Control of Groundwater for Temporary Works,

CIRIA(Construction Industry Research and Information Association) Report No.113.

Teng, V.C.(1962) Foundation Design, 466 p., Ch.5, Prentice-Hall, IAC.,Englewood

Cliffs, N.J.

Tomlinson, M., & Woodward, J. (n.d.). Pile design and construction practice (Sixth ed.).

US Army Corps of Engineers engineer manual, Geotechnical Investigations.

68

F. APPENDIX

Table 28. Typical Floor Column Loads Exported from Etabs

TYP

SLS ULS

SW SIDL DL LL SUM DL LL SUM

80.6 36.4 C1 117 16.1 133.1 140.4 25.76 166.16

137.5 67.6 C2 205.1 30 235.1 246.12 48 294.12

175.8 89.4 C3 265.2 39.6 304.8 318.24 63.36 381.6

181.8 94.1 C4 275.9 42 317.9 331.08 67.2 398.28

240.3 124.5 C5 364.8 56.8 421.6 437.76 90.88 528.64

235 121.6 C6 356.6 55.5 412.1 427.92 88.8 516.72

156.5 76.1 C7 232.6 35.8 268.4 279.12 57.28 336.4

227.8 114.7 C8 342.5 47.9 390.4 411 76.64 487.64

178.7 87.6 C9 266.3 36 302.3 319.56 57.6 377.16

82.5 36.6 C10 119.1 19 138.1 142.92 30.4 173.32

169.8 86.9 C11 256.7 35.5 292.2 308.04 56.8 364.84

129 56.8 C12 185.8 35.5 221.3 222.96 56.8 279.76

91.3 42.82 C13 134.12 22.2 156.32 160.944 35.52 196.464

183.7 97.7 C14 281.4 39.4 320.8 337.68 63.04 400.72

135.4 62 C15 197.4 38 235.4 236.88 60.8 297.68

143.5 68.8 C16 212.3 33.5 245.8 254.76 53.6 308.36

170.4 88.7 C17 259.1 36.4 295.5 310.92 58.24 369.16

207 101.9 C18 308.9 41.6 350.5 370.68 66.56 437.24

91 38.3 C19 129.3 17.1 146.4 155.16 27.36 182.52

197.4 92.4 C20 289.8 43.9 333.7 347.76 70.24 418

206 96 C21 302 45.8 347.8 362.4 73.28 435.68

72.8 30.7 C22 103.5 13.1 116.6 124.2 20.96 145.16

68.4 28.9 C23 34.2 12.5 46.7 41.04 20 61.04

69

Table 29. Maximum Pu for all Columns

Analysis Etabs Max

WT2 WT1 TYP WT2 WT1 TYP WT2 WT1 TYP

C1 0.0 0.0 77.8 0.0 0.0 166.2 0.0 0.0 166.2

C2 0.0 0.0 284.7 0.0 0.0 294.1 0.0 0.0 336.8

C3 0.0 0.0 328.7 0.0 0.0 381.6 0.0 0.0 394.6

C4 0.0 0.0 348.3 0.0 0.0 398.3 0.0 0.0 427.5

C5 0.0 0.0 536.9 0.0 0.0 528.6 0.0 0.0 536.9

C6 0.0 0.0 525.0 0.0 0.0 516.7 0.0 0.0 525.0

C7 0.0 0.0 305.9 0.0 0.0 336.4 0.0 0.0 375.3

C8 0.0 323.7 532.4 0.0 510.0 487.6 0.0 510.0 532.4

C9 0.0 363.9 479.0 0.0 268.7 377.2 0.0 493.2 479.0

C10 0.0 0.0 146.2 0.0 0.0 173.3 0.0 0.0 173.3

C11 0.0 552.5 539.9 0.0 406.7 364.8 0.0 559.0 539.9

C12 194.0 552.5 190.9 289.0 204.1 279.8 289.0 557.6 279.8

C13 0.0 0.0 150.3 0.0 0.0 196.5 0.0 0.0 196.5

C14 0.0 633.1 555.4 0.0 444.6 400.7 0.0 633.1 555.4

C15 194.0 633.1 196.3 239.6 218.6 297.7 246.6 633.1 297.7

C16 0.0 0.0 164.4 0.0 0.0 308.4 0.0 0.0 308.4

C17 0.0 179.0 538.4 0.0 304.0 369.2 0.0 312.4 538.4

C18 0.0 427.8 499.1 0.0 539.2 437.2 0.0 559.0 499.1

C19 0.0 0.0 89.3 0.0 0.0 182.5 0.0 0.0 182.5

C20 0.0 0.0 417.6 0.0 0.0 418.0 0.0 0.0 418.0

C21 0.0 0.0 414.3 0.0 0.0 435.7 0.0 0.0 435.7

C22 0.0 0.0 138.7 0.0 0.0 145.2 0.0 0.0 163.6

C23 0.0 0.0 84.8 0.0 0.0 61.0 0.0 0.0 141.2

70

Figure 46. Architectural Design

71

Figure 47. Building Section

72

Figure 48. Parking Lots

73

Figure 49. Borehole No. 1

74

Figure 50. Borehole No. 1 (continued)

75

Figure 51. Borehole No. 2

76

Figure 52. Borehole No. 2 (continued)

apappa lagos geotechnical design

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