cantonment au

GEOTECHNICAL REPORTFOR

APARTMENT WITH BASEMENTAT

AFRICAN UNION (AU) VILLAGE, CANTONMENT- ACCRA

SUBMITTED TO: PREPARED BY:WONDER WORLD ESTATES HOLDFAST VENTURESP.O. BOX 8743 P.O. BOX TN 464ACCRA TESHIE/NUNGUA ESTATES

ACCRA

FEBRUARY, 2014

THE PROPOSED FOUR-STOREY RESIDENTIAL

HOLDFAST VENTURES Proposed four-Storey Residential Apartments with Basementat the AU Village, Cantonment-Accra, Accra

Geotechnical Report Page: ii

TABLE OF CONTENTS

1.0 INTRODUCTION ............................................................................................................ 1

2.0 THE SITE ......................................................................................................................... 2

2.1 Location....................................................................................................................... 2

2.2 Region Geology............................................................................................................ 22.2.1 Topography.......................................................................................................................................... 22.2.2 Climatic Conditions ........................................................................................................................... 22.2.3 Stratigraphy ......................................................................................................................................... 3

2.3 Seismicity...................................................................................................................... 43.0 INVESTIGATION............................................................................................................... 5

3.1 Fieldwork ..................................................................................................................... 5

3.1.1 Dynamic Cone Penetrometer Test (DCPT-DIN 4094)............................................. 5

3.1.2 Test Pitting ................................................................................................................... 6

3.2 Laboratory Testing ..................................................................................................... 64.0 DISCUSSION................................................................................................................... 8

4.1 Ground Condition ......................................................................................................... 8

4.2 Engineering Characteristics ........................................................................................ 10

4.3 Groundwater Condition.............................................................................................. 11

4.4 Chemical Analysis ..................................................................................................... 115.0 FOUNDATION DESIGN............................................................................................... 12

5.1 Foundation Depth ....................................................................................................... 12


Geotechnical Report Page: iii

5.2 Bearing Capacity ....................................................................................................... 12

5.3 Settlement................................................................................................................... 15

5.4 Foundation Type ....................................................................................................... 15

5.5 Excavations ................................................................................................................ 156.0 SUMMARY AND RECOMMENDATIONS ................................................................. 16


Geotechnical Report Page: 1

1.0 INTRODUCTION

with Basement at the African Union (AU) Village, Cantonment in Accra.

The client has requested for a Geotechnical Investigation to be carried out at the site to assessthe soil structure.

This report is a record of the work carried out at the site as revealed by Dynamic ConePenetration (DCP) tests and trial pits.

The report gives recommendations covering the following:-

Geology and seismic information of the site and recommendation for seismic design.

Foundation design to include Bearing Capacity, depth and type of foundation.

Groundwater condition if encountered.

Chemical composition of Soil/Groundwater

Hydrology of the area with respect to rainfall intensity

Excavation characteristics in respect of stability of open pits/trenches

Recommendation for any necessary site improvement works

Messrs Wonder World Estates has proposed to construct a Four-Storey Residential Apartments



2.0 THE SITE

2.1 Location

The site is located at the African Union (AU) Village, Cantonment in Accra. The site is walledand the excavation for the basement is in progress.

Picture 2.1: The project site2.2 Region Geology

2.2.1 Topography

The site fairly flat therefore drainage should be managed effectively with the construction oflined drains.

2.2.2 Climatic Conditions

The area lies in the Savannah zone. There are two rainy seasons. The average annual rainfall isabout 730mm, which falls primarily during the two rainy seasons. The first begins in May and



ends in mid-July. The second season begins in mid-August and ends in October. Rains usuallyfalls in intensive short storms and give rise to local flooding where drainage channels areobstructed.

There is very little variation in temperature throughout the year. The mean monthlytemperature ranges from 24.7C in August (the coolest) to 28C in March (the hottest) withannual average of 26.8C.

As the area is close to the equator, the daylight hours are practically uniform during the year.Relative humidity is generally high varying from 65% in the mid-afternoon to 95% at night.

The maximum wind speed record in Accra is 107.4 km/hr (58 knots). Strong winds associatedwith thunderstorm activity often cause damage to property by removing roofing material.Several areas of Accra experience micro climatic effects.

2.2.3 Stratigraphy

The geology of the project area can be divided into three distinct groups i.e., Accraian, Togoand Dahomeyan series known as Accra formation. The Accraian series belonging to theDevonian age are sedimentary deposits, and consists of upper Interbedded sandstone andshales, Middle clay shale and lower sandstone.

The Togo series belongs to the Upper Proterozoic age. They generally consist of metamorphicrocks of quartzites, shales and phyllites.

The Dahomeyan system belongs to the Middle Precambrian Age and consists mainly of acidand basic Hornblends gneiss and quartz mica schists, muscovite-biotite gneiss, and biotitegneiss. The soils here are derived from the decomposition of the Dahomeyan system and Togoseries, which were facilitated by the warm and humid climate. This has resulted in theformation of grey, white, reddish and yellowish-brown coloured soils.

On the surface there is a thin layer of hard sandy or silty clay, which is considered a driftmaterial. A thick layer of laterized quartz gravel underlies this layer. It is considered that quartzin these layers is a weathered product of bedrock mixed with transported sandy and silty



clay.The lateritic gravel layers are very hard, with fines varying in texture from silty sands tosandy clay. Due to the leaching of upper layers of gravel the material is generally less plasticnear the surface, but plasticity increases with depth.

2.3 Seismicity

Studies on historical earthquakes and micro-earthquake activities have not indicated that theproject area falls within a high seismic active zone. There are no observed faults existing in thearea of the site. The site is believed to be in a geologically stable zone. Irrespective of this, itis advised that the necessary precautions be taken in the design of the foundation, particularlywhere high storey structures are expected.

Though the project area is not known to have seismic activities, the area is put in zone 3 ofSeismic risk map of Ghana (Figure 2.1).

Recent research assigns Horizontal Design Ground Acceleration of 0.20g, which isrecommended for the design.

ZONE 0 ZONE 1 ZONE 2 ZONE 3Figure 2.1: Seismic Risk Map of Ghana



3.0 INVESTIGATION

3.1 Fieldwork

Fieldwork was started on February 7, 2014 and was completed on the same day.

The investigation comprised the following activities: -

Excavating two (2) number excavated Trial Pits

Conducting seven (7) number Dynamic Cone Penetrometer Test (DCPT - DIN 4094)

Laboratory testing of disturbed samples collected.

3.1.1 Dynamic Cone Penetrometer Test (DCPT-DIN 4094)

The ground investigation was undertaken in accordance with the BS 5930 Standard Code ofPractice for site investigation. Seven number (7 No) Dynamic Cone Penetrometer Test (DCPTDIN-4094) were performed to give a better indication of foundation depth and an assessmentof the soil strength (Bearing Capacity). They were distributed so as to give a fair representationof the ground consistency over the site and also to determine competent stratum.

Picture 3.1(A&B): Sinking of DCPT DIN 4094 at the site

A B



3.1.2 Test PittingTwo test pits were sunk to a maximum depth of 4.2m using an excavator. These were done toexamine the soil profile of the excavated depth.Bulk disturbed samples were taken for soil classification.

Picture 3.2: Excavator sinking of Trial pit at the site Picture 3.3: Measuring the soil profile after pitting

3.2 Laboratory TestingRelevant laboratory tests for the classification and determination of the appropriate designparameters were carried out on disturbed soil samples in accordance with BS 1377 and ASTMStandards.These tests include:-

Natural moisture content Atterberg limits Particle size analysis Chemical test on soil sample Free Swell test

Summary of the laboratory test results are presented in Tables 3.1.



PROJECT:

TITLE Free SwellSAMPLE 20 14 10 5 2 1 600 425 300 150 75 NMC LL PL PI

IDENTIFICATION mm mm mm mm mm mm m m m m m % % % %

TP DS DEPTH (m)1 1 0.65-2.10 100 - 100 100 99 95 - 91 87 73 56 8.3 28 23 5 0

1 2 2.10-2.70 90 - 80 64 48 42 - 39 37 31 22 4.5 29 20 9 3

2 3 2.70-4.40 100 - 95 88 83 80 - 76 72 57 41 7.1 33 22 11 10

2 4 0.30-1.90 100 - 100 100 99 96 - 92 87 75 74 10.0 26 19 7 0

3 4 1.90-4.10 100 - 92 74 50 47 - 46 44 39 30 8.1 34 22 12 8

%

TABLE 3.1: Summary of laboratory tests results

Date: 12/02/2014ATTERBERG

TEST

Proposed Four-Storey Residential Apartment withbasement at AU Village, Cantonment in Accra

Sieve Sizes



4.0 DISCUSSION

4.1 Ground Condition

Three subsurface units have been defined in this report. The Soil Units are as follows: -Unit 1 clayey silty SAND/sandy - Loose to medium dense, reddish brown

CLAY The thickness of this layer is between1.45m and 1.60m.The soil is of low plasticity.

Unit 2 clayey sandy GRAVEL/sandy - Dense, reddish brownclayey GRAVEL (quartzitic) Thickness of this layer varies between

0.6m and 2.2m.The material is inorganic clay with lowplasticity.

Unit 3 gravelly clayey SAND/ - mottled, reddish/yellowish brownweathered SANDSTONE/sandy homogeneousclayey GRAVEL (quartzitic)

Records of the pits logs are presented below:-

Trial Pit 1

Depth (m) Description0.00 0.65 Light brown Topsoil0.65 2.10 Loose to medium dense, reddish brown clayey silty SAND (S1)2.10 2.70 Dense, reddish brown clayey sandy GRAVEL (S2)2.70 4.40 Compact, mottled, reddish/yellowish brown homogeneous gravelly

clayey SAND/weathered SANDSTONE (S3)No groundwater was encountered



Trial Pit 2

Depth (m) Description0.00 0.30 Light brown Topsoil0.30 1.90 Loose to medium dense, reddish brown sandy CLAY (S4)1.90 4.10 Compact, mottled, reddish/yellowish brown homogeneous sandy clayey

quartzitic GRAVEL (S5)No groundwater was encountered.

Summary of laboratory test results for S1- S5 are presented in Table 3.1.

Peck, Hansen and Thorburn (1974) related the plasticity index to the swelling potential of soilsin a simple relation shown in Table 4.1.

Table 4.1: Relationship between Plasticity Index and swelling potential

SWELLING POTENTIAL PLASTICITY INDEX

Low 0-15

Medium 10-35

High 20-35

Very High 35 and above

The degree of expansiveness of cohesive soils and the possible damage to light loadedstructures may be quantitatively assessed from Table 4.2.



Table 4.2: Relationship between Differential Free Swell and Degree of Expansiveness

Differential Free Swell (%) Degree of ExpansivenessLess than 20 Low20-35 Moderate35-50 HighGreater than 50 Very High

The recovered samples from the foundation area are of low plasticity and have low potential toswell in the presence of moisture.

4.2 Engineering Characteristics

Plot of Plasticity (PI) against Liquid Limit (LL) on the Cassagrande Plasticity Chart put thesoils S2, S3, S4 and S5 above the A-Line, indicating soils of inorganic clay with low plasticity.However, soil S1 is below the A-Line, indicating soil of inorganic silt with lowcompressibility.

Figure 4.1 presents the plots on the Cassagrande Plasticity Chart.

0 10 20 30 40 50 60 70 80 90 100 1100

10

20

30

40

50

60

70

S5

S1

S3S2S4

EXTREMELY HIGH (E)VERY HIGH (V)HIGH (H)

INTERMEDIATE (I)LOW(L)

A-Line, Tested Sample

PLAS

TICITY

INDE

X (PI)

- %)

LIQUID LIMIT (LL) - %

Figure 4.1: Cassagrande Plasticity Chart



4.3 Groundwater Condition

No groundwater was encountered in the trial pits. It is expected that no groundwater would beencountered during excavations but seepage water through the fissures of the ground cannot beruled out especially in the rainy season.

4.4 Chemical Analysis

The result of chemical analysis on soil samples indicated a pH value of maximum 5.9. pHvalue of 5-7 requires no special protection for concrete and that Ordinary Portland Cement(OPC) and Rapid Hardening Portland Cement (RHPC) are recommended for the manufactureof concrete.

The presence of chlorides in soil/groundwater usually has little effect on ordinary good qualityconcrete, but may affect steel reinforcing bars if high.

Maximum chloride concentration obtained for the soil is 7.3mg/kg will require no specialprotection. Sulphate as SO3 content of about 12.4mg/kg maximum would require no specialprotection.

Summary of the maximum chemical test results on the soil samples are as follows:

Test Result LimitpH 5.9 5-7Sulphate Content (mg/kg) 12.4 200Chloride Content (mg/kg) 7.3 300

The concentration of salts in the soil is low. This may not pose any serious threat toburied concrete and steel. However, it is advised that very dense concrete must be usedfor the construction to make the foundation concrete impermeable and less vulnerable toattack.



5.0 FOUNDATION DESIGN

5.1 Foundation Depth

The foundation footings should be carried below topsoil or organic material. These zones tendto have high volume changes due to moisture fluctuations and unconsolidated material.

Indication from the DCP test is that refusals depths were fairly uniform, ranging between 3.0mand 4.7m.

Where localized incompetent strata are encountered, such areas will require groundimprovement. Such ground improvement will require excavating the unsuitable material to agreater depth and replacing with selected granular fill, compacted to a high density and footingplaced in it. The compacted granular fill, when properly carried out can be brought up in layersto reduce the foundation depth.

It is therefore recommended that the foundation for the building be placed minimum 4m ordeeper in the weathered sandstone or clayey quartzitic gravel reference Trial pit 1and 2 (TP1and TP 2). This will depend on the type of foundation considered by the designer.

5.2 Bearing Capacity

The correlation for obtaining bearing capacity from DCP is given as;

qs = 30r

Where qs is safe bearing capacity and r is blows/100mm penetration

It is expected that the proposed building will be founded in Soil Unit 3.Very high safe bearing capacities values (excess of 500KN/m2) were obtained in soil unit 3,indicating very compact material in all DCP points sunk. The minimum safe bearing capacityat a depth of 4.0m is 540kN/m2.



Table 5.1 shows probe results of minimum safe bearing capacities with depth for the soil at thesite.

Compact gravel/silty SAND/weathered Sandstone (soil unit 2 and 3) is expected to havepresumed bearing pressure greater than 1000kN/m2 (M. Carter- Geotechnical EngineeringHandbook).

DCP test results at refusal gave safe bearing capacity in excess of 600kN/m2.

The Accraian formation is very much unpredictable and therefore the use of very high bearingcapacities for design is not recommended.

A graph of the overall minimum safe bearing capacities with depth is illustrated in Figure 5.1below.

0 50 100 150 200 250 300 350 400 450 500 550 600 650-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.50.0

Proposed Four-Storey Residential Apartments at AU Village, Cantonment, AccraDCP [DIN 4094] TEST RESULTS

DEPT

H (m)

MINIMUM SAFE BEARING CAPACITY (kN/m2)Figure 5.1 : DCP Tests Results (Min for all)



Table 5.1: Minimum Safe Bearing Capacities at Different LevelsDepth(m)

P1(kN/m2)

P2(kN/m2)

P3(kN/m2)

P4(kN/m2)

P5(kN/m2)

P6(kN/m2)

P7(kN/m2)

qmin(kN/m2)

0 . . . . . . 0 0-0.1 . . . . . . 48 48-0.2 . . . . . . 48 48-0.3 . . . . . . 60 60-0.4 . . . . . . 84 84-0.5 . . . . . . 96 96-0.6 . . . . . . 432 432-0.7 . . . . . . 456 456-0.8 . . . . . . 456 456-0.9 . . . . . . 492 492-1 . . . . . . 456 456-1.1 . . . . . . 528 528-1.2 . . . . . . 516 516-1.3 . . . . . . 492 492-1.4 . . . . . . 480 480-1.5 . . . . . . 540 540-1.6 . 456 480 . . . 576 456-1.7 . 432 576 . . 336 552 336-1.8 420 444 516 . . 348 540 348-1.9 456 444 480 . . 360 528 360-2 444 480 504 . . 444 540 444-2.1 456 516 444 . . 420 564 420-2.2 564 480 420 . . 480 564 420-2.3 588 540 480 . . 480 564 480-2.4 564 540 492 . . 504 588 492-2.5 576 588 516 . . 552 576 516-2.6 576 576 588 . . 540 564 540-2.7 552 576 576 . . 492 540 492-2.8 588 588 564 . . 552 540 540-2.9 564 576 564 . . 528 564 528-3 600 540 552 . . 540 564 540-3.1 576 576 . . 528 552 528-3.2 540 588 . . 528 552 528-3.3 540 588 . . 564 576 540-3.4 540 576 . . 588 552 540-3.5 588 564 . . 552 552 552-3.6 564 576 . . 576 552 552-3.7 564 576 504 480 576 576 480-3.8 564 552 540 492 564 576 492-3.9 576 588 564 480 588 588 480-4 588 564 540 552 576 576 540-4.1 564 588 576 540 576 576 540-4.2 540 576 564 540 600 600 540-4.3 576 576 588 540 540-4.4 576 600 588 576 576-4.5 588 600 588 588-4.6 600 588 588-4.7 600 600

It is therefore recommended that a safe bearing capacity between 350 and 400kN/m2 beused for the design.



5.3 Settlement

Plot of plasticity index and liquid limit on the Cassagrande Plasticity chart is shown in Figure4.1. From the plot, the soils in the foundation area are of low to intermediate plasticity andhave low potential to swell in the presence of moisture, therefore settlement is expected to bewithin limits.

5.4 Foundation Type

Raft foundation may be used to bring the depth of foundation to shallow depth. In addition,Raft foundation will reduce the net loading on the soil by removal of material within thefoundation and differential settlement will be tolerable.

Here the mat is expected to be stiffened and columns raised on the mat.

5.5 Excavations

Materials encountered on site are dense and compact, giving an indication that the ground is beself- supporting.

It is expected that no groundwater would be encountered during excavations but seepage waterthrough the fissures of the ground cannot be ruled out especially in the rainy season, henceprovision of some form of dewatering system may be provided where necessary.



6.0 SUMMARY AND RECOMMENDATIONS

A geotechnical investigation has been carried out at the site for the proposed Four-storeyResidential Apartments at the AU Village, Cantonment in Accra and the following are therecommendations made:

A safe bearing capacity between 350 and 400kN/m2 may be used for the footingdesign.

The footings are to be placed at a depth of 4.0m or deeper.

Horizontal Design Ground Acceleration of 0.20g is to be used for seismicdesign considerations since the site is in zone 3 of the Seismic Risk map ofGhana

Settlement is expected to be within tolerance.

Raft foundation is recommended. However, the choice depends on the designer.

Concrete for the substructure should be produced to a dense consistency.

Drainage should be managed effectively with the construction of lined drains

Some form of dewatering system may be provided during excavation wheregroundwater is encountered.

cantonment au

Documents

storey residential apartments

accrageotechnical report

cantonment accrasubmitted

box tn

test pitting

ground condition

groundwater condition

foundation design