advanced foundation engineering report

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INTRODUCTIONThis report is prepared to analyze all geotechnical conditions of the railway bridge

which is planning to be built at Tpra zone in zmit, Turkey. This research contains many

detailed information about the project such as site condition reports, stability

analyses,possible problems about the construction, alternative solutions for all problems etc.

At this project, two boreholes were opened and taken samples so that the soil profile

and the engineering properties of the soil could be determined. Many in-stu and laboratory

tests were applied on this samples. CPT is the main test in order to obtain engineering

properties for the bearing capacity, settlement, liquefaction and heaving capacity. SPT is also

applied for determining soil profile of the site and deciding the soil type of the site. Correlated

SPT values were used for calculations and classifications. General geology of the site is also

investigated and the seismisity of the zone is explained in detail.

Possible soil problems such as bearing capacity problem, settlement problem and

permeability problem were considered to be appear in the site while construction. Therefore

many alternative solutions were determined and advised for a safe structure with a long

service life without any problem.


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PROJECT INFORMATION

1) Location: Project is planned to be applied in Tpra-zmit

2) Climate: Marmara type of climate is usually observed. Very hot and very low amounthof precipitation can be seen in summer whereas cold and rainy winters. Yearly average

precipitation is about 700 mm and average temperature 14.6 C.


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3) Structural Informations: An approacing way which will be approximately 160 mtlength to be built for the railway overpass section.Kilometres for the approaching way

is 0+060 to 0+221.35. Height of the approaching way is about 7.5 metres nearby the

bridge abutment. The possible stress is considered to be occured in soil is estimated as

160-170 kPa at this section.

CPT TESTS

Cpt tests were applied at four different zone at the site. Tip area of the cone is 10 cm

and the surrounded area is 150 cm. Determination of the soil layers, bearing and

settlement behaviours are observed by these tests. Undrained shear strength and

angle with elevation and depth is shown below.


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SPT TESTS

SPT tests were applied at two points where is close to the bridge abutment. The

location of the boreholes are so important that these varies are critical for deciding soil

properties nearby the bridge abutments. Correlated SPT N values with depth and

elevation is shown in the graphs below. See borehole reports(Appendix)


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Seismisity of The Area:

zmit is at the first degree earthquake region. Structures must be designed related to

the possible strong earthquake forces. Therefore, all design criteria about the

seismisity are listed below:

1) Active horizontal earthquake acceleration() should be taken as 0.4 g2) Local soil class can be taken as Z3 according to CPT and the other test results3) Soil group is classified as D-C4) Spectrum Periods are taken as =0.15 , =0.60 according to Z3 type soils.ENGINEERING INVESTIGATIONS

Engineering properties of the construction site is evaluated in bearing capacity,

settlement, heaving potential.

Bearing Capacity

Bearing capacity analyses are applied according to Terzaghi bearing

capacity formula.

Terzaghi Method:

The bearing capacity for shallow foundations:

. . . . . . .

, = Shape Factors

=Unit Weight Above the Foundation Base

= Foundation Depth

Foundation Width

, , = Bearing capacity Factors

Undrained Shear Strength


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For Clays;

For Sands;

Bearing capacity for sands are greater than clays. Therefore critical bearing capacity

can be calculated according to clays.

At first 8 metres foundations is considered to be constructed:

SPT/N~8, c=50 kPa, K=1, =0 >>>Nc=5.7, Nq=1.0, 18 kN/m3, min Df= 0.7m

Factor of Safety can be taken as 2.0.Then;

Qall=300/2=150 kPa

When the structural loads are analysed for the areas the approaching way height (H) is

greater than 6 mt, the bearing capacity problem is about to occur. Therefore soil

stabilization is necessary for these regions.

Settlement:

For settlement analysis CPT values are used. For the expecting foundation base

stresses are calculated as CPT-1,H=7.3m>>160 kPa, CPT-2,H=5.8m>>130 kPa,

CPT-3, H=2.3m>>60 kPa, CPT-4, H=1 m>>35 kPa. Foundation Width(B) is taken as 8

mt at settlement analysis by using Janbus tangent modulus method.


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CPT No Wall Height(m)

Stresses at the

Foundation Base

(kPa)

Calculated

Settlements(cm)

1 7.3 160 19.9

2 5.8 130 7.8

3 2.3 60 16.7

4 1 35 18.5

According to the results, settlements at some sections are not in acceptible values(s=10

cm). Therefore,at this regions soil improvement should be applied to prevent large

settlements.

Soil Improvement Methods:

1) Dynamic Compaction Method2) Prefabricated Sand Drains+ Preloading Method3) Low Densification Potential Granular Backfill (Soil Replacement)4) Jet Grouting or Deep Mixing (DSM) high modulus columnsDynamic compaction method is not advised because of the existance many gas and oil

tanks around. Sand drains+preloading method is not a suitable choice for improvement

because of the limited time of the construction. Therefore, soil replacement and jet-

grouting methods can be choosen.

1) Soil Replacement MethodIf this method can be applied settlements will be limited according to decreasing the

soil layer can be consolidated. Therefore, 2 mt excavation will be applied under the

foundation, then compacted granular backfill is constituted. Ground water table level

should be considered while application too. For a uniform stres distribution with

increasing bearing capacity and limited settlements, geogrid can be used between

foundation base level and granular fill.


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Calculations according to criteria above are listed:

H= Potential Shear Depth

Weight Factors:

Upper Layer: 2.0m/B=2.0/8.0=0.25

Lower Layer:6.0/8.0=0.75

Cumulative Soil Parameters:


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Factor of safety can be taken as 2.0:

Qall= Qult/2=440/2=220 kPa

According to bearing capacity results after soil replacement, approaching way is safe

and the method is convenient.

Settlement Analyzes:

Using tangent modulus, settlements after 2 mt replacement is calculated as below:


Stresses at the

Foundation Base

(kPa)

Calculated

Settlements(cm)

1 7.3 160 5.5

2 5.8 130 3.1

3 2.3 60 4.4

4 1 35 5.3

Settlements is also limited by this soil improvement technique.

During the application,because of the clay existance in lower layers, some compaction

problems may occur. In order to prevent these problems. Geogrid application at the

base of the granular fill base, 50 cm rock fill layer above the geogrid, and the geotextile

application on the rock layer fort he purpose of seperation is advised. The backfill

should be constructed on this layers to avoid possible compacting problems.


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2) Jet Grouting-Deep Mixing (DSM) Method:According to this application,the general informations and results are listed below:

a) Jet Grouting Method- Diameters of jetgrout columns: 0.6 m- Distance between jetgrout columns (centre): 2.0m- Length of the columns: 10 m- Vertical Allowable Bearing Capacity of Each Column: 40 tonb) Deep Mixing (DSM) Method- Diameters of columns: 0.8 m- Distance between columns: 2.5m- Length of columns: 10 m- Vertical allowable Bearing Capacity of Each Column:70 tonSettlements after jetgrout-deep mixing(DSM) application is shown below:


Stresses at the

Foundation Base

(kPa)

Calculated

Settlements(cm)

1 7.3 160 9.6

2 5.8 130 6.8

3 2.3 60 3.7

4 1 35 2.4


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APPENDIX

A-BOREHOLE DOCUMENTS

B-CROSS SECTION OF THE BRIDGE

C-PLAN VIEW OF THE AREA

D-KEY PLAN

E-REINFORCED EARTH WALL PROFILE

F-TANGENT MODULUS METHOD

G-PHOTOS OF THE AREA


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Tangent Modulus Approach

Settlements in cohesive and cohesionless soils can be analysed using the tangentmodulus method, which takes into account the non-linearity of the load-deformation

relationship of most soils. Janbu (1963) has shown that the tangent modulus Mt can bedefined by the following relationship,

Mt = m .r ( /r)(1 - j) (14)

where m is a dimensionless modulus number, r is an arbitrarily chosen reference stress

(100 kPa), is the vertical effective stress and j is a stress exponent. The strain

increase of a soil layer caused by an increase of the vertical effective stress canbe calculated from

= {[ ( 0 + ) /r] j - [0 /r] j }/( m . j ) (15)

where 0 is the initial vertical effective overburden stress and is the increase of thevertical effective stress. For cohesive soils, values of the modulus number m and the

stress exponentjcan be determined by conventional laboratory tests. For cohesionlesssoils, however, it is often difficult to obtain undisturbed soil samples and the results fromlaboratory tests are therefore uncertain. Thus, empirical values are often used toestimate the modulus number and the stress exponent, cf. Canadian FoundationEngineering Manual (1985), Table 4.

Soil Type Stress Exponent,j

Modulus Number,

m

Gravel 0,5 40 - 400

Dense Sand 0,5 250 - 400

Compact Sand 0,5 150 - 250

Loose Sand 0,5 100 - 150

Dense Silt 0,5 80 - 200

Compact Silt 0,5 60 - 80

Loose Silt 0,5 40 - 60

Table 4. Typical Values for the Stress Exponent j and the Modulus Number m for

granular soils, (after Canadian Foundation Engineering Manual, 1985)

Copyright 1998-2009 GRV ABLegal disclaimer


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