Foundation Design

Building structural systemBy Dr. Sompote Youwai

Contents• Fundamental of Soil Mechanics• Interpretation from Soil Report– Subsurface investigation– Field and laboratory testing

• Pile Foundation Design– Single Pile– Pile Group

• Fundamental of retaining structure– Sheet pile– Diaphragm wall

Additional text book

• Das M. B., Foundation Engineering.• Tomlinson, M. J. Foundation Design &

Construction • Hunt, Geotechnical Engineering Investigation

Handbook.• Handout

Method for Pile Design

• Hand Calculation• Finite Element Analysis

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2. Foundations for Signature Towers Dubai

75-F Office65-F Hotel

55-F Residential

• Nicknamed “Dancing Towers”

• Office 351 m, Hotel 305 m, Residential 251 m high

• Piled raft foundations

• Bored piles 483 nos., 1.5 m dia, 45 m long

• Ground conditions:

0-10 m: Sand

10-25 m: Very/Weak Sandstone

25-30 m: Very/Weak Siltstone

30-40 m: Very/Weak Conglomerate

>40m: Very/Weak Claystone

6

Foundation Layout

Office(168 nos)

Hotel(126 nos)

Residential(184 nos)

7

3DF Mesh

505m

590m

150mNo of elements = 32,000

• Pile rafts 5.5 m thick, located at 10 metre below ground level

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3DF Mesh

168 nos. 126

nos.

184 nos.

Embedded piles: 1.5 m dia. 45 m long

Pile raft

Load

Office Tower

Hotel Tower

Residential Tower

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3DF Outputs

Office Tower Hotel Tower

Residential TowerContours of Settlements

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3DF Outputs

Office ResidentialHotel

Office Hotel Residential

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3DF Outputs

Deformations of Office piles Axial forces of Office piles

Fundamental of Soil Mechanics

Bangkok Subsoil condition

Bangkok Subsoil condition

Keyword from boring log

• ST, SS• Atterberg’s limits• Water content• Unit weight• Sieve analysis• Unconfined shear• Standard penetration test

• Soil is generally a three phase material• Contains solid particles and voids• Voids can contain liquid and gas phases

Solid

Water

Air Vs

Vw

Va

• Soil is generally a three phase material• Contains solid particles and voids• Voids can contain liquid and gas phases

Solid

Water

Air Vs

Vw

Va

• Soil is generally a three phase material• Contains solid particles and voids• Voids can contain liquid and gas phases

Solid

Water

Air

Phase Volume Mass Weight

Air Va 0 0

Water Vw Mw Ww

Solid Vs Ms Ws

Vs

Vw

Va

Units

• Length metres• Mass tonnes (1 tonne = 103 kg)• Density t/m3

• Weight kilonewtons (kN)• Stress kilopascals (kPa) 1 kPa= 1 kN/m2

• Unit weight kN/m3

• Accuracy Density of water, rw = 1 t/m3

Stress/Strength to 0.1 kPa

Weight and Unit weight

• Force due to mass (weight) more important than mass• W = M g

• Unit weight

Weight and Unit weight

• Force due to mass (weight) more important than mass• W = M g

• Unit weight

g = r g

W

V

M g

V

Weight and Unit weight

• Force due to mass (weight) more important than mass• W = M g

• Unit weight

g = r g

W

V

M g

V

svz sv = r g z

sv = g z

Specific Gravity

• Gs @ 2.65 for most soils

• Gs is useful because it enables the volume of solid particles to be calculated from mass or weight

GD e n s i t y o f M a t e r i a l

D e n s i t y o f W a t e r w

GU n i t W e i g h t o f M a t e r i a l

U n i t W e i g h t o f W a t e r w

This is defined by

Moisture Content

• The moisture content, m, is defined asm

Weight of Water

Weight of Solids

W

Ww

s

Moisture Content

• The moisture content, m, is defined as

In terms of e, S, Gs and gw

Ww = gw Vw = gw e S Vs

Ws = gs Vs = gw Gs Vs

mWeight of Water

Weight of Solids

W

Ww

s

Procedure for grain size determination• Sieving - used for particles > 75 mm

• Hydrometer test - used for smaller particles– Analysis based on Stoke’s Law, velocity proportional to diameter

Sieve analysis

Atterberg Limits• Particle size is not that useful for fine

grained soils

Moisture content versus volume relation during drying

•Liquid Limit – The minimum water content at which the soil can be flow under its own weight

•Plastic Limit – The minimum water content at which soil can be roller into a thread 3 mm diameter with out breaking up

•Shrinkage – The maximum water content at which further loss of moisture does not cause a decrease in the volume of soil

Atterberg’s Limit

LL - Liquid limit

PL – Plastic limit

SL – Shrinkage limit

Atterberg Limits

SL - Shrinkage LimitPL - Plastic LimitLL - Liquid limit

Plasticity Index = LL - PL = PI or Ip

Liquidity Index = (m - PL)/Ip = LI

Moisture contentmassof water

massof solids

Definition of Grain Size

Boulders CobblesGravel Sand Silt and

ClayCoarse Fine Coarse FineMedium

300 mm 75 mm

19 mm

No.4

4.75 mmNo.10

2.0 mm

No.40

0.425 mm

No.200

0.075 mm

No specific grain size-use Atterberg limits

Symbols• Soil symbols:• G: Gravel• S: Sand• M: Silt• C: Clay• O: Organic• Pt: Peat

• Liquid limit symbols:• H: High LL (LL>50)• L: Low LL (LL<50)• Gradation symbols:• W: Well-graded• P: Poorly-graded

Example: SW, Well-graded sand

SC, Clayey sand

SM, Silty sand,

MH, Elastic silt)sandsfor(

6Cand3C1

)gravelsfor(

4Cand3C1

soilgradedWell

uc

uc

Plasticity Chart

(Holtz and Kovacs, 1981)

LL

PI

HL•The A-line generally

separates the more claylike materials from silty materials, and the organics from the inorganics.

•The U-line indicates the upper bound for general soils.

Note: If the measured limits of soils are on the left of U-line, they should be rechecked.

Soil Classification Procedure

Effective stress theory

u

- Fully Saturated: Sr=100%

- = Total stress to boundary

- u = pore water pressure

-u = Effective stress which is transmitted to the soil structure

Bishop (1954):

’ = -u : No change in soil strength if no change in ’.

f=c’ + ’ tan(’)

c’ and ’ are effective cohesion and friction angle of soil.

- Fully Saturated: Sr=100%

- = Total stress to boundary

- u = pore water pressure

-u = Effective stress which is transmitted to the soil structure

Bishop (1954):

’ = -u : No change in soil strength if no change in ’.

f=c’ + ’ tan(’)

c’ and ’ are effective cohesion and friction angle of soil.

- Equilibrium condition

- impermeable membrane

- Equilibrium condition

- impermeable membrane

0 50 100 150

0m

2m

4m

6m

8m

kPa

pore waterpressure Effective

stress

TotalStress (5m)

Depth

Stresses acting on a soil element

x

y

z

xz

yz

zz

yy

xy

zy

xx

yx

zx

z

x