Topic 2.3 ive geo

Geotechnical and Foundation Engineering SCE5331

Geotechnical and Foundation Engineering Dr. Hong Chengyu, Joey Office: 301, Tel: 2176-1545 Email: [email protected] 1

TOPICS & SYLLABUS: Topic 1: Review of Soil Mechanics

Topic 2: Shallow Foundations Topic 3: Lateral Earth Pressure and Retaining Walls

Topic 4: Pile Foundations Topic 5: Subsoil Exploration Topic 6: Slope Stability Textbook: Braja M. Das. (2007). Principles of Foundation Engineering, 6th Edition, ISBN 0-495-08246-5. Reference book: Foundation Design and Construction (2006), GEO Publication No. 1/2006, 376 p. 2

2.1 Bearing Capacity of Shallow Foundation • • • •

• •

2.3 Primary 2.2 Settlement Consolidation Settlement and Creep of Shallow Settlement Foundation

• Stresses From Elastic Theory • Types of Foundation Settlement General Concept • Elastic Settlement Terzaghi’s Bearing Capacity Based on the Theory Theory of Elasticity Factor of Safety • Elastic Settlement of Modification of Bearing Foundations on Capacity Equations for Water Saturated Clay Table • Range of Material The General Bearing Capacity Parameters for Equation Computing Elastic Settlement Eccentrically Loaded Foundations

• Primary Consolidation Settlement Relationships • Three-Dimensional Effect on Primary Consolidation Settlement • Vertical Stress Increase in a Soil Mass Caused by Foundation Load • Allowable Bearing Pressure in Sand Based on Settlement Consideration • Field Load Test • Tolerable Settlement of Buildings

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2.3 Primary Consolidation Settlement and Creep Settlement Primary Consolidation Settlement Relationships Three-Dimensional Effect on Primary Consolidation Settlement Vertical Stress Increase in a Soil Mass Caused by Foundation Load Allowable Bearing Pressure in Sand Based on Settlement Consideration Field Load Test

Tolerable Settlement of Buildings 4

3.14 Primary Consolidation Settlement Relationships (One-Dimensional Straining – Vertical Compression Only)

Scoed  settlement in oedometer condition Hc

S c ( p )    z dz

if  z  constant



zH c

0

e  z  vertical strain  1  eo e  void ratio; eo  initial e (a)  z  mv  z' (b)  z  f ( z' ,  c' ,  o' , Cc , Cs ) 1 '  av  ( t'  4 m'   b' )  6 1 ( t'   b' )   m' 2

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For normally consolidat ed clay  Area (ii ) with    ' 0

Sc( p )

' av

 o'

 : ' c

(i )

' Cc  0'   av   z Hc  log Hc ' 1  e0 0

' with  0'   av   c' :

Sc( p )

Cs     log 1  e0  0'

' av

 c'

 z'

(ii )

Cs or Ce

For over  consolidat ed clay  Area (i ) ' 0

 z'

Cc Hc

For over  consolidated clay  Areas (i )  (ii ) with  o'   c'   0'   av' : Sc( p )

Cs  c' Cc  0'   av'  log ' H c  log H ' 1  e0 0 1  e0 c

where C c  compression index Cs  Ce  swelling / elastic index

 c'  preconsolidation pressure   initial verticial effective stress ' o

e

 z'   o'   z'

Scoed  settlement in oedometer condition 6

3.15 Three-Dimensional Effect on Primary Consolidation Settlement

u   (3)  A[ (1)  7(3) ]

B: diameter of a circular foundation or width of a continuous foundation.

S c  KScoed S coed  settlement in oedometer condition K  settlement ratio from Figure 3.22

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Primary Consolidation Settlement and Creep Settlement Primary Consolidation Settlement Relationships Three-Dimensional Effect on Primary Consolidation Settlement Vertical Stress Increase in a Soil Mass Caused by Foundation Load (Self review) Allowable Bearing Pressure in Sand Based on Settlement Consideration Field Load Test

Tolerable Settlement of Buildings 14

3.16 Vertical Stress Increase in a Soil Mass Caused by Foundation Load (for Consolidation Settlement Calculation)

Self review Stress due to a Concentrated Load Boussinesg (1885) equation is Vertical stress increase : 3P   5/ 2 2  r  2 2z 1       z   where r  x 2  y 2

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Stress due to a Circularly Loaded Area

Vertical stress increase below centre :       1   q0 1  3/ 2  2    B    1   2 z         

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Stress below a Rectangular Area The vertical stress increase below the corner : 3q0 (dxdy ) z 3    q0 I y 0 x 0 2 ( x 2  y 2  z 2 ) 5 / 2 I  influence factor  f(m, n)

  L

B

B L m ,n z z Use Table 3.8

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Table 3.8 Variation of Influence Value I

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Table 3.8 Variation of Influence Value I

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Below any point say “O”

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Calculate stress increase below the centre of a rectangular area  I qo   Iq o I

from Fig .3.28

Same as Table 3.8

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qo  B  L   ( B  z )( L  z )

Calculate the average stress increase of a soil layer

1  av'  ( t'  4 m'   b' ) (5.84) 6

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Example: A flexible rectangular area measures 1.5m×3m in plan. It supports a load of 100kN/m2. Determine the vertical stress increase due to the load at a depth of 3.75 m below the center of the rectangular area.

Solution 1: using Table 3.8

  13.12kPa

Solution 2: using 2:1 method

  12.6kPa 24

For a OC soil, c=60kPa, what is Sc ?

16.5

eo Cc

3.30

What is immediate settlement for the clay layer, Se ? 25

(5.84)

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For normally consolidat ed clay  Area (ii ) ' with  0'   av   c' :

Sc( p )

' Cc  0'   av   z Hc  log Hc ' 1  e0 0

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S c  KScoed S coed  settlement in oedometer condition K  settlement ratio from Figure 3.22

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For each layer Hj, if mv and ’ are constant with depth z, then: Scj   v H j  mv  ' H j

In case of normally consolidated clay, using Cc: Cc  1' Scj   v H j  log ' H j 1  e0 0 For multi-layer Hj (j=1,2,3, …n), summation of settlements in all layers : called “分層縂和法”

j n

sc 

s

cj

j 1

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St  U v S c Uv

U v  average degree of consolidat ion

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Settlement due to Secondary (Creep) Consolidation e e C  Ce   log t2  log t1 log( t2 / t1 )

Ce  C 1 ep

Sc ( s )

Ce t2  zH  log H 1 ep t1

t2  C log H t1

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For Hong Kong Marine Clays: C = (0.3% to 1%) w (in %)

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Why a clayey soil creeps? Creep is due to –viscous adsorbed water (double layers) on clay particles –viscous re-arrangement/sliding/deformation of clay particles/plates –viscous deformation of clay plates Adsorbed water is NOT free water Adsorbed water is NOT free to flow under gravity. 34

Creep movement !

Under effective stress

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• Creep always exists under the action of effective stresses (loading), independent of the excess pore water (or pore pressure). • Therefore, creep has nothing to do with the “primary” consolidation. • Creep exists during and after “primary” consolidation. • Creep rate depends on stress/strain state: –Creep rate is large in a normally consolidated state. –Creep rate is small in a over-consolidated state. 37

Example 3.11: a. Determine the primary consolidation settlement of a foundation with 1.5m ×3m in plan. b. Assume the pore water pressure parameter A for the clay is 0.6, estimate the consolidation settlement considering the 3D effect. c. Assume that the primary consolidation settlement is completed in 3 years. Also let C=0.006. Estimate the secondary consolidation settlement at the end of 10 years.

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3.17 Allowable Bearing Pressure in Sand Based on Settlement Consideration Meyerhof (1956) proposed a correlation for the net allowable bearing pressure for foundations with SPT (N1)60. Original Meyerhof method, for 25mm estimated maximum settlement:

qnet ( all)  qall  D f qnet ( all) (kN / m 2 )  11.98 N 60

( for B(in meter )  1.22m)

 3.28B  1  qnet ( all) (kN / m )  7.99 N 60    3.28B  2

2

( for B(m)  1.22m)

Researchers observed Meyerhof’s results are rather conservative….

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Example 3.12: A shallow foundation measuring 1.75m ×1.75m is to be constructed over a layer of sand. Given Df = 1m; N60 is generally increasing with depth, the average value of N60 is 10. The estimated elastic settlement of the foundation is 14.7mm. Use Meyerhof’s method (modified form by Bowles) to calculate the allowable bearing pressure of the sand.

qnet(all) = 115.6 kPa

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Standard Penetration Test (SPT) SPT N-Value: • Standard hammer weight is 622.72 N(62.3 kg or 140 lb) • Hammer drop height is 762 mm (or 30 in) • Number of blows for spoon penetration of three 152.4mm (6 in) is recorded • The blow number of last 2 penetrations (2 x 152.4=304.8mm) is the SPT N-Value: (1) 152.4mm → 4 blows (2) 152.4mm → 5 blows (3) 152.4mm → 7 blows

SPT N-Value=5+7=12

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Es  0.8N

(in MPa )

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Example 3.13: A shallow square foundation for a column is to be constructed on sand. The foundation must carry a net vertical mass of 102,000 kg. The standard penetration numbers (N60) obtained from exploration are given in the figure. Assume that the depth of the foundation will be 1.5m and the tolerable settlement is 25mm. Determine the size of the foundation.

Consider the non-homogeneous nature of soil deposits:

Es

Depth (m)

N60

2

3

4

7

6

12

8

12

10

16

12

13

14

12

16

14

18

18

E  

s (i )

z

(average )

z 45 z  H or 5B ( whichever smaller )

3.18 Field Load Test

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Plate load test simulates field loading conditions and predicts settlement on proposed foundation. Bearing capacity and modulus of subgrade reaction. 48

For tests in clay : qu ( F )  qu ( P ) (Independent of the size of the plate) where qu ( F )  ultimate bearing capacity of proposed foundation qu ( P )  ultimate bearing capacity of test plate

For tests in sandy soils : qu ( F )  qu ( P )

BF BP

where BF  width of proposed foundation BP  width of test plate 49

3.20 Tolerable Settlement of Buildings   gradient between two successive points

  angular distortion 

ST(ij) lij

(lij is distance between points i and j)   relative deflection from reference line ( A'  E ' )   deflection ratio L

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Prof. A.W. Skempton

In Hong Kong: (a)25mm – for important structures; (b) 50mm – less important (c) 100 mm for walk road, and (d) 200mm for gardens etc.

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