Solutions Manual for Principles of Foundation Engineering 8th ...

1

© 2016 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.

Chapter 2

2.1 d. 3kN/m 17.17

)05.0)(1000(

)81.9)(5.87(

c. 3

kN/m 14.93

15.01

17.17

1 wd

a. Eq. (2.12): 0.76

e

ee

G ws ;1

)81.9)(68.2(14.93 .

1

b. Eq. (2.6): 0.43

76.01

76.0

1 e

en

e. Eq. (2.14): 53%

)100(

76.0

)68.2)(15.0(

e

Gw

V

VS s

v

w

2.2 a. From Eqs. (2.11) and (2.12), it can be seen that,

3

kN/m 16.48

22.01

1.20

1 wd

b. e

G

e

G sws

1

)81.9(

1kN/m 16.48 3

Eq. (2.14): ).)(22.0( ss GwGe So,

2.67

s

s

s GG

G;

22.01

81.948.16

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Chapter 2

2


2

2.3 a. 0.55

e

ee

wG ws ;1

)12.01)(4.62)(65.2(119.5 .

1

)1(

b. 0.355

55.01

55.0n

c. 57.8% 10055.0

)65.2)(12.0(

e

GwS s

d. 3

lb/ft 106.7

12.01

5.119

1 wd

2.4 a. w

eGs . 0.97

ee

e

e

w

ew

d ;1

)4.62(36.0

85.43 .1

)(

b. 0.49

97.01

97.0

1 e

en

c. 2.6936.0

97.0

w

eGs

d. 3

lb/ft 115.9

97.01

)4.62)(97.069.2(

1

)(sat

e

eG ws

Chapter 2

3


2.5 From Eqs. (2.11) and (2.12): 3lb/ft 108

08.01

64.116

d

Eq. (2.12): 53.0 ;1

)4.62)(65.2(108 ;

1

e

ee

G wsd

Eq. (2.23): 0.94

max

max

max

minmax

max ;44.0

53.082.0 e

e

e

ee

eeDr

3lb/ft 85.2

94.01

)4.62)(65.2(

1 max

(min) e

G wsd

2.6 Refer to Table 2.7 for classification.

Soil A: A-7-6(9) (Note: PI is greater than LL 30.)

GI = (F200 – 35)[0.2 + 0.005(LL – 40)] + 0.01(F200 – 15)(PI – 10)

=

=

(65 – 35)[0.2 + 0.005(42 – 40)] + 0.01(65 – 15)(16 – 10)

9.3 9

Soil B: A-6(5)

GI =

=

(55 – 35)[0.2 + 0.005(38 – 40)] + 0.01(55 – 15)(13 – 10)

5.4 5

Soil C: A-3-(0)

Soil D: A-4(5)

GI =

=

(64– 35)[0.2 + 0.005(35 – 40)] + 0.01(64 – 15)(9 – 10)

4.585 ≈ 5

Soil E: A-2-6(1)

GI = 0.01(F200 – 15)(PI – 10) = 0.01(33 – 15)(13 – 10) = 0.54 ≈ 1

Soil F: A-7-6(19) (PI is greater than LL 30.)

GI =

=

(76– 35)[0.2 + 0.005(52 – 40)] + 0.01(76 – 15)(24 – 10)

19.2 ≈ 19

Chapter 2

4


4

2.7 Soil A: Table 2.8: 65% passing No. 200 sieve.

Fine grained soil; LL = 42; PI = 16

Figure 2.5: ML

Figure 2.7: Plus No. 200 > 30%; Plus No. 4 = 0

% sand > % gravel – sandy silt

Soil B: Table 2.8: 55% passing No. 200 sieve.

Fine grained soil; LL = 38; PI = 13

Figure 2.5: Plots below A-line – ML

Figure 2.7: Plus No. 200 > 30%

% sand > % gravel – sandy silt

Soil C: Table 2.8: 8% passing No. 200 sieve.

% sand > % gravel – sandy soil – SP

Figure 2.6: % gravel = 100 – 95 = 5% < 15% – poorly graded sand

Soil D: Table 2.8: 64% passing No. 200 sieve

Fine grained soil; LL = 35, PI = 9

Figure 2.5 – ML

Figure 2.7: % sand (31%) > % gravel (5%) – sandy silt

Soil E: Table 2.8: 33% passing No. 200 sieve; 100% passing No. 4 sieve.

Sandy soil; LL = 38; PI = 13

Figure 2.5: Plots below A-line – SM

Figure 2.6: % gravel (0%) < 15% – silty sand

Soil F: Table 2.8: 76% passing No. 200 sieve; LL = 52; PI = 24

Figure 2.5: CH

Figure 2.7: Plus No. 200 is 100 – 76 = 24%

% gravel > % gravel – fat clay with sand

Chapter 2

5


2.8 43.01117

)4.62)(68.2(1e ;

1

d

wswsd

G

e

G

Eq. (2.37):

43.01

43.0

63.01

63.0

22.0;

1

1

3

3

2

2

32

1

31

2

1

k

e

e

e

e

k

k; k2 = 0.08 cm/s

2.9 From Eq. (2.41):

n

nn

e

e

e

e

k

k)6316.0(1667.0 ;

9.1

2.1

2.2

9.2

1091.0

102.0or ;

1

16

6

2

1

1

2

2

1

898.31996.0

778.0

)6316.0log(

)1667.0log(

n

cm/s 10 0.075 6-

)10216.0(9.1

9.0

1

10216.02.1

)2.2)(102.0()1(

6998.3

3

6

998.3

6

1

11

e

eCk

e

ekC

n

n

Chapter 2

6


6

2.10 The flow net is shown.

/m/sm 1017.06

36-

8

)4)(25.5(

10

105.6

So, m. 25.575.17 cm/s; 105.6

2

4

21max4

q

HHhk

2.11 a.

7825.03

2

7825.03

210

6.01

6.0)2.0(4622.2

)1(4622.2

e

eDk cm/s 0.041

b. 32.2

6.0332.2

106.0

3

)2.0(2.0

4.0

6.01

6.0)35()(

135

DC

e

ek u cm/s 0.171

Chapter 2

7


2.12 3

dry(sand) kN/m 16.8455.01

)81.9)(66.2(

1

e

G ws

3

sat(sand) kN/m 81.0248.01

)48.066.2(81.9

1

e

eG wws

3

sat(clay ) kN/m 18.55)74.2)(3478.0(1

)3478.01)(81.9)(74.2(

1

)1(

s

ws

wG

wG

At A: σ = 0; u = 0; σ = 0

At B: σ = (16.84)(3) = 50.52 kN/m2

u = 0

σ = 50.52 kN/m2

At C: σ = σB + (20.81)(1.5) = 50.52 + 31.22 = 81.74 kN/m2

u = (9.81)(1.5) = 14.72 kN/m2

σ = 81.74 – 14.72 = 67.02 kN/m2

At D: σ = σC + (18.55)(5) = 81.74 + 92.75 = 174.49 kN/m2

u = (9.81)(6.5) = 63.77 kN/m2

σ = 174.49 – 63.77 = 110.72 kN/m2

2.13 Eq. (2.54): Cc = 0.009(LL – 10) = 0.009(42 – 10) = 0.288

Eq. (2.65):

mm 87.2

110

155log

82.01

mm) 10007.3)(288.0(log

1 o

o

o

ccc

e

HCS

2.14 Eq. (2.69):

mm 56.69

128

155log

82.01

)3700)(288.0(

110

128log

75.01

mm) 3700(5

288.0

log1

log1 c

o

o

cc

o

c

o

csc

e

HC

e

HCS

Chapter 2

8


8

2.15 a. Eq. (2.53): 0.392

150

300log

792.091.0

log1

2

21

eeCc

From Eq. (2.65): o

o

o

ccc

e

HCS

log

1

Using the results of Problem 2.12,

2kN/m 87.88)81.955.18(

2

5)81.981.20(5.1)84.16)(3( o

953.0)74.2)(3478.0( so wGe

mm 194.54

87.88

5087.88log

953.01

mm) 5000)(392.0(cS

b. Eq. (2.73): 2H

tCT v

v . For U = 50%, Tv = 0.197 (Table 2.11). So,

days 609 sec 105262;cm) 500(

1036.9197.0 4

2

4

tt

2.16 a. Eq. (2.53): 0.377

120

360log

64.082.0

log1

2

21 eeCc

b. 0.736

2

2

1

2

21 ;

120

200log

82.00.377 ;

log

eeee

Cc

Chapter 2

9


2.17 Eq. (2.73): 2H

tCT v

v . For 60% consolidation, Tv = 0.286 (Table 2.11).

Lab time: min 6

49 min 8

61 t

/minin. 0788.0;)5.1(

6

49

286.0 2

2

v

v

C

C

Field: U = 50%; Tv = 0.197

days 6.25 min 9000;

2

1210

)0788.0(197.0

2

t

t

2.18 5.060

30U

tt

H

tCT

v

v6

221

)1(

)1( 102

2

10002

))(2(

tt

H

tCT

v

v6

222

)2(

)2( 108

2

10001

))(2(

So, Tv(1) = 0.25Tv(2). The following table can be prepared for trial and error

procedure.

Tv(1) Tv(2) U1 U2

UHH

HUHU

21

2211 (Figure 2.22)

0.05

0.10

0.125

0.1125

0.2

0.4

0.5

0.45

0.26

0.36

0.40

0.385

0.51

0.70

0.76

0.73

0.34

0.473

0.52

0.50

So, Tv(1) = 0.1125 = 2 10-6

t; t = 56,250 min = 39.06 days

Chapter 2

10


10

2.19 Eq. (2.84): .2H

tCT cv

c tc = 60 days = 60 24 60 60 sec;2

2H m = 1000 mm.

0415.0)1000(

)60602460)(108(2

3

cT

After 30 days: 0207.0)1000(

)60602430)(108(2

3

2

H

tCT v

v

From Figure 2.24 for Tv = 0.0207 and Tc = 0.0415, U = 5%. So

Sc = (0.05)(120) = 6 mm

After 100 days: 069.0)1000(

)606024100)(108(2

3

2

H

tCT v

v

From Figure 2.24 for Tv = 0.069 and Tc = 0.0415, U 23%. So

Sc = (0.23)(120) = 27.6 mm

2.20

N

S1tan

Normal force, N (lb) Shear force, S (lb)

NS1tan (deg)

50

110

150

43.5

95.5

132.0

41.02

40.96

41.35

From the graph, 41

2.21 Normally consolidated clay; c = 0.

245tan2

31 ; 38

;

245tan309630 2

Chapter 2

11


2.22

245tan2

31 ; 30

;

245tan204020 2

2.23 c = 0. Eq. (2.91): 2kN/m 387.8

2

2845tan140

245tan 22

31

2.24 Eq. (2.91):

245tan2

24531 c

245tan2

245tan140368 2 c (a)

245tan2

245tan280701 2 c (b)

Solving Eqs. (a) and (b), = 24; c = 12 kN/m2

2.25 25

1332

1332sinsin 1

31

311

31

311sin

23

23 lb/in. 5.75.513 ;lb/in. 5.265.532

34

5.75.26

5.75.26sin 1

Normally consolidated clay; c = 0 and c = 0

2.26

245tan2

31 . 22

1 kN/m 305.92

2045tan150

Chapter 2

12


12

2kN/m 61.9

u

u

u ;

2

2845tan

150

9.305 ;

245tan 22

3

1

2.27 a. )log(6.14.01026 50DCD ur

30.7 )]13.0)[log(6.1()1.2)(4.0()53.0)(10(26

b. bae

1

327.209.0

21.0097.0101.2097.0101.2

15

85

D

Da

081.0)327.2)(398.0(845.0398.0845.0 ab

33.67

081.0)68.0)(327.2(

1

1 © 2016 Cengage Learning Engineering. All Rights Reserved.

Principles of Foundation Engineering, 8th edition Das

Chapter 2

Geotechnical Properties of Soil


© 2016 Cengage Learning Engineering. All Rights Reserved. 2

Introduction

∗ Designing foundations for structures such as buildings, bridges, and dams generally requires a knowledge of the following:

∗ 1. The load that will be transmitted by the superstructure to the foundation system

∗ 2. The requirements of the local building code

∗ 3. The behavior and stress-related deformability of soils supporting the foundation system

∗ 4. The geological conditions of the soil under consideration



Introduction – Soil Testing

∗ Geotechnical properties of soil such as grain-size distribution, plasticity, compressibility, and shear strength can be assessed by laboratory testing.

∗ In situ determination of strength and deformation properties of soil are considered because these processes avoid disturbing samples during field exploration.

∗ Not all of the needed parameters can be/are determined, because of economic or other reasons.



Engineer Experience∗ To assess the soil parameters the engineer must have a good

grasp of the basic principles of soil mechanics.

∗ Natural soil deposits are not homogeneous in most cases. Thus the engineer must have a thorough understanding of the geology of the area, such as the origin and nature of soil stratification and ground water conditions.

∗ Foundation engineering is a clever combination of soil mechanics, engineering geology, and proper judgment derived from past experience.



Introduction – Chapter Summary

∗ This chapter serves primarily as a review of the basic geotechnical properties of soils.

∗ Focus includes grain-size distribution, plasticity, soil classification, hydraulic conductivity, effective stress, consolidation, and shear strength parameters.



Grain-Size Distribution

∗ Sizes of the grains vary greatly in any soil mass. To classify a soil properly, you must know its grain-size distribution.

∗ Grain-size distribution for coarse-grained soil is determined through sieve analysis.

∗ Grain-size distribution for fine-grained soil is conducted by hydrometer analysis.



Sieve Analysis

∗ Conducted by taking a measured amount of dry, well-pulverized soil and passing it through a stack of progressively finer sieves with a pan at the bottom.

∗ The amount of soil retained on each sieve is measured, and the cumulative percentage of soil passing through each is determined and is referred to as percent finer.



Sieve AnalysisTable 2.1 contains a list of U.S. sieve numbers and the corresponding size of their openings.

These sieves are commonly used for the analysis of soil for classification purposes.



Sieve Analysis∗ The percent finer for each sieve is determined by a sieve

analysis and plotted on semilogarithmic graph paper, as shown here.

∗ Notice that grain diameter, D is plotted on a logarithmic scale and the percent finer is plotted an arithmetic scale.



Sieve Analysis

∗ Two parameters can be determined from the grain-size distribution curves of coarse-grained soils

∗ 1. The Uniformity coefficient ( ),

∗ 2. The coefficient of gradation, or coefficient of

∗ curvature ( )

∗ D10, D30, and D60 are the diameters corresponding to percents finer than 10, 30, and 60%, respectively.

Cu

Cc

Cu

=D

60

D10

Cc

=D

30

2

(D10

)(D60

)



Hydrometer Analysis

Based on the principle of sedimentation of soil particles in water.

1. A deflocculating agent is added to dry, pulverized soil.

2. The soil soaks for a minimum of 16 hours.

3. Add distilled water and transfer to 1000 ml cylinder and fill sample with distilled water to 1000 ml mark

4. Place hydrometer in solution to measure the specific gravity of soil and water over 24 hour period.



Hydrometer Analysis∗ Hydrometers show the amount of soil that is still in suspension at any

given time t.

∗ The largest diameter of the soil particles still in suspension at time t can be determined by Stokes’ law

= diameter of the soil particle

= specific gravity of soil solids

=dynamic viscosity of water

=unit weight of water

=effective length (i.e., length measured from the water surface in the cylinder to the center of gravity of the hydrometer)

∗=time

D =18h

(Gs-1)g

w

L /t

DG

s

hg

w

L

t



Hydrometer Analysis

∗ With hydrometer readings taken at various times, the soil percent finer than a given diameter D can be calculated and a grain-size distribution plot prepared.

∗ The sieve and hydrometer techniques may be combined for a soil having both coarse-grained and fine-grained soil constituents.



Size Limits for Soils∗ Several organizations have attempted to develop the size limits for

gravel, sand, silt, and clay on the basis of the grain sizes present in soils.

∗ This table presents the size limits recommended by the American Association of State Highway and Transportation Officials (AASHTO) and the Unified Soil Classification systems (Corps of Engineers, Department of the Army, and Bureau of Reclamation)



Weight-Volume Relationships

∗ Soils are three-phase systems consisting of solid soil particles, water, and air.

∗ The phases can be separated

∗ Based on this separation, the volume relationship can be defined.



Weight Volume Relationships

∗ The void ratio ( ) is the ratio of the volume of voids to the volume of soil solids in a given soil mass.

∗ The porosity, n, is the ratio of the volume of voids to the volume of the soil specimen.

∗ = volume of voids

∗ = volume of soil solids

∗ = total volume

e

e = Vv

/Vs

Vv

Vs

n = Vv

/V

V




∗ The degree of saturation, S, is the ratio of the volume of water in the void spaces to the volume of voids.

∗

∗ = volume of water

S(%)=V

w

vv

(100)

Vw




∗ Moisture content,

∗ Moist unit weight,

∗

∗ Dry unit weight,

∗ = weight of the soil solids

= weight of water

∗ = total weight of the soil specimen

w(%)=W

w

Ws

(100)

g = W /V

gd

= Ws

/V

Ws

Ww

W W = Ws+W

w




∗ For moist unit weight of a soil specimen,

∗ For dry unit weight, of soil specimen,

∗ For saturated unit weight soil,

= specific gravity of soil solids

∗ = units weight of water

Gs

gw

g =w

v=

Ws+W

w

Vs+V

v

=G

sg

w(1+ w)

1+ e

gd

=w

s

v=

Ws

Vs+V

v

=G

sg

w

1+ e

gsat

=w

s+ w

s

vs+ v

v

=G

sg

w+ eg

w

1+ e




∗ For further relationships




∗ Specific gravities for certain materials can be found in the table below



Relative Density

∗ For granular soils the degree of compaction in the field can be measured according to relative density,

∗ = void ratio of the soil in the loosest state

∗ = void ratio in the densest state

∗ = in situ void ratio

−=

−max

max min

(%) (100)r

e eD

e e

emax

emin

e



Relative Density

∗ Clean sand ( = 0 to 5%)

∗ Sand with fines ( )

∗ Sands with fines and clay ( ; = 5 to 20%)

∗ Silty Soils ( = 5 to 20%)

∗∗ = fine fraction for which grain size is smaller that 0.075 mm

∗ = clay-size fraction (<0.005 mm)

Fc

emax

= 0.072+1.53emin

5 < Fc

£ 15% emax

= 0.25+1.37emin

15 < Pc

£ 30% Fc

30 < Fc

£ 70% Pc

emax

= 0.44 +1.32emin

emax

= 0.44 +1.21emin

Fc

Pc



Relative Density

∗ The general equation relation for and emin e

max

emax

- emin

= 0.23+0.06

D50

(mm)



Atterberg Limits

∗ When clayey soil is mixed with an excessive amount of water, it may flow like a semiliquid.

∗ If the soil is gradually dried, it will behave like a plastic, semisolid, or solid material, depending on its moisture content.

∗ Liquid limit (LL): The moisture content, in percent, at which the soil changes from a semiliquid to a plastic state.

∗ Plastic limit (PL): The moisture content, in percent, at which the soil changes from a plastic to a semisolid state.

∗ Shrinkage limit (SL): The moisture content, in percent, at which the soil changes from a semisolid to a solid state.

∗ These limits are referred to as Atterberg limits.



Atterberg Limits∗ The liquid limit of a soil is determined by Casagrande’s liquid device

and is defined as the moisture content at which a groove closure of 12.7 mm (1/2 in.) occurs at 25 blows.

∗ The plastic limit is defined as the moisture content at which the soil crumbles when rolled into a thread of 3.18 mm (1/8 in.) in diameter.

∗ The shrinkage limit is defined as the moisture content at which the soil does not undergo any further change in volume with loss of moisture.

∗ The difference between the liquid limit and the plastic limit of a soil is defined as the plasticity index (PI)

PI = LL- PL



Liquidity Index∗ Liquidity Index: The relative consistency of a cohesive soil

in its natural state.

= in situ moisture content of soil

∗ The in situ moisture content for a sensitive clay may be greater than the liquid limit. In this case

∗ The soil deposits that are heavily overconsolidated may have a natural moisture content less than the plastic limit. In this case

LI =w - PL

LL- PL

w

LI >1

<0LI



Activity

∗ Plasticity of soil is caused by the adsorbed water that surrounds the clay particles. It is expected that the type of clay minerals and their proportional amounts in a soil will affect the liquid and plastic limits.

∗ Plasticity index of a soil increases linearly with the percentage of clay-size fraction (% finer than 2 micrometers by weight) present.

∗ Activity: Slope of the line correlating PI and % finer than 2 micrometers.

∗ Activity is used as an index for identifying swelling potential of clay soils

of clay-size fraction, by weightA =

PI

%



Soil Classification Systems∗ Soil classification systems divide soils into groups and

subgroups based on common engineering properties such as the grain-size distribution, liquid limit, and plastic limit.

∗ The two major classification systems presently in use:

∗

∗ (1) the American Association of State Highway and Transportation Officials (AASHTO) System

∗ (2) the Unified Soil Classification System (also ASTM). The AASHTO system is used mainly for the classification of highway subgrades. It is not used in foundation construction.



AASHTO∗ According to the present form of this system, soils can be classified

according to eight major groups, A-1 through A-8, based on their grain-size distribution, liquid limit, and plasticity indices.

∗ Soils listed in groups A-1, A-2, and A-3 are coarse-grained materials.

∗ Soils in groups A-4, A-5, A-6, and A-7 are fine-grained materials.

∗ Peat, muck, and other highly organic soils are classified under A-8. They are identified by visual inspection.



AASHTOFor qualitative evaluation of the desirability of a soil as a highway subgrade material, a number referred to as the group index has also been developed.

The higher the value of the group index for a given soil, the weaker will be the soil’s performance as a subgrade.

A group index of 20 or more indicates a very poor subgrade material.

The formula for the group index is

= percent passing No.200 sieve, expressed as a whole number

= liquid limit

= plasticity limit

GI = (F200

-35)[0.2+ 0.005(LL- 40)]+0.01(F200

-15)(PI -10)

F200

LL

PI



AASHTO∗ When calculating group index for a soil of group A-2-6 or

A-2-7, use only the partial group-index equation relating to the plasticity index.

∗ The group index is rounded to the nearest whole number and written next to the soil group in parentheses.

∗ The group index for soils which fall in groups A-1-a, A-1-b, A-3, A-2-4, and A-2-5 is always zero.

GI = 0.01(F200

-15)(PI -10)



Unified System

∗ The plasticity chart and the table show the procedure for determining the group symbols for various types of soil.

∗



Unified System



∗ When classifying a soil be sure to provide the group name that generally describes the soil, along with the group symbol.

∗ The flowcharts are used for obtaining the group names for coarse-grained soil, inorganic fine-grained soil, and organic fine-grained soil, respectively.

Unified System



Unified System



Unified System

∗ Symbols of the Unified System

G= Gravel

S = Sand

M = Silt

C = Clay

∗ O = Organic silts and clay

∗ Pt = Peat and highly organic soilsH = High plasticityL = Low plasticityW = Well gradedP = Poorly graded



Hydraulic Conductivity

∗ Water flows between void spaces or pores between soil grains.

∗ Knowing how much water is flowing through soil per unit of time is important for soil mechanics and foundation engineering.

∗ Water flow knowledge is required for designing earth dams, determining seepage under hydraulic structures, and dewatering foundations before and during construction.




∗ The equation is used to calculate the velocity of flow of water through a soil.

∗ =Darcy Velocity (unit: cm/sec)

∗ =hydraulic conductivity of soil (unit: cm/sec)

∗ = hydraulic gradient

∗ The hydraulic gradient is defined as

∗ = piezometric head difference between the sections at AA and BB

∗ = distance between the sections at AA and BB

v = ki

vki

i = Dh/ L

Dh

L




∗ For granular soils, the hydraulic conductivity (k) depends on the void ratio.

∗ Although several equation have been proposed, it is recommended that the equation be used to relate k to the void ratio in granular soils.

∗ = hydraulic conductivity = void ratio

The range of hydraulic conductivity for various soils is given in this table.

k µe3

1+ e

ke




∗ In determining the hydraulic conductivity of consolidated clays, use the equation

∗

∗ and are constants determined experimentally.

k = cen

1+ e

cn



Steady-State Seepage

∗ For most cases of seepage under hydraulic structures, the flow path changes direction and is not uniform over the entire area.

∗ One of the ways for determining the rate of seepage is by a graphical construction referred to as the flow net.




∗ The flow at any point A can be determined by the equation

∗ =hydraulic conductivity of the soil in the x, y, and z directions, respectively

∗ = hydraulic head at point A (i.e., the head of water that a piezometer placed at A would show with the downstream water level as datum, as shown in Figure 2.11)

kx

¶2h

¶x2+ k

y

¶2h

¶ y2+ k

z

¶2h

¶z2= 0

kx,k

y,k

z

h




∗ Laplace’s equation

∗ Laplace’s equation is valid for confined flow and represents two orthogonal sets of curves known as flow lines and equipotential lines.

∂ ∂+ =

∂ ∂

2 2

2 20

h h

x z




∗ A flow net is a combination of numerous equipotential lines and flow lines.

∗ A flow line is a path that a water particle would follow traveling from the upstream side to the downstream side.

∗ An equipotential line is a line along which water, in piezometers, would rise to the same elevation.




∗ In drawing a flow net, you need to establish the boundary conditions.

∗ The ground surfaces on the upstream (OO’) and downstream (DD’) sides are equipotential lines.

∗ The base of the dam below the ground surface, O’BCD, is a flow line. The top of the rock surface, EF, is also a flow line.




∗ Once the boundary conditions are established, a number of flow lines and equipotential lines are drawn by trial and error so that all the flow elements in the net have the same length-to-width ratio (L/B).

∗ In most cases, L/B is held to unity, that is, the flow elements are drawn as curvilinear “squares.”

∗ All flow lines must intersect all equipotential lines at right angles




∗ Once the flow net is drawn, the seepage (in unit time per unit length of the structure) can be calculated using the equation

∗ = number of flow channels

∗ = number of drops

∗ = width-to-length ratio of the flow elements in the flow net (B/L)

∗ = difference in water level between the upstream and downstream sides

Nf

Nd

q = khmax

Nf

Nd

n

n

hmax



Effective Stress

∗ The total stress at a given point in a soil mass can be expressed as

∗ = total stress

∗ = effective stress= pore water pressure

s =s '+ us

us '



Effective Stress

∗ The effective stress is the vertical component of forces at solid-to-solid contact points over a unit cross-sectional area.

∗ In reference to point A of the figure Effective stress is calculated by the equation

=effective or submerged unit weight of soil.

s '

s ' = g h1

+g 'h2

g '



Effective Stress

∗ If there is no upward seepage of water in the soil then again referring to point A

∗ = unit weight of water

∗ = saturated unit weight of soil

s = g h1

+gsat

h2

u = h2g

w

gsat

gw



Effective Stress

∗ If upward seepage of water does occur in soil than at point A in the figure

∗ =hydraulic gradient ( )

s = h1g

w+ h

2g

sat

u = (h1

+ h2

+ h)gw

s '= h2(g '-

h

h2

gw

)= h2(g '- ig

w)

i h/h2



Effective Stress∗ If the hydraulic gradient is very high, then the effective

stress will become zero.

∗ If there is no contact stress between the soil particles, the soil will break up.

∗ This situation is referred to as the quick condition, or failure by heave.

∗ For the heave

= Critical hydraulic gradient

i = icr

=g '

gw

=G

s-1

1+ e

icr



Consolidation ∗ When stress on a saturated clay layer is increased—for

example, by the construction of a foundation—the pore water pressure in the clay will increase.

∗ The hydraulic conductivity of clays is very small and some time will be required for excess pore water pressure to dissipate and for the increase in stress to be transferred to the soil skeleton.



Consolidation

∗ If is a surcharge at the ground surface over a very large area, the increase in total stress at any depth of the clay layer will be equal to .

At time t = 0 (i.e., immediately after the stress is applied), the excess pore water pressure at any depth

∗ will equal .

∗

∗ (at time t=0)

Ds

Ds

Du Ds

Du = Dhig

w= Ds



Consolidation

∗ At time t = all the excess pore water pressure in the clay layer should dissipate as a result of drainage into the sand layers.

∗ (at time t= )

∗ Then the increase in effective stress in the clay layer is

∗ The gradual increase in the effective stress in the clay layer will cause settlement over a period of time and is referred to as consolidation.

¥

Du = 0 ¥

Ds '= Ds



Consolidation

∗ Laboratory tests on undisturbed saturated clay specimens can be conducted to determine the consolidation settlement caused by various incremental loadings.

∗ Based on the laboratory tests, a graph can be plotted showing the variation of the void ratio e at the end of consolidation against the corresponding vertical effective stress . σ '



Consolidation

From the e–log curve shown in Figure 2.16b, three parameters necessary for calculating settlement in the field can be determined.

∗ The parameters are preconsolidation pressure , compression index , and the swelling index .

σ '

σ 'c

Cc

Cs



Preconsolidation Pressure

∗ Preconsolidation pressure, , is the maximum past effective overburden pressure to which the soil specimen has been subjected.

σ '



Preconsolidation Pressure∗ Determining the preconsolidation pressure involves a 5

step process. (Use figure on previous slide for reference)

1. Determine the point O on the e–log curve that has the sharpest curvature (i.e., the smallest radius of curvature).

2. Draw a horizontal line OA.

3. Draw a line OB that is tangent to the e–log curve at O.

4. Draw a line OC that bisects the angle AOB.

5. Produce the straight-line portion of the e–log curve backwards to intersect OC. This is point D. The pressure that corresponds to point D is the preconsolidation pressure .

∗

sc

'

s '

s '

sc

'




∗ Natural soil deposits can be normally consolidated or overconsolidated (or preconsolidated).

∗ If the present effective overburden pressure is equal to the preconsolidated pressure the soil is normally consolidated.

∗ If the soil is overconsolidated.

s ' =s0

'

sc

'

s0

' <sc

'




∗ Preconsolidation pressure can be correlated with the liquidity index by the equation

∗ = atmospheric pressure

∗ = liquidity index

sc

'

Pa

= 10(1.11-1.62LI )

Pa

LI



Compression Index

∗ Compression index is the slope of the straight-line portion of the loading curve and is determined by

∗

∗ and are the void ratios at the end of consolidation under effective stress and respectively.

∗ The compression index, as determined from the laboratory curve, will be somewhat different from that encountered in the field.

Cc

=e

1- e

2

logs2

' - logs1

'

1es

1

' s2

'2e



Compression Index

∗ The swelling index is the slope of the unloading portion of the curve and is determined by

∗ The swelling index is also referred to as the recompression index.

Cs

e - logs '

σ

σ

−= 3 4

'4'3

log( )s

e eC



Calculation of Primary Consolidation Settlement

∗ The one-dimensional primary consolidation settlement (caused by an additional load) of a clay layer having a thickness Hc may be calculated by the equation

∗ =primary consolidation settlement

∗ =total change of void ratio caused by the additional load

application

∗ =void ratio of the clay before the application of load

Sc

=De

1+ e0

Hc

Sc

De

e0




∗ For a normally consolidated clay use the equations

Sc

=C

cH

c

1+ e0

logs

0

' + Ds '

s0

'De = Cclog

so

' + Ds '

so

'




Sc

=C

sH

c

1+ e0

logs

0

' + Ds '

s0

'De = Cslog

so

' + Ds '

so

'

For overconsolidated clay with use equations s0

' + Ds ' £sc

'




σ σ σ

σ σ

+ ∆= +

+ +

' ' '0

' '0 0 0 0

log log1 1

s c c c cc

C H C HS

e e

σ σ σ

σ σ

+ ∆∆ = ∆ + ∆ = +

' ' '0

1 2 ' 'log logc

s c

o c

e e e C C

For overconsolidated clay with use equations

σ σ σ σ< < + ∆' ' ' '0 c o




∗ Consolidation is the result of the gradual dissipation of the excess pore water pressure from a clay layer.

∗ The dissipation of pore water pressure increases the effective stress, which induces settlement.

∗ To estimate the degree of consolidation of a clay layer at some time t after the load is applied, the rate of dissipation of the excess pore water pressure must be determined.




∗ Referring to the figure (see next slide), vertical drainage (drainage in the z direction only) is determined by the equations.

=hydraulic conductivity of the clay

∗ =total change of void ratio caused by an effective stress

∗ increase of

= average void ratio during consolidation

=volume coefficient of compressibility=

¶(Du)

¶t= C

v

¶2(Du)

¶z2γ γ

σ

=∆

∆ +'(1 )

v

v ww

av

k kC

em

e

av

1+ eav

=De

Ds '(1+ eav

)

kDe

eav

mv

�




∗ The average degree of consolidation of the clay layer can be defined as

∗ = settlement of a clay layer at time t after the load is applied

∗ = maximum consolidation settlement that the clay will undergo under a given loading

U =S

c(t )

Sc(max)

Sc(t )

Sc(max)




∗ If the initial pore water pressure ( ) distribution is constant with depth the average degree of consolidation can be expressed with relation to time as

Du0

U = 1- (2

M2)

m=0

m=¥

å e-M2T

v




The variation of with have been calculated in this tableTv U



Degree of Consolidation Under Ramp Loading

∗ Previous discussion considered surcharge load per unit area as being applied instantly at t=0. However, surcharge is usually applied over time till max surcharge is met.

∗ In this situation where use equation Tv

£Tc

=∞

=

= − −∑ 2

40

2 1{1 [1 exp( )]}

mv

vmc v

TU M T

T T M



Degree of Consolidation Under Ramp Loading

∗ If then use equation

∗ =nondimensional time factor =

∗ =

U = 1-2

Tc

1

M4m=0

m=¥

å [exp(M2Tc)-1]exp(-M2T

c)

Tv

³ Tc

Cvt / H2

[(2m+1)p ]/2M

Tv

Tc

=C

vt

c

H2



Shear Strength

Shear strength of soil is defined in terms of effective stress

(Morh-Coulomb failure criterion)

∗ = effective normal stress on plane of shearing

∗ = cohesion, or apparent cohesion (0 for sands and normally consolidated clays and >0 for overconsolidated clays)

∗ = effective stress angle of friction

s = c ' +s ' tanf '

s '

c '

f '



Shear Strength

Shear strength parameters of a soil are determined by two standard laboratory tests:

Direct shear test

Triaxial test



Direct Shear Test

∗ Dry sand can be tested by direct shear tests.

∗ The sand is placed in a shear box that is split into two halves

∗ First a normal load is applied to the specimen.

∗ Then a shear force is applied to the top half of the shear box to cause failure in the sand.



Direct Shear Test

∗ Using the equations and give values that are plotted as against the angle of friction can be determined by the equation

∗

s ' = N / As = R / A

f ' = tan-1(s

s ')

s s '



Direct Shear Test



Direct Shear Test

∗ For granular soils the friction angle can be determined by the equation

=void ratio

∗ =

∗ =

and = diameters through which respectively 85% and 15% of soil passes

f '(deg)= tan-1(1

ae + b)

e2.101+0.097(

D85

D15

)a0.845- 0.398ab

D85

D15



Triaxial Test

∗ Triaxial compression tests can be conducted on sands and clays.



Triaxial Test∗ Consists of placing a soil specimen confined by a rubber

membrane into a lucite chamber then applying an all-around confining pressure to the specimen by means of the chamber fluid (generally, water or glycerin).

∗ An added stress can also be applied to the specimen in the axial direction to cause failure.

∗ Drainage from the specimen can be allowed depending on the conditions being tested.



Triaxial Test

∗ The three main tests are

∗ Consolidated-drained test (CD test)

∗ Consolidated-undrained test (CU test)

∗ Unconsolidated-undrained test (UU test)



Triaxial Test

∗ Consolidated Drained Test

∗ Used on various clay soils

The test will help determine:

Major principal effective stress=

Minor principal effective stress=

s1

'

s3

'



Triaxial Test

∗ With this test the shear strength can be determined by plotting Mohr’s circle as failure and drawing a common tangent to the Mohr’s circles. This is the Mohr-Coulomb Failure Envelope

∗ Failure = s1

' =s3

' tan2(45+f '

2)+2c ' tan(45+

f '

2)



Triaxial Test

∗ Consolidated Undrained Test

∗ Used on various soils


Major principal effective stress=

Minor principal effective stress=

∗ Major principal total stress =

∗ Minor principal total stress =

s1

'

s3

'

s1

s3



Triaxial Test

Consolidated-Undrained Tests:

∗ The total stress Mohr’s circles at failure can now be plotted. Then a common tangent can be drawn to define the failure envelope.

∗ This total stress failure envelope is defined by the equation

∗ and are the consolidated-undrained cohesion and angle of friction, respectively.

s = c +s tanf

c f



Triaxial Test



Triaxial Test

∗ Unconsolidated-Undrained triaxial test

∗ Used on various soils


Major principal total stress =

∗ Minor principal total stress =

s1

s3



Triaxial Test

∗ The total stress Mohr’s circle at failure can now be drawn.

The tangent to these Mohr’s circles will be a horizontal

line called the condition.

∗ The shear strength for this condition is determined by the equation

∗ = undrained cohesion (or undrained shear strength)

∗ = Failure stress

f = 0

s = cu

=Ds

f

2

Dsf

cu



Triaxial Test



Unconfined Compression Test∗ The unconfined compression test is a special type of

unconsolidated-undrained triaxial test in which the confining pressure .

∗ In this test, an axial stress is applied to the specimen to cause failure ( )

∗ The axial stress at failure, , is referred to as the unconfined compression strength.

∗ The shear strength of saturated clays under this condition

∗ ( ) is determined by the equation

s3

= 0

Ds = Dsf

Ds

Dsf

= qu

f = 0s = c

u=

qu

2



Unconfined Compression Test

∗ The unconfined compression test is a special type of unconsolidated-undrained triaxial test in which the confining pressure .

∗ In this test, an axial stress is applied to the specimen to cause failure ( )

∗ The axial stress at failure, is referred to as the unconfined compression strength.

∗ The shear strength of saturated clays under this condition

∗ ( ) is determined by the equation

s3

= 0

Ds = Dsf

Ds

Dsf

= qu

f = 0

s = cu

=q

u

2



Unconfined Compression Test

∗ The unconfined compression strength can be used as an indicator of the consistency of clays.

∗ Unconfined compression tests are sometimes conducted on unsaturated soils.

∗ With the void ratio of a soil specimen remaining constant, the unconfined compression strength rapidly decreases with the degree of saturation.



Comments on Friction Angle,

∗ In general, the direct shear test yields a higher angle of friction compared with that obtained by the triaxial test.

∗ The failure envelope for a given soil is actually curved.

∗ The Mohr–Coulomb failure criterion is only an approximation. Because of the curved nature of the failure envelope, a soil tested at higher normal stress will yield a lower value of .

f '

f '



Sensitivity

∗ For many naturally deposited clay soils, the unconfined compression strength is much less when the soils are tested after remolding without any change in the moisture content.

∗ This is known as sensitivity and is the ratio of unconfined compression strength in an undisturbed state to that in a remolded state or

= u(undisturbed)

t

u(remolded)

qS

q

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