Undrained shear strength versus Qver- consolidation ratio for isotrQpieally

SID (3

5.10 Undrained shear strength versus Qver- consolidation ratio for isotrQpieally

'Consolidated Dhaka clay.

'I

..

[},'--- 1_ - _~.._. L_. '__ ._. . .J _

" 3; G ~~ 12 '!5

--- .•••••-OVER CONSOLIDATION RATIO, n

Fig. 5.11 Normalised relationship between Over- consolidation ratio and undrained shear strength.

::J

Vl VIc..

z

10 15

,- r--l

I

Thus in Fig. 5.12 the 5

/0

for

u vo

tbpredict the undrained shear strength (oc) .., ₅(nc;;)

5.-u u

=

0vo .0v

may be used as a mo del

For ko consolidated samples similar type of graph has been also plotted in Fig. 5.12 and the relation between 5 /~

U'.- va

and bverconsolidation ratio in a logarithmic plot isrepresen- thOse were also straight lines •.

Of remoulded Dhaka clay for isotropically consolidated and pressure_. S /_u

a

_va and overconsolidation r.atio' n, in a plain

straight line. Similar graphs were also plotted by Mitachi and Kitago (1976), Mayne (1980) for other types of clay and

graph for isotropically consolidated Dhaka .E~ay. This relation- ship is not. also a straight line. But if these data are plotted in logarithmic scale as shown in Figure 5.12 the plot is a

ted by a straight line. These suggest that the equation

ko consolidated samples.

overconsolidation ratio 1 is the intercept of the curve. The slope Of the line represented i. gives a DOmplete linear

o

equation in log-log scale. The slope of the line ~o thus determined from the graph is henceforth referred .to as

experimentally determined value. For Dhaka clay in isotropi- cally consolidat~d and k consolidated condition the~ values

. 0 0

obtained in above manner are 0.746 and 0.83 respectively and

40

,

\:

.,

.

^'.-

k o consolidated

Isotropically consolidated

Variation of normalized shear strength • with overconsolidation ratio for Dhaka clay.

'\

'

fi

"4 S

--- .•••-OVER- CONSOLIDATION RATIO. n 10

8

Legend

6

0-()

*--*

4 3

Fig. 5.12

1.

0 I-

«f.

n:

""lib'?

₁

II} ,-

1

,$1

.0

.]

,(;

.

^'

"(.

\., ,

, of 11

o pore pressure parameter. Table 5.5 shows for,_comparison

referred as experimentally determined values of critical state

From the experimentally determined values shown in Table 5.5 it caRnbe seen that 110 for isotropically consolidated samples. For isotropically consolidated samples 110 obtained from experimental curves is 14.3 percent less than the value- determined from isotropic consolidation test data. For ko Consolidated samples the 11 obtained from the experimental

o

curve is 8.5 percent less than the value determined from con-

A plot of 5_u

15

_va- ^versus overconsolidation ratio fOr undis- ventional consolidation test data. Mitachi and Kitago (1976) had also. found the. value of 11 determined experimentally less

o

determined from other methods such as by the relati~nC1-Cs/Cc) or (1-C_Sl

./c

_Cl

.)

and henceforth referred as conventional conso- lidation and isotropic consolidatio~ method respectively.

than the value determined from consolidation test,data, but the values were close.

turbed natural soil samples of Dhaka clay collected from two

natural sample were obtained from a comparison of probable maximum overburden pressure that would be required to develop

loc<;ltionsare presented in Fig. 2.17 in page 45 after 5afiullah (1977) and Islam (1980). The overconsolidation for these

the current undrained shear strength (5 ) of the samples from

. u

5kempton'srelation,

In this equation 5u is the undrained shear strength of the

Table ^5.5 Critical' state pore pressure parameter ^AD by various method;

Determined Deter~ined Determined Determined Determined

from slop e from f:rom from the from the slop e

of 5 /0 1 C . /C .

i:..c

/C slope of of 5

/'0

and

and_Un ¥or ^{s~ eJ.} se" c

5 /0 and ^{. u va}

, ) nl.n

natural soil ^{.• I} U' v

,n in iso- condition tropic _-

condition

1 ₂ 3 _{4 5}

.

I

0.958 0.87 O. 907 _0.7/,6 0.83

I !

I

I

,

the undisturb~d sample. The value of ~o for these natural Dhaka clays is 0.958 while the experimentally determined values for remoulded Dhaka clay are 0.85 in ko condition.

and .746 for isotropic condition. This discrepancy between natural soils and remoulded soil is probably due to the some amount of uncertainties in the evaluation of overconsolidation ratio of the natural deposit. The other reason for the diffe- rence is the values ~f A . obtained for natural and reconsti-

Q)

tuted Dhaka soil may be due to the difference in fabric or

the final soil structure of the soils obtained 1"n two processes, and also due to sampling disturbance. This aspects however

require further investigation.

Table 5.6~ compares the results of undistuibed undrained shear str~ngth

ot

^{natural Dhaka clay with that predicted from}

the model by Mitachi&'Kitago(lg76~epicted by the equation.

2.19. The undrained shear strength data have been collected after 5afiullah (19~7) and Isl~m (1980). The model assumes that the water content and in-situ vertical stresses are known. In this model the overconsolidation ratio has been computed on the basis of water content versus iog of consolidatioR;:pressure v curve shown. in Fig. 5.13. For example a point corresponding to

in-situ vertical stress and water content of the soil is first located at point A.

A

line parallel to swelling curve and

through that point corresponding to k condition cut the o

normally loaded state' consolidation curve at point 8. The

.,..,I U I f.<'"

I I ....• .,.., 0 aJ C

0 U 0 o ~ D- Ul ^{-0 ~ •.-I} U

~ ~ ^{en ill •.-4} Ul aJ Ul .,.., co C C aJ"- •...• .,..,

.'

^....•

~ ""- ""- ^{....• Cf.<D- C f.;l C Ul o f.< o 0 croD- D- aJ}

""- ^{~ ~ ro o :J 0 o :J 0 D- .,..,-I-> .,..,U .,.., E f.<0 f.<D}

~ U U Ul f.< UCI)U,-" -1->0 -I-> aJ ro.c ro ro f.< ro 0

x -I-> .,.., Ul-l-> Ul ro Ul ro 0" f.<-I->Ul aJ-I-> aJ E

C -I-> aJ aJ C -I->~ E aJ 0 E aJ o aJ D.,..,

D -'"f.< D '" ~ ^{'£ Q} .c

0 .,.., D .c aJ f.<.,..,^{:J~(J1 __} :Jf.<-'"

"

^{.,.., .,.., :J C C-0 ••..• UlUl~ Ul 0}

.,..,

.

^{E C -I-> -I-> aJ Ul E 0...-1 .,.., E D-} f.< •...•E •...•EU :J ^OJ Q) Ul .,-i'.-I

-'"

-I-> 0 .,.., _H C " D- .,.., Ul .,.., f.< :J 00 o 0 ^{~ .0. 0.} D Ul D ~

ro _z ....•

"-

^{0 ~ XCE'o. X C 0 U Ul f.< Ul f.<C} "D-IJk..., aJ.c D- ID .c ..-i

U U 0 U :J ^ttl a a ... roo"- c"- C"-o aJ Ul^:J C-I->~ C -I->Ul

0 aJ D .,.., aJ -I->Ul ^{E'..-I ~} E."" C 0 0 .,.., f.< -I-> .,..,

'"

^{• ..-i (J1- a.}

..J ....• .,.., -I-> .c •...• f.< .,..,Ul -1->"- aJ -I-> 0 U 0 aJ Uo-l-> ^:J f.<Ul roc •...• roc~

D- :J Ul -I->D- ro Ul aJ -I->ro _{x "} -I->ro ro."" ^{k'r-1 :;::. k ...-t} co Ul co '..-1 f.<aJ ro f.<aJ E ^IT ro D-E -I-> ^I f.< UlD ro f.< UlD E-I-> aJ-I->f.< aJ-I->D aJaJD D f.<D D f.<

ro

...•

....• aJ ro ro C-I-> ro.,..,E :J CO'..-I :::.co " ro :J ^{:::. co ...-1} aJ.ca: C -I->0 C-I->

(f) ..J D- o Ul _- 3 Hen. a. ...•,'0> u a. ...•^{I\) -0} o f.<U of.< •...•'" Ul :J ^:::J Ul E :::J Ul

1 41 1 9 81-5" to 9'-8" 22.27 8.33 73.0 115.0 9.0 1 3.8 11.39 12.88 13.98

2 43 1I) 10'-2" to 11 ' - 5" 23.63 9.58 56.0 90.0 5.84 9.39 10.83 10.70 11.68

"

.,..,f.< 3 46 22 12' -0" to 1 3' -0" 25.29 10.85 40.0 60.0 3.69 5.53 12.50 8.62 8.52

ro

.D 4 43 22 15' -4" to 16' - 8" 24.03 12.45 50.0 80.0 4.02 5.42 12.01 10.55 11.78 ....•

:J -

LL 5 42 19 17' -2" to 18' -5" 21.34 13.24 65.0 1 20.0 4.90 9.06 10.40 13.00 15.67

6 43 24 6' -811 to 8'-2" 19.00 6.65 1 40.0 195.0 21.06 29.34 10.50 19.36 20. 85

7 51 21 1 3' -0" to-14'-6" 21 .0 13.47 80.0 1 20.0 5.93 8.9 9.50 15.25 15.71

•

Cl 8 46 21 32'-0" to 33' - 6" 20.0 19.48 105.0 115.0 5.39- 7.95 1 2. 50 20.53 20. 68 zCl

i

^.

~o

N

200

300

\

\

\

,

•

c\ ,

\ \

\

\

ko consolidated

Isotropically ~onso- lidated

0--0

legend

'",

30 40 50 60 '00 100 20

\

\

\ ,

-

^{"X __}

--~ ^\

\

\ "

^\

0--:---_-...0--_

o

--<>---

.\

\

x__

\

\

---- ^\

.

---.: _\

\

--~ ••-VERTICAl CONSOLIDATION PRESSURE lo-v) IN PSI

X-.-_

- _{-- --}

^A

--

""-

--- ^...

Fig. 5.13 Relationship between water content and consolidation pressure for Dhaka clay to compute overconsolidation ratio.

21

20 21 18

3 4 5 6 7 8 910

32 31 30

~ .29

f--

2B z

w

ucr

27

'wa.

~

26

f--

z

w

f--

25

z

a u cr

24

w

~

~

23

has been (0 c) _'

::'stjo 5 /eJ

U __ \iU-

condition resp~cl~ve.Ly.

J.:'ounc! pressure ..

ca.:I:cu.lated r:c'om GFf::':r:cOrtsol:idat.,)Dll .~.-)-c.:Lo,< [=InC i.he experimental consolidation pressure corresponding to that point is the past maximum overbruden pressure. The ratig between the

pressure corresponding to this point ,C i,s the past, maximuffl",all past maximum consolidation pressure and the present in-situ vertical pressure is the overconsolidation ratio. Similarly the line correspondingfto isotropic condition ~ut the normally state consolidation c0rve at another point t, The consolidation

pr'S[11o;ted fromisotrop5,cally conso,U.r.Jated model and k conso ..

, 0

lid8iced mod,eJ. and the strength or ne.tLJra,lsoil' after LJndist'LJr- b'ed s~mplingo nverconsolidatio'n. r"tio has besn determined

from the water content and exlsting vertical pressure from the generalized curves for Oh, a soil shown in Figo 5-14, There is. some scatter in the estimated strength of the soil to actual strength a~ measured 'by )aboratory testing of the in-situ

sampleso This scatter may be due to the variability of the soil data and samp'ing effect. The plasticity ~ndex for the soil varied from 19 to 29 and the liquid limit varied from 41 to

51,

for these soils. Moreover some of these field samples were not fully saturatedo These results sug~~st that inorder

,0 .40 20

e.

;

I

• •

•

Isotropic.

4 S 678910

o

,

105

100 9 8 70

GO •

Measured values. overconsolidation ,ratio , computed from ko condition

o Measured values. overconsolidation ratio

'0

computed from isotropic condition

---"-OVERCONSOLIDATION, n

Fig. 5.14 Comparison of measured values of undrained shear strength wi th those predicted from the mOdel.

2 ..30

!'!

to generalize !I",~parameter for Dhaka soil, further tests are required for other vari ties of Dhaka clay with different plasticity indices and different liquid limits. At the same time further study is also required to obtain more controlled field data with actual field strength tests to verify the applicability of'the proposed mOBelfor Dhaka clay.

Fig. 5~15>'represents the water content versus undrained shear strengtb;-,for isotropically cons'olidated and kD consoli- dated Dhaka clay at normally loaded state. The plot is a straight line in semilogarithmic scale. For London clay and Weald clay straight line relationship in semi-logarithmic

~cale was also found by Henkel (1959) shown in Fig. 2.£. It' can be seen from the Fig. 5.1S'that'the relationship between water content Bnd undrained 5hear strength is not unique for isotrD~~cally consolidated and'k

o

c0060liDated s~mples.This Suggest that the undrained strength in normally loaded state is also dependent on stress histoFybecause different micro- structures may have been build up during dissimilar conso- lidatiofl history. It also_appears from this figure that the difference between the curves of isotropicallyconsolidated and ko consolidated is initially smaller but increases with the decrease of water content. The reason may be due to the fact that initially the isotropically consolidated samples were consoli,dated in ko condition.

Fig. 5;16 shows the(S

It.. /Ci

versusoverconsolidatioA:

LJ "tv va

ratio. 5imilar normalized g~aph was also drawn by Mahar and

100

;

I

~ Isotropically consolidat

0-0

ko consolidated.

I

.fi

I Legend

45 6 7 I'>910 3

I

2

-- •. UNDRAINED SHEAR STRENGTH IN. 'P:'>I (Su1

Fig. 5.15 Water content versus undrained shear strength

for isotropically consolidated and k .consolidated

Dhaka clay at .normallyloaded state.o

\

^,

.

I

3D 40 SO

,

20

I 3

,

/

Legend

, Beaumont (eH) l'lontogmary(CL)

If--4(

at'ter tlahar "(1983). . 'c

^I,

. !r-:-ti

i,Isotropically c.nsolidated Dhaka clay " •

e>-,-o'k consolidated Dhaka Clay

8. . "

ei

Natural Dhaka clay

---~ OVERCONSOLIDATION RATIU, n

General normalized undrained shear strength versus overconsolidation rati,.

I 2

Fig., 5.16

O'Neill (198:5) for desiccated clay. From this figure it can be observed that the curve for isotropically consoli- dated Dhaka clay is very close to the normalized curve drawn by them. Data points for natural Dhaka clay are also shown in this figure. Thes~ points are close to the curve shown by Mahdr and O'Neill (1983). The curve .for ko consolidated

samples is different but parall~l to the c~rve drawn by them.

This probably answers ~n important question regarding the mechanism through which Dhaka clayh~ve been overconsolidated, that is thou~b desipcatioR~

5.4 Coefficient of-Lateral Earth Pressure at Rest ko

Table

5.7

shows the lateral stress and the coefficient of lateral earth pressure at rest (k ) corresponding to

. _ . 0

vertical. stress for loading and unloading condi tion in remoulded state. The lateral earth pressure are measured from strain-

gauges fiLted to the consolidation cell wall at the completion of consolidation and swelling. The details of these measure- ments and calculations fork- has ~een discussed in Appendix-C

. .0. .

It may be mentioned that out of two electric strain-gauges one strain-gauge gave inconsistent readings and therefore the results tabul~ted in Table

5.7

are from one strain gage onl y.

Table 5.7 Results of special consolidation test for determination of k

o

Vertical Height of the

o

^verconso- Lateral Coefficient effective sample at the lidation stress on of lateral consoli- end of consoli- Ratio the soil earth pressure

dation dation psi at rest

pressure inches k

psi ⁰

1 2 3 4 5

0 1.5

- - -

20 0.8539 1 9.6 0.48

40 - 0.8133 1 18.15 0.454

60 0.7905 1 26.98 0.459

80 0.7726 1 :'6.10 .0.451

100 0.7574 1 45.48 0.456

120 O. 7471 1 54.87 _ .0.457

,J,'".

100 0.7483 _.~)JJ-~ 1.2 52.53 0.525

80 0.7495 1.5 .50.33 0.629

-

60 0.7512 2 .45.86 0.764

40 0.7529 3 41 .40 1.03

.

---

_.'--

20 0.7552 6 34. 76 1. 74

10 O. 7607 12 32.23 3.23

The data of Table 5.7 is plotted in Fig. 5.17(a). From this figure it can be observed that the lateral stress

.increases in direct proportion with increase in vertical stress. The lateral stress also decreases when the vertical stress is reduced. But the lateral stress during unloading is greater than the lateral stress when the soil was first loaded. That is during unloading lateral stress decreases at a lower rate than the vertical stress:

From the Figure 5.17:b) it can be observed that coeffi- cient of lateral earth press~re at rest (k ) remains essentially

o

constant for normally loaded state. The value of ko increases with the unloading. That is ko increases as the overconsolida_

tion ratio increases.

Table 5,8 compares the experimental values of eatth pressure co-efficient at rest .for noimallyloaded condition k(nc) with

. ;. 0

the values determined from empirical relations such as suggested in article 2.4.1. It is seen from the table that measured

values of ko is less than the value determined by available emp~rical relations for Dhaka clay.

F'_19. 5_. 1E_: repreSen. st the re1a lOnt' b te ween k(oc)/k(nc)o. 0 wl.th overconsolidation ratio n in a log-lo~~scale. The relation

has been found to be a straight line, which is similar to that given by Alpan (1967)

cx ..

n

\

100' IN PSI

pressure

80

100

&0 Loading Unloading

40

Loading

•. EFFECTIVE OVER BURDEN PRESSUR, lYyIN PSI

I

20

20 40 &0 80

--- __ EFF ECTIVE OVER BURDEN PRESSUR~, lYy

5.17(a) Overburde~ pressure versus lateral for remoulded Dhaka clay.

4

o o

Fig.

&0

'Fig. 5.17(b) Overburden pressure.versus coefficient ko~

o o

0 3 !

::.::

,

. .z

•...

ILl

u u..u.. 2

W 0u

1

¹

WILl

.

>0::

;::::>

UVl

ILl Vl 20

u..1LI

u..0::

1LI11.

k determined ko determined ko determined ko determined e~p erimentall y from empirical from the curve from the Curve

relation of Massarch of Flavigny

.95-5in _<P (1 979f (1980)

Brooker and Irdend (1 965)

. .

1 ₂ 3 4

0.46 0.53 0.54 0.51

,

Table 5.8 Values of k for normally loaded Dhaka clay determined

o .

by various method.

•

. I!~:;."

"',.- -'"

20

Relationship betwe~D overcons9lidatioD .ratio andratiG k.loc) and.kolnc)

for Dhaka clay. • .

• OVERCONSOLIDATION RATIO, n 80

30

50 ko(nc) = Coefficient .f earth pressure at rest for normally loaded condition

AD k (oc)

=

Coefficient of earth pressure o at rest for prel@aded condition

20

Fig. 5.18

2

100

11 1.5

0

«

I-

cr 10

u u 9

0 c

o. 0

8

7

~ ~

5

,

I

5 4 3

The value of a determined from the Fig. 5.18 i~ 0~755. The value recommend by Alpan (195.7; was .46 ::.•06, and calculated on the basis of equation recommended by Flavigny 41~8o)is .55. That is the experimentally determined value of a for Dhaka clay is greater than that recommended by Alpan (1967) and Flavigny (1980).

CONCLUSION AND RECOMMENDATION FOR FUTURE