effect of the initial state of skeletal-clay soil on its deformability and strength

EFFECT OF THE INITIAL STATE OF SKELETAL-CLAY

SOIL ON ITS DEFORMABILITY AND STRENGTH

G. M. Lomize and V. G. Fedorov UDC 624.131.22:624.131.54

Experimental investigations show the effect of the i u ida l state of a rd~e i a l l y compacted c lay soil on the operation of a core or other watertight element in a high-head earth dam. The problem of designing the init ial state is considered using as the example the experience of c o n s ~ c t i n g the Nurek dam core and is solved on the basis of a comprehensive study of the mechanical properties of the soil for the case of a three-dimensionalstressed state of t r iaxial compression. As a result of layer -by- layer mechanical compaction of the soil i t is necessary to obtain the required ar t i f ic ial structure and state of the soil for the design grain-size distribution. Its mechanical and seepage properties, which determine the regularities of development of the stress-strain state in the core during construcrion, introduction into operation, and subsequent long-term operation, depend to a considerable extent on this structure. A special feature of this art icle is the comprehensive study of the mechanical properties of clay at different stages of load resistance in a state of tr iaxial compression. The results of long-term investigations of soils in the prel imit ing stage of loading and during flow, carried out at the V. V. Kuibyshev Moscow institute ofC ivi l Engineering, are used. In this lies the theoretical interest of the presented solution of the applied problem of constructing impervious elements in high-head earth dams.

The general applied scheme of selecting the initial state to be used when constructing high-head dams con-

sisrs in the following. The consmaction material is selected on the basis of information on local sources, the gen-

eral characteristics of its composition and properties are established, and then the consistency when placing and

the degree of compaction of a clay soil with design granulometric and mineralogic compositions, determined by

the assigned work, are selected. This task is carried out with consideration of the requirements imposed on the

mechanical and seepage properties of the soils in a structure of given design being constructed in relation to the

natural conditions. This problem does not have a universal solution and the existing recommendations of the

standards do not adequately meet the requirements of modem dam construction.

The Safedobsk soil in the core of the Nurek dam is typical skeletal-clay soil, consisting of fine earth (diam-

eter of the fractions less than 2-5 rnm) and a coarse-granular component (diameter greater than 2-5 ram). For

such soils it is necessary to design the composition and properties such that the structure of the compacted soil

obtained precludes the possibility of piping. For this purpose it is necesssary to keep a proportion between the fine

earth and coarse-granular component such that r.he formation of a rigid skeleton is impossible wir2~ the compaction

technology used. In this case the coarse-granular f~actions of the soil will be separated by fine earth.

To meet the indicated requirements the mechanical and seepage properties of the compacted soil, deter-

mining the strength of the structure, will depend mainly on the fine earth, composed of clay [I]. The content of

fine earth in the Safedobvsk soil amounts to 50-80~ In physical properties and granulometric composition it be-

longs to sandy loam, close to the lower limit of the granulometric composition allowed for placement in the core

of the Nurek dam. The content of silty-clay particles (less than 0.05 ram) in the fine earth is 40.6%, uniformity

coefficient U = 80 (Table i).

Three initial states, determined by the density, water content, and specific work expended on compaction,

were investigated experimentally in the study of the mechanical properties (T able I). The first initial state of the

soil (I) with water content W = 9.5% and density 7 = 2.14 g/cm 3 is the design state for the core, which was selected on the basis of the latest edition of the method * of selecting the parameters of the initial state of skeletal-

�9 Developed in the research department of the All-Union Plannir~g. Surveying and Research Institute (Gidroproek0

on the basis of the experience in constructing the Nurek dam.

Translated from Gidretekhnicheskoe Stroitel 'stvo, No. 12, pp. 19-2% December, 1975.

1153

1154

TABLE I

G. M. LOMIZE AND V. G. FEDOROV

Initial Bulk spe- soil eific state gravity,

g / c m ~

1 I1

1II 2,74

Density, Natural water

g/ern3 I content,

I 9o

2,14 2,04 2,04

9.5 12,0 9,5

Void ratio

O. 280 O. 343 O. 343

Liquid limit of tin,earth.

15,5

Plastic 11mitof t in,earth.

11,5

Plasticity nUl~o~er,

4.0

Coeff.of s a t u r - a t i o n

0.93 O, 96 0,70

Spec. work Rel. of compo-

consistency sition, g- / c m 3

--0,500 28 0C0 0,125 28 000

--0.500 10 000

Grain-size distribution (%) of fractions, mm

5.0-3.0 3.0-1,0 I 1.0-o.n 0 .5--0 .25 o.z~-ojo 0,,0-0.05 [ ~ 0~-o l 0.0,-0.00~ I <o.oos

4.9 16. I 4,0 6.75 10.5 17.5 10,4 !1,8 20,4

TABLE 2

�9 "~ .5 =

o ~q

II

1II

Instmrnent

T riaxial apparatus

Stand No. 1 with independent control of trtree principal Stresses

Stand No. 2 with independent control of three pnncipal stresses or strains

Triaxial apparatus

T riaxial apparatus

Test conditiom Os-s, kg /

c ln z

For assigned stTesses

For assigned s t r e s ses

For a.ssigned s t r a l u s

For assigned sl~esses

For assigned s t r e s ses

0; 5; 10; 15; 20; 25

5and 10

20

5and 10

5 and 10

a a

--I ,5; --0,7; --0.286; 0; 0,333; 0.5

--0,286; 0; 0,333

--0.286; 0; 0,333

--0,286; 0; 0,333

a0 K e = Ae i

--0,59; --0,33; --0,12; 0

I%

--1

- - ~ I 0;

--1

txe

clay soil. According to this method during construction of the dam and subsequent saturation with water upon filling the reservoir the compacted soil should not pass into a plastic state and the pore-water pressure from the load should be as small as possible. Simultaneously it is required that compaction does not produce a cohesion- less structural composition as a consequence of tmdercompaction of the soil. A solid consistency was taken for the fine earth. To improve the mechanical properties of the soil both during construction and operation of the structure, the fine earth was compacted in the laboratory by a modernized AASHO (American Association of State Highway Officials) method, which is equivalent to compacting the soil under field conditions by heavy machines [2]. For a given compaction and consistency we determined the corresponding optimal water content and maximum density. The second initial soil state (IT) was taken with a density T = 2.04 g/ca'a s and water content W = 12%, close to the plastic limit. Such an initial state corresponds to that adopted and achieved in the first stage of constructing the core. The third initial state has the same water content as the first, but the density is 7 = 2.04 g /e ra s, i.e.. the soft is undereompaeted in comparison with the first state. Undereompaction corresponds to a coefficient of relative compaction determined by the equation

EFFECT OF THE INITIAL STATE OF SKELETAL-CLAY SOIL i155

8

fO

%

a K#0,~3!~ , ~ . . ~ . . -6 . . . ._ ._A_ _ _ ~ [] K~=o ~Os. s = ' ~

�9 ,

8 I " I I I ~ ,

Fig. I . Relation O = 0(o) between volumetric change and total uniform compression for Os- s equal to 5 and 10 kg /cm z in different loading trajectories for the investigated init ial states (/~o = - t , n'iaxial apparatus).

k~

32

28

2~

ZO

~6

12

8

r

O

cl-n 2

F i 2 ~ 6 8 I0 CZ r 16 18 20 22 %

Fig. 2. Relation ei = ei(o i) between intensity of strain and shear for trajectories w~th Ko =0.888 foros-s = 6 and 10 kg /cm z and with K c =-0 .286 for Os- s = 10 kg / c m z for the investigated init ial states (go = - 1 , triaxial apparatus). The encircled symbols correspond to the start of flow of clay soils (the legend is given in Fig. 1).

D = y d / y dmax =0.95, (1)

where 7 d is the dry density in the case of undercompaction; 7 d max is the dry density corresponding to the optimal compaction for a given water content W = 9.5%.

The experiments for studying the mechanisms of stress-strain state development in the prelimiting region, in the state of I lmit (plastic) equilibrium, and subsequent flow were carried out in a triaxial apparatus (cylindrical specimen, d = 60 mrn and h = 127 mm), when o1>_ o z = o 3 for a value of the Lode parameter /~o = - 1 , and on stand No. 1 (hollow cylindrical specimen, o.d. = 60 ram, i .d. = 36 mm and h = 80 mm). The experiments made it possible to investigate three-dimenstonaI stress for different vaIues of ~o [3, 9]. The experiments were performed under assigned stress conditions. In the first stage of all investigated trajectories of stress development the specimen was subjected to isotropic compression to the assigned value Os-s.

The variable stage of loading was accomplished for a constant value in each experiment of the parameters of the trajectories Ko = C a / o i and go . Thus the trajectory was determined by three parameters Os-s, Ko, and

/Jo, constant in each experiment.

1156 G. M. LOMIZE AND V. G. FEDOROV

kgtem z

~oet

/< = 0,33~/~'~- $2

28 / ~ /(=O : / / ~

eo /<= - o~ 7

16 - i, 5 �9 ~-(a--O,7 Cltr a =- 0,286

i2 m Ka = 0

A Ka= 0,33 8 �9 K~= 0,5

@for ~=o

4

t ' ; 6 ~

. 0 ~. 8 12 16 20 2* 28 J2 36 *0 "-~ kg/crri z

Fig. 3. Relation r o c t = l"oct(Ooct) in different loading trajectories for the in - vestigated ini t ia l states (/1 o = - 1 , triaxial apparatus). The heavy solid lines correspond to the state of l imi t equilibrium for the first in i t ia l state (W = 9.5~ ~t = 2.14 g / c r n s) of Safedobsk sandy loam; the heavy dashed line, for the second state (W = 12%, 7 = 2.04 g/cmS); the thin solid line, for the third state (W = 9.5%. T = 2.04 g/cmS). The dot-dashed lines 1, 2, and 3 correspond to the l imi t of transition from compaction to dilatancy disintegration (for X =0) for the corresponding ini t ial soft states.

kg I cm ~-

~0 ~'oct /L~=-~ 6 =0

. r me=-o,286] / ~ . . . . . I - > - " J / . , ~ Fe =-L..'-(/-~'~-~

e / I " ~ T , IoKa=~ |

0 2 , 6 8 10 12 r kg/crn z

Fig. 4. Relation roc t = ~'oct(Ooct) for different values of the parameter of the stressed state (Po = - 1 , 0, and +1) for the design ini t ial state of Safedobsk sandy loam (stand No. 1 with independent control of the three principal stresses). The solid lines correspond to the state of limit equilibrium; the dot-dashed lines correspond to the limit of

transition from compaction to dilatancy disintegration (for k = 0).

Special attention was devoted to an investigation of dilatancy as the main factor determining the combined

effect of invariants o and o i on volumetric change [4]. For this purpose the dilatant component of volumetric

change was isolated from the total by the eq'zation

EFFECT OF THE INITIAL STATE OF SKELETAL-CLAY SO~.

TABLE 3

--0,091 --0,159 --0,254 [ 1.52 [ 3,90

1157

0,286

O O

O 0,

- ! -1

+~

Fig. 5. Projection of characteristic lines of the stress-strain state onto the deviator plane for Os-s = 10 k g / e m 2. Solid lines: state of l imi t equilibrium for K o = 0.333. 0, and --0.286; dashed lines: for ei = const with K o = 0; dot-dashed lines: for>. = 0. Other designations according to Fig. 4.

0=0is(o) +0a(Cr, cr~), (2)

where 8is(O) is the part of the total volumetric change corresponding to isotropic compression; e a(O, a i ) is the additional change developing as a consequence of the combined effect of o and o i . The extent of the experiments performed and the ranges of change of the indicated parameters of the trajectories are presented in Table 2.

The plastic volumetric change for the investigated ini t ia l states corresponding to the effect of isotropic compression alone is given in Fig. 1 by lines 1, 2, and 3. which are satisfactorily approximated by the function

6 i ~ m~L (3)

the form of which, as follows from experimental investigations [4], is characteristic for the given type of clay soilsin the impervious elements o f thedam and also for granular softs. The coefficients m and r are functions of soil porosity and water content. All following figures present graphs covering the prel imit ing state, state of l imi t equilibrium, and subsequent flow.

Distortion and volumetric change for the deviatoric stage of the loading trajectories are illustrated by the graphs in Figs. 1 and 2 in the form of the relations

O = O(~, ~, ~.) (4)


o

-0~/8

-o,2o

-O, JC

-0,~0

-0,50

-0,50

-0,70

4 8 12 16 20

o's-s;10 kg/emi

_ ~ T / / I / i ~ O : . s - - 2 0 kg/crn'

iii: '//

2~ 28 32 36 40 ~.z,

m 2

�9 ua=0,5

A K6 =0,33

,, K~=o t [] Ka----o, zo6

�9 K6=-0~7 I

o K~'I=-/~ 5

. . . . . . p

[ I

kg/cm 2 61in

Fig. 6. Relation k l im = Xlim(olim) between the index of development of d i la taney and the spherical tensor under conditions of l imi t equilibrium for the design in i t ia l state (/so = - 1 , t r iaxial apparatus).

and

et = e~(~, *t, ~ ) (5)

on the base invariants o and o i for a constant value of the form of three-dimensional stress ~o equal t o - 1.

Figure 3 shows the cross section of the l imit ing region of deformation in the space of principal stresses ol , o z, and o 3. The condition of l im i t equilibrium is obtained in the form of the following family of straight lines

~lim = n ~ l i m + b. (6)

For the desig n in i t ia l state of soil the coefficients n and b, depending on the parameter of the trajectories Ko, are approximated by the expressions:

(7)

b = SiK~ + Sa, (8)

where B I, B z, B s, S1, and S z are experimental coefficients whose values are given in Table 3. This same graph shows straight lines 1, 2, and 3 corresponding to the adopted in i t ia l states of the soil, which are of considerable interest for further analysiSo Each Line is the relation between o i and o in the ease of a change of sign of dilataney, i .e. , when the differential index of di la taney k = 0 for any investigated trajectory. This straight l ine can be written in the form of the equation

~0~ =lcr0, (9)

the coefficient of which l = tan cx corresponds to the value k = 0 (the point of maximttm compaction).

Equations (6) and (9), obtained in the 1972 investigations [3, 5], are of considerably interest for analyzing the regttlarities of deformabi l i ty . These lines divide the entire region of the l imit ing state of the soil into compaction and expansion zones, the la t ter being clnser to the state of l imi t equil ibrium.

Figures 4 and fi character ize the spatial changes of the l imit ing values of the stress intensities. The longi- tudinal sections, obtained for values o f / s o equal t o - l , 0, and +1, are combined in the plane of Fig . 4. F igures gives the condirional picture of the l imi t surface obtained for different values o f / so but constant in each experiment . The same figures show lines corresponding to the intermediate l imit ing state for which k = 0.

EFFECT OF THE INITIAL STATE OF SKELETAL-CLAY SOn. 1159

- ! 0 ! 2 3 ~ 5 %

Fig. 7. Relations e i = ei(O) between distortion and total volumetric change in different loading trajectories for the design ini t ia l state with os- s = 5, 15, and 25 kg/cmZ(#o = - 1 , triaxial apparatus). Solid lines 1, 2, and 3: state of l imit equilibrium for corresponding values K o = 0.33, 0, and - 0 . q ; dot- dashed lines 1', 2', and 3': respectively for the same trajectories for X = 0. The legend of the symbois is given in Fig. 3.

We note that the experimental points in the investigated trajectories with #o = const in each experiment of o~ lira for trajectories with ~o = - 1 and minimum for /~o = 1. All other give m axLrnum values loading trajec-

tories determined by a different possible law of change of the intermediate principal stress o z will have o~ m be-

tween the indicated limits. Thus the results of investigating the influence of the form of stressed state /~o pre-

sented in Figs. 4-5 broaden the generalization of the condition of limit equilibrium to the general case of three-

dimensional stress when the loading trajectories develop not only in one plane when ~z o =-I, which is charac-

teristic in the performance of laboratory experiments, but also in planes determined by other values of #o in the

range from - 1 to 1.

The experimental data obtained on the state of l imit equilibrium permitted establishing graphic relarions, presented in Fig. 6. The approximation of the experimental values

l im dSa

l l i m = d-~Hm

for different loading trajectories has the form of the equality

A (10)

where A = a + ~ o s . s.

The from Eq. (10) follows the equation


k g / c m 2 % e~

18

16

i z B

i

_ Ostrair - ! 0 i 2 3 ~" 5 % -1 0 I 2 o r .4 5 %

Fig. 8 Fig. 9

Fig. 8. Relation 8 = 8(oi) between the total volumetric change and intensity of stresses in different loading tra-

jectories for the design initial stale for Os- s = 5, 15, and 25 kg/cm 2 (~o =-I, triaxial apparatus). See Fig. 7 for legend.

Fig. 9. Relation 0 a = 0 a(ei) between the dilatant component of volumetric change and intensity of strain in dif- ferem loading trajectories for the design initial state for Os- s = 5, 15, and 25 kg/crnZ(/~o = - 1 , triaxial apparatus). See Fig. 7 for legend.

( ) ~ - g - + s (11) ] ) lira] = = t im '

or

I 2,1imi

(' T -~ ~S- e(~eq q- ~ s-s) (12)

aS-s~- K=~ lira -- ~s ' sq- K=%lim

where a and c~ are empirical parameters depending on the initial state; Oeq is the equivalent average stress corresponding to the initial compaction of the soil; Os_ s and Ko are parameters of the loading trajectory. Equations (11) and (12) establish the relation between the index of dilatancy I klim I and the limiting stress state characterized by o lim or o ~ n for different loading trajectories and go = - l -

Using gcts. (11) and (12), we determine the limit value of o~trn:

~ ilim.=_ ~ q- ~s-s (= - - I ~ l iml ) IMiml K= (13)

Deserving of considerable attention for an analysis of the mechanimas of development of the stress-strain state in the limiting region in connection with dilatancy are the generalizing graphs in Figs. 7 and 8, which show, respectively, the relations of the total volumetric change 0 to the intensity of strain ei and stresses o i for different loading trajectories, determined by the parameters Os- s and Ko, for the design initial state.

EFFECT OF THE INITIAL STATE OF SKELETAL-CLAY SOIL

TABLE 4

1,55.10-2 I 1-10-~ ] --0,78 3.2.10-~ ] 0,62 I --0,345

1161

kg / cm z 6t

35

32

24

20

~6

'2

8

I

I l

I . 4 c9

'/ i i

~% He = -0~59 o Ke=-O,33- [] K e =-O, ig.

~O KeFO

12 16 20 2~

Fig. I0. Characteristic development of

stresses o and o i for different trajectories

in prelimiting and limiting stressed states

under conditions of assigned strains (el =

const, O = const) and under conditions of

stresses for the design initial state of Safe-

dobsk sandy loam (stand No. 2 with independent control of the three principal stresses

or strains). The blackened experimental

points pertain to the stress conditions.

same show curves I, 2, and 3 of the rela- The graphslim lim tions e l~(o lim) and O (o i ) corresponding to the state

I of limit equilibrium and curves I', 2', and 3' of the relations

e~ ~ and O~176 corresponding to the value of the index

of dilatancy k = 0, by means of which the entire region of limiting deformation is divided into a zone of compaction A

enclosed between the coordinate axis 8 and the curve (for

I = 0) and the remaining zone of expansion B.

As a result of processing the experimental data we also

obtained graphs showing the development of dilatancy during

shear. As an illustration we present a graph (Fig. 9) of the re-

larion of the dilatant component of volumetric changeO a to

distortion e i in the case of different loading trajectories for

the design initial state of the soil. The same graph shows the the relation of the limit values of OaKm as a function of e~ m

and curves I, 2, and 3 corresponding to the limit of transi-

tion from compaction to dilatancy disintegration (for k = 0).

An analogous graph was obtained in coordinates Oa--O i. As

we see from the graphs of Figs. 7-9, the size of the zones of

compaction A and expansion B in the limiting region of def-

ormation changes considerably depends on the trajectories,

particularly on transition to the trajectories with negative

values of the parameter K•

For an analysis of the state of limit equilibrium, of

considerable interest are the limit curves i, 2, and 3 of the

relation between the two base invariants ehl m and O lira (Fig.

7). For this purpose we found the expressions e. lira and olim

as a function of k lira and the most characteristic index of

dilatancy on transition from a limit equilibrium state to flow:

lira e i = (b~ + b~%. s) I lliml h3, (14)

c~ (15) Blim = C~s_s] i l imtC';

here 51, b2, b3, CI, Cz, and C 3 are experimental coefficients whose values are given in Table 4.

From Eqs. (14) and (15) by eliminating I kliml , we obtain the analytic expression of curves i, 2, and 3,

representing the strength condition during deformations, namely:

lim b3 lim / 8 \~

(~6)

Using Eqs. (12) and (15), we find the equation describing the limit curves in coordinates olim-oilim (Fig.8),


TABLE 5

% ei.

r

I

I

i I

"~" 10 \

-4 -J -3 -I 0 ! 2 3 ~ %

Fig. 11. Relation 0 = O(ei) between total volume change and distortion in the investigated trajectories and loading conditions for the design initial state (independent control of the three principal stresses or strains). Solid lines: condition of assigned strains (ei = consr, O = ccnst); dashed line: stress condition. The other symbols are according m Fig. 10.

m

r

a

Buml

Safedobsk sandy loam

W=9,5%, W:=19% r--2,14g/cm: r=2,04g'/crn: W=9,5%, T =2,o4g/era ~

Charvak loam

II ~ II -

Inhomogcneous coarse- grained sand

average deme den~ty e=0,533 e-=0,5~

0,0115 0,60

O, 82 0,13 6,3

O, 08--0, 30

0,017 0,65

0,15 0,045 3.34

0,03--0,15

0,017 0,68

0,14 0,046 3,05

0,03--0, 13

0,016 0,58

0,23 0,061 3,8

)--0,2(

0,017 0,67

0,0148 0,34

1,12 0,54 2,1

0,22--0,74

o,o156 0.40

0,12 0.24 0,5

O, 13--0,34

Note

Eq. (3)

Eq. (i2)

Range of change of [ Xliml is shown for Sgfedobsk sandy loam for Os=s = 10 [kg/ern ~ and for Charvak loam and inhomogeneo~ sand for Os= s = 9 k g / e m

01.ira = C,osC_'sl ~liml c ' =

a Ca

+ o,-,)1 C a CS/ . ; I

= ' os- + Ko* LL J "

(rT)

EFFECT OF THE INITIAL STATE OF SKELETAL-CLAY SOIL 1163

The information obtained on the state of l imit equilibrium broadens the possibilities both of a theoretical examination of the regularities of the stress-strain state and the solution of certain applied problems, particularly the main theme of the a r t i c l e - the effect of the initial state of compacted soil on deformability and strength.

After reaching the state of l imi t equilibrium, characterized by the investigated E~ . (6), (12), (16), and (lq), the further development of soil deformations obeys mechanisms of flow when the relation between stresses and strains in an invatiant form is determined in the form of the relarions between the corresponding invariants of stress and strain. With approach to the state of l imit equilibrium the index of development of dilatancy X approaches the l imi t value Xli m, which remains a constant during subsequent flow and is determined by the function:

~,lim= ~(~lim, lim K=). ~i , ~,,, ~s-s' (18)

Since Xli m = const for the state of flow in the investigated loading conditions, the relation between stress and distortion in an invariant form can be taken * in the form of the following equations:

et'= ~ } i m, (19)

~} = ~ l i m . (20)

The observed flow under conditions of loading by assigned stresses occurred with a progressive increase in the rates and strains di(t) and 0(t) and constant values of a lira, a ilia, po lira, kli m corresponding to the condition

of limit equilibrium. It was established from an analysis of the experimental data that the flow rates can be

approximated by the expressions

b,(t) = e'.z + . ~ t - - to) (21)

o(t) = 60 + L ?-limla(t - to). (22)

where ~ and ~0 are the rates of distortion and volumetric change of the first stage of flow with constant rates, and the second terms show the relation between the process of acceleration for the second uniformly accelerated stage to flow occurring with acceleration a.

In the unsteady stage of flow, where acceleration of deformation of form (distortion) and volume change are observed, the coefficients 11 and ~ will be functions of not only the stressed state and trajectories but also of the distortion rates. Using Eqs. (12) and (19)-(22), we obtain the equations

~(t) = z[boi + 7 ( t - - to)]

and

~(t) = N'0o + . f fh ( t - - to),

(23)

(24)

where

lira Z = I/~ i

1 1 N = ~ = %s+ ~ o}im;

Z lim ~ lira M =-- alim-- ~s_s_l.. K==}im"

*A satisfactory observance of similarity of the deviators of the stress and strain rate tensors in the investigated

loading trajectories was established experimentaily.


Thus the flow mechanisms are represented as functions of the distor~on rates, and the indicated parameters Z, N, and M determine the effect of the history of soil loading in the prel imit ing region on the flow meehanimas.

Let us consider the problem of the effect of the loading conditions. Earlier investigations [6, 7] showed the considerable effect of loading conditions on the mechanimas of deformabil i ty in the prelimiting region under con= ditions of l imi t equilibrium and dialing subsequent flow. A special cyc le of investigations was carried out to establish the effect of the given factor on the deformabil i ty of $afedobsk soil with different ini t ia l states. I t was assumed hypothet ical ly that the main effect on the test conditions is the t ime factor, i .e . , the development of the stressed state as a function of t ime . Four experiments were conducted under conditions of assigned strains, when the mechanisms of the development of the stress-strain state result from applicat ion to the cubic specimen of the three assigned principal stresses e A, e e, and for thedesignini t ia l state(W = 9.5%, 7 = 2.14g/cm3) at a value of pre= l iminary isotropic compression a s - s = 20 kg / c m z with parameters of the deviator stage of the trajectory K e ectual t o - 0 . 5 9 , - 0 . 3 3 , - 0 . 1 2 . and 0. The development of the stressed state with t ime in various regions of deformation was observed earef ta ly during the experiment. Then experiments under conditions of assigned stresses were carried out for the same in i t ia l state and with the former level of isotropic compression Os= s. In these experiments we complete ly reproduced the entire process of development of the stressed state established by observa- tions under conditions of assigned strains, i .e . , the stress invariants o and a i were changed with respect to the t ra- jectories obtained under conditions of strains according to Fig. 10, and the rates of change of these invariants were assumed equal to the measured rates of development of s~esses under the first investigated conditions.

According to the graph in Fig . t l , the experimental investigations showed that with such conduclion of experiments under stress conditions the developing strains in the prel imit ing region, characterized by the relat ion- ships between e i and O exact ly dupl icate the graph of the strains of the condi6ons of assigned strains. The diver- gences of the curves for the two investigated conditions in the flow section are explained by the fact that at the given stage of deformation the strain rates developed differently at constant stresses owing to the different method of applying the boundary conditions to the specimen. Thus it was established that the t ime factor is responsible for the different conditions. Therefore, the same trajectory but %vith different functional relations to t ime leads to different results. The effect of the drne factor investigated above consists in a substantial influence of therates of development of the stressed state on the noted mechanisms. Long-term investigations carried out in the laboratory of construction properties of soils showed the considerable effect of the rates of development of the stress- strain state on deformabil i ty and strength of soil [8, 9].

We wilt apply the results of the investigation of the mechanica l properties to an evaluation of the effect of the ini t ial state of e iay soils ar t i f ic ia l ly compacted during placement on the strength and deformabil i ty of the core of high-head dams. For disturbed c lay the shear resistance is determined by the internal friction, cohesion, volume change during isotropie compression, by di lataney (of compaction or expansion) in shear and the loading trajectory. Al l investigated loading trajectories consist of the following sections: a section of isotropie compression and a section of deviator loading. Only isotropic compression develops in the first section; in the second, in addirlon, d i la tancy and distortion develop. As follows from the experiments, ini t ia l rnechanieal compaetion of the soil has a quite considerable effect on a l l types of deformation.

An examination of the graphs in Fig. 1 shows that isotropic compression of Safedobsk soil (curves 1 and 3) referred to the same average stress o decreases by 2-9..5 times upon an increase in the density from 2.04 g / e r a 3 (third state of the soil) to 2.14 g / e r a 3 (first design state). Such a volumetric change corresponds to an increase of isotropic shrinkage 3=4 t imes. The same effect is illustrated by the changes of coefficients m and r in Eq. (3) (Table 5). For comparison T able 5 gives data for Charvak loam and two states of density of inhomogeneous coarse-grained sand [3, 5, 10].

Also considerable is the effect of in i t ia l compaction on deform abil i ty during the deviator stage of d evelop- ment of the t rajectory. For the same densities 2.04 and 2.14 g / e m 3 corresponding respectively to void ratios 0.343 and 0.280, the value of distortion e i in different states of prel imit ing loading differs as much as 3 t imes and el~ n as much as 1.6 t imes (Fig. 2). In this ease the di latancy due to shear increases the volumetric change sl ightly. The indicated effect is not diff icult to trace in the graph in Fig . 9, where famil ies of curves of the relat ion O a(ei) are shown for different levels of isotropic compression a s - s. For example, d i la taney of compaction with a change of Ko f r o m - 0.7 to 0.33 for a s - s = 25 k g / c m z increases by 1.2%, and di la tancy of expansion, being a consequence of in i t ia l compaction, changes, depending on the trajectory parameter Ko as much as 1%. The indicated values are less than values of volumetric change from isotropic compression, but they lead to a quali tat ive change in the

EFFECT OF THE INITIAL STATE OF SKELETAL-CLAY SOIL 1165

work of the soil in the entire zone of developing expansion. As a consequence of this, the resistance of the soil to the acting load increases substantially. Such an effect, depending on the change of the trajectory parameter Ko from - 0 . 7 to 0.33 in the range Os. s = 5-25 k g / c m z, increases oilim to 20 k g / e m z.

In coordinates o i and o for #o = - 1 the indicated region of expansion for different values of Ko is included between the straight lines corresponding to the lower l imi t of expansion and the lines for o . Equations (6) and (9) give the analytic expression of this region. A slightiy different expression of the expansion zone is given by Eq. (12), which shows a considerable change of Xl.tm, a, c~, andoeq (Table 5) depending on the ini t ial compaction~ As a consequence of this the l imi t values of ol~ m and o lira increase. An examination of the noted effect of the ini t ia l state on the compaction and expansion zones shows a considerable influence of the in i t ia l state of the ~ i l both on deformabil i ty and on strength.

For the construction of the Nurek dam a comprehensive study of the mechanical properties of Safedobsk soil in the investigated class of loading trajectories object ively showed the advantages of the decision made by Gid- roproekt concerning the selection of the iui t ial state related with transition to a higher density with a solid consistency of the fine earth. The relationships presented here between the stresses and strains in the prelimiting region, state of l imi t equilibrium, and during subsequent flow with consideration of the effect of the ini t ial state and loading trajectory are recommended by the authors for use in designing high-head earth dams and for further investigations related with their construction.

LITERATURE CITED

I. S.V. Bortkevich, "Main requirements imposed on the quality of skeletal-clay cores of rock-fill dams, = Gidrotekh. Stroitel'., No. 8 (1973).

2. B.K. Howe, Principles of Engineering Soil Science [Russian translation], N. N. Maslov (editor), Stroiizdat,

Moscow (1966). 3. G . M . Lomize and E. I . Sukhanov, "Limit stress and failure of clays," Gidrotekh. Stroi l te l ' . , No. 8 (1973). 4. G . M . Lomize, "Problems of deformabili ty and strength of s o i l . " in: Problems of Strength and Deformabil-

i ty of Soils [in Russian], Azerneshr, Bairn (1966). 5. G . M . Lomize, "Strength and deformabili ty of soils in the cores of high-head dams and foundations of hy-

drauiic structures," Gidrotekh. Stroitel ' . , No. 8 (1973). 6. G . M . Lomize and E. I . Sukhanov, "Laws governing the flow of soils that have failed under loading," Gidro-

tekl~ S t ro i t e l ' , No. 6 (1974). 7. G . M . Lomize and V. G. Stolyarov, "Mechanisms of deformabili ty and strength of clay cores of high-head

dams," Gidrotekh. StroiteI ' . , No. 11 (1974). 8. G.M. Lomize, A. A. Muzafamv, and V. Yu. Novikov, "Limiting stress of clay soils in cores of high-head

dams and foundations of hydraulic structures," Gidrotekh. Stroitel'. No. 4 (1975). 9. G.M. Lomize, I. N. Ivashchenko, M. N. Zakharov, and A. A. Isakhanov, "Deformability, strength, and

creep of clayey-soil core walls of high dams," Gidrotekh. Stroitel'., No. II (1970). I0. G.M. Lomize, E. I. Sukhanov, and V. G. Fedorov, "D eformability and strength of sand under various load-

ing trajectories and conditions," in: Investigations of Soil Mechanics, Bases, and Foundations [in Russian],

Kalmyskoe Gos. Izd. Elista (1974).

effect of the initial state of skeletal-clay soil on its deformability and strength

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