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Effects of soil disturbance in pressuremeter tests
Law, K. T.; Eden, W. J.
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recherches en Mtiment
Effects of Soil Disturbance in
Pressuremeter Tests
by
K.T.Law and W.J. Eden
Appeared in
Proceedings of "Updating subsurface samplings of soils
and rocks and their in-situ tesing"
Santa Barbara, CA, January 3-8, 1982
p. 291 -303
(DBR Paper No. 1330)
Reprinted with permission
Price $3.00
ABSTRACT An experitnencdl s t u d y of t h e e f f e c t s of v a r i o u s components of s o i l d i s t u r b a n c e . The t e s t s were c a r r i e d o u t u s i n g t h e Cambridge s e l f b o r i n g p r e s s u r e m e t e r a t a s i t e i n Ottawa, O n t a r i o . The r e s u l t s a r e compared w i t h o t h e r s r e c o r d e d i n t h e l i t e r a t u r e and some g u i d e l i n e s a r e s u g g e s t e d . i t u d e e x p e r i m e n t a l e d e s e f f e t s de p e r t u r b a t i o n du s o l p a r d i v e r s composants. Les e s s a i s o n t s t 6 r e a l i s e s a v e c un a p p a r e i l de mesure d e s p r e s s i o n s Cambridge a u t o f o r e u r s u r un c h a n t i e r s i p
-
~ ~ - -'
- I - - -x-=lfats s ~ n t _ c o m p a r 6 s 1 ceux d ' . ' Q ~ S s o n t f o r mEFFECTS OF SOIL DISTURBANCE I N PRESSUREMETER TESTS by
K.T. Law* and W.J. Eden* INTRODUCTION
Successful design and c o n s t r u c t i o n of foundations r e q u i r e a thorough understanding o f t h e engineering behaviour o f s o i l as i t e x i s t s i n t h e f i e l d . I n s i t u t e s t s t r e a t t h e s o i l as i s and thus remove t h e
u n c e r t a i n t y t h a t a r i s e s from sampling and approximating f i e l d
c o n d i t i o n s d u t - ~ n g la b o r a t o r y t e s t s . U f t h e many i n s i t u t e s t devices, the pressuremeter probably has t h e best p o t e n t i a l f o r studying t h e important aspects of s o i l behaviour since t h e t o t a l i n s i t u h o r i z o n t a l s t r e s s as w e l l as t h e s t r e s s - s t r a i n r e l a t i o n s h i p can be obtained. L i k e o t h e r t e s t equipment, however, t h e pressuremeter produces r e s u l t s t h a t are influenced by f a c t o r s such as disturbance, s t r e s s path, a n i s o t r o p y and r a t e e f f e c t . T h i s paper deals w i t h t i l e disturbance aspect.
Tiiere a r e two components o f disturbance. The f i r s t causes a change
i n
t i l e i n s i t u stress, hence modifying t h e s t a r t i n g s t r e s s reference. This s t r e s s m o d i f i c a t i o n may change t h e s o i l behaviour and as a r e s u l tt r u e behaviour i s n o t measurable. The second component generates a
mechanical s o f t e n i n g o f an annular zone o f s o i l around t h e pressure- meter probe. This may a l s o l e a d t o r e s u l t s t h a t do n o t r e f l e c t t h e
t r u e behaviour. Considering undrained strength, t h e o r e t i c a l work
(Baguelin e t a l . 1975 and Prevost 1979) shows t h a t both disturbance
components l e a d t o an overestimation o f t h e undrained strength. T h i s
overestimation derived from disturbance i s i n c o n t r a s t t o o t h e r t e s t
experience and leads t o unsafe design i f t h e r e s u l t s a r e taken a t fact?
value.
An o r d i n a r y pressuremeter, such as t h e Clenard type, r e q u i r e s a
p r e d r i l l e d borehole and disturbance i s t h e r e f o r e unavoidable. The s e l f
b o r i n g pressuremeter, considered s u p e r i o r t o t h e o r d i n a r y type, does n o t completely e l i m i n a t e disturbance e i t h e r . As t h e r e i s a growing use of pressuremeters t h e need t o study disturbance e f f e c t s i s q u i t e evident.
An experimental study o f t h e e f f e c t s o f botll disturbance components i s r e p o r t e d i n t h i s paper. The t e s t s \rere c a r r i e d o u t using t h e Cambridge s e l f boring pressureneter a t a s i t e i n Ottawa, Ontario. The r e s u l t s a r e compared \ i i i h o t h e r s recorded i n t h e 1 it e r a t u r e anci based on t h i s some g u i d e l i n e s a r e suggested.
*Research O f f i c e r s , Geotechnical Section, D i v i s i o n o f B u i l d i n g Research, National Research Council of Canada, Ottawa, Canada KIA OR6
292 UPDATING SUBSURFACE SAMPLINGS SOIL CONDITIONS
The s i t e s t u d i e d i s l o c a t e d i n an open f i e l d w i t h i n t h e grounds of t h e
d a t i o n a l Research Council of Canada (NRCC). The s u b s o i l
profile
c o n s i s t s o f Champlain Sea c l a y and i s shown i n F i g u r e 1 . The surface f i s s u r e d c r u s t i s about
7
m
t h i c k . Underlying i t i s a l a y e r of medium grey c l a y o f h i g h s e n s i t i v i t y . It i s overconsolidated w i t h an over- consul i d a t i o n r a t i o of about 2.2. A l l t h e pressuremeter t e s t s r e p o r t e d here were conducted i n t h i s l a y e r .WATER C O N T E N T , ?+ I > P R E S S U R E . k p a
F i g u r e 1 Geotechnical P r o f i l e a t ;4ational Research Council of Canada, Ottawa. Canada
K. T.
Law andW.
J . Eden TEST DESCRIPTIONEquipment
The Cambridge s e l f b o r i n g pressuremeter was used i n t h i s study. I t s o p e r a t i n g p r i n c i p l e was described i n d e t a i l by Wroth and Hughes (1973, 1974). Some m o d i f i c a t i o n s were made so t h a t t h e equipment could be adopted t o a hydraul ic-powered, truck-mounted d r i l l r i g . The d e t a i l s were given by Eden and Law (1980).
The pressuremeter probe was 80 mm i n diameter and 0.90 m long. It
has a r o t a t i n g c u t t i n g blade i n s i d e the c u t t i n g shoe. Above t h e shoe i s t h e membrane p o r t i o n , which i s about 0.61 m long. The o r i g i n a l c u t t i n g shoe was replaced by a removable hardened t o o l s t e e l shoe t h e same s i z e as t h e membrane section. A Kaye System 800 data logger was used t o record t e s t data and a Texas-SR-52 programable c a l c u l a t o r was employed f o r p r e l i m i n a r y data a n a l y s i s . Figure 2 shows t h e setup i n a covered s e r v i c e t r u c k .
Tests w i t h d i f f e r e n t c u t t i n g shoe sizes
I n t h i s t e s t s e r i e s , t h r e e borehole sizes were studied. They a r e d e f i n e d by R, t h e r a t i o o f c u t t i n g shoe (hence borehole) diameter t o t h e membrane s e c t i o n diameter. The values o f R are 1.011, 1.00 and 0.995 and t h e c u t t i n g shoes used a r e c a l l e d oversized, matched and undersized r e s p e c t i v e l y .
The t e s t procedures g e n e r a l l y consisted o f t h e f o l l o w i n g :
1 ) measurement o f membrane r e s i s t a n c e before i n s e r t i n g the probe i n t o t h e ground; 2) s e l f boring t o depth; 3 ) w a i t i n g f o r t h e pore pressure
294 UPDATING SUBSURFACE SAMPLINGS
t o come t o e q u i l i b r i u m ; 4 ) i n f l a t i n g t h e membrane i n c r e m e n t a l l y a t a r a t e o f 9.8 kPa/min u n t i l the c i r c u m f e r e n t i a l s t r a i n reached 7 per cent; and 5 ) complete unloading.
Tests w i t h d i f f e r e n t degrees o f s o f t e n i n g
Softening o f t h e s o i l around t h e probe was introduced i n a t h i s t e s t s e r i e s by s u b j e c t i n g t h e s o i l t o a c e r t a i n s t r a i n b e f o r e t l l e t e s t . To do so a procedure o f loading, unloading and r e l o a d i n g was used. A f t e r t h e probe was s e l f bored t o t h e a p p r o p r i a t e depth and the excess pore water pressure dissipated, t h e membrane was i n f l a t e d t o 1 p e r Cent and then unloaded. A t t h i s p o i n t excess pore water pressure was generated and a f t e r w a i t i n g f o r i t s d i s s i p a t i o n , r e l o a d i n g t o 3 p e r c e n t was c a r r i e d out. T h i s process o f unloading, w a i t i n g and r e l o a d i n g was
repeated w i t h maximum s t r a i n reaching 7 and 9 per c e n t r e s p e c t i v e 1 y . For each l o c a t i o n , therefore, f o u r loading curves were obtained, each being s u b j e c t t o a d i f f e r e n t p r i o r s t r a i n .
Throughout t h i s t e s t s e r i e s , a matched c u t t i n g shoe was used. A constant s t r a i n r a t e of 0.16 per cent/min bras used i n t h e l o a d i n g stage and a constant l o a d decrement o f 50 kPa was a p p l i e d every h a l f minute i n t h e unloading stage. Three depths were studied, i .e., 8.78, 10.30 and 11.82 m.
TEST RESULTS
Several methods e x i s t f o r i n t e r p r e t i n g t l i e pressuremeter data. Some y i e l d t h e shear s t r e s s - s t r a i n r e l a t i o n s h i p from t l l e measured pressure- expansion curve (Baguel i n e t a l . 1972; Palmer 1972 and Ladanyi 1972). Others r e q u i r e an assumption of t h e s t r e s s - s t r a i n r e l a t i o n s h i p (Gibson and Anderson 1961 ; Prevost and Hoeg 1975 and Denby 1978). The f i r s t approach i s used here because o f i t s simp1 i c i t y and g e n e r a l i t y .
There a r e a l s o a number o f ways t o determine t h e i n s i t u h o r i z o n t a l s t r e s s aho. Wroth and Hughes (1974) suggested t h a t oh0 i s t h e pressure when t h e excess pore pressure s t a r t s t o develop d u r i n g t h e i n f l a t i n g stage. Denby (1978) used t h e pressure corresponding t o t h e e a r l y movement of t h e membrane w i t h c o r r e c t i o n by i n s e r t i n g an i n i t i a l s t r a i n i n t h e governing pressuremeter equation. Lacasse e t
a l .
(1981) took t h e pressure a t t h e s t a r t o f t h e l i n e a r v a r i a t i o n o f r a d i a l s t r a i n w i t h time. The oho r e p o r t e d i n t h i s paper corresponds t o t h e pressure r e q u i r e d t o j u s t l i f t t h e membrane o f f t h e probe. T h i s p o i n t can be i d e n t i f i e d e a s i l y by p l o t t i n g t h e pressure a g a i n s t t h e l o g a r i t h m o f t h e s t r a i n as shown i n F i g u r e 3. The pressuremeter i n t h i s case bras loweredSURROUNOING 7-O-O WAIER PRESSURE F i g u r e 3 Measuring a given t o t a l o 0 1 o I 1 0 l o s t r e s s u s i n g S T R A I N . L pressuremetei
K.
T.
Law andW.
I .
Eden
295to the bottom of an oversized predrilled borehole and filled with water.
The horizontal pressure was known and equal to the hydrostatic pressure
which corresponds to the sharp kink followed by a relative flat section.
When a test is performed in soil, the kink is less sharp but is still
readily identifiable.
An initial modulus is commonly reported from the pressurmeter
results. For some soils, the Champlain Sea clay in particular, the
initial pressure-expansion curve rises steeply, leading to considerable
scatter in the deduced stress-strain relationship. The initial modulus
reported here corresponds to the secant modulus at the point where the
relationship begins to be well defined.
Tests with different cutting shoe sizes
Different pressure-expansion curves (Figure 4) were obtained from tests
with different shoe sizes at the same depth. The derived engineering
quantities are sumnarized in Table I. For the oversized borehole
1 I I I
I
0 I I 1
0 1 2 1 4 5 6 7 8
S T R A I N , +
I
2% UPDATING SUBSURFACE SAMPLINGS( R
l.O),
a gap was created between t h e s o i l and t h e probe. T h i s caused a r e d u c t i o n o f h o r i z o n t a l s t r e s s i n t h e s o i l around the probe, hence t h i s t e s t r e g i s t e r e d a lower value than t h e matched shoe. For theundersized borehole (R < 1.0), t h e s o i l was pushed l a t e r a l l y by
the
membrane s e c t i o n l e a d i n g t o an inmediate r i s e of h o r i z o n t a l s t r e s s i n t h e s o i l . T h i s s t r e s s r i s e , however, v;as p a r t l y removed by t h e
stress
r e l a x a t i o n process i n t h e ensuing w a i t i n g p e r i o d .
The deduced shear s t r e s s - s t r a i n r e l a t i o n s h i p s a r e shown i n Figure 5.
A pronounced peak i s found i n t h e oversized case w i t h t h e undrained TABLE I
RESULTS OF PRESSUREilETER TESTS WITH DIFFEREiJT S I Z E RATIO OF CUTTING SI-IOE TO MEl1BRAIJE
Size r a t i o o f T o t a l
c u t t i n g shoe h o r i z o n t a l I n i t i a l shear Undrained Test Depth t o membrane, s t r e s s oilo modulus, G strength, Su
No. m R kPa MPa kPa
K. T. Law and W. J.
Eden
297I
shear s t r e n g t h exceeding t h a t of t h e s o i l because a s t r e s s r e l e a s e and deformation had taken p l a c e before t h e t e s t . T l i i s c o n d i t t o n has k e n shown i n theory (Prevost 1979, Law and Eden 1980) t o produce an a p p a ~ t I peak much higher than t h e r e a l s t r e n g t h o f t h e m a t e r i a l . Such b l n g the
case, t h e shear s t r e n g t h deduced from t h e pressuremeter t e s t i n an oversfzed borehole, p a r t i c u l a r l y i n one p r e d r i l
led,
w i l l lead t o unsafe design.Tests from an undersized borehole a1 so y i e l d a shear S t r e s s - s t r a i n r e l a t i o n s h i p d i f f e r e n t from t h e matched case ( F i g u r e 5). The r e s i s t a n c e i s lower a t small s t r a i n s and beyond a moderate s t r a i n ( -
5
per c e n t ) t h e d i f f e r e n c e i s small.The i n i t i a l modulus i s highest i n the case o f
R
> 1.0, again due t o t h e s t r e s s release e f f e c t w h i l e the modulus from t h e undersized borehole i s lower than from t h e matched case.Tests w i t h d i f f e r e n t degrees o f s o f t e n i n g
F i g u r e 6 shows a t y p i c a l s e t o f pressure-expansion curves f o r t h i s t e s t s e r i e s and t h e r e s u l t s a r e sunnnarized i n Table 11.
The l a r g e r t l t e softened zone created p r i o r t o t h e expansion of t h e probe, t h e g r e a t e r t h e deformation a t a given pressure. The d i r e c t consequence i s , therefore, an underestimation o f t h e i n s i tu horizontal; s t r e s s and t h e i n i t i a l shear modulus as i n d i c a t e d by t h e t e s t r e s u l t s . The apparent shear strength, however, increases w i tli increased degree of
softening. Such a phenomenon i s c o n t r a r y t o o t h e r s t r e n g t h t e s t s t h a t y i e l d decreasing s t r e n g t h w i t h i n c r e a s i n g softening. Theories have
been p u t f o r t h (Baguel i n e t a l . 1975 and Prevost 1979) t o e x p l a i n t h i s
500 'loo m ' 300 u D. = UI 2 ZOO LL a I00 0 ' I I
-
-
MAXIMUM P A S T-
-
' 1 0 . ) l o s 2 10-l S T R A I NF i g u r e 6 Pressure-expansion curves f o r t e s t s w i t h d i f f e r e n t
degrees
c o n t r a d i c t i o n . E s s e n t i a l l y , t h e reason l i e s i n t h e assumption of homogeneous m a t e r i a l used i n t h e conventional theory o f i n t e r p r e t a t i o n . This assumption i s n o t compatible w i t h the presence of a softened annulus zone o f m a t e r i a l around t h e probe.
TABLE I 1
RESULTS OF PRESSUREMETER TESTS WITH DIFFERENT DEGREE OF SOFTENING P r e v i o u s l y
appl i e d T o t a l I n i t i a l
s t r a i n before h o r i z o n t a l shear Undrai ned Test No. Depth expansion s t r e s s , aho modulus, G strength, Su
m % kPa
.
MPa kPa.
M A X I M U M P A S 1 APPLIED STRAIN 1%&-
r
2
-
O l Z f 4 5 6 7 E P I O S I R A I N . %Figure 7 Shear s t r e s s - s t r a i n curves f o r t e s t s w i t h d i f f e r e n t degrees o f s o f t e n i n g
K.
T.
Law and W. J. Eden 299Figure 7 shows t h e apparent shear s t r e s s - s t r a i n r e l a t i o n s h i p s deduced from t h e pressure-expansion curves. As t h e degree o f s o f t e n i n g increases, t h e s o i l becomes more compressible and reaches f a i l u r e a t a higher s t r a i n w i t h higher shear strength. There is, a l s o a more pronounced decrease i n the post-peak shear strength.
DISCUSSION
Based on the foregoing experimental r e s u l t s , i t i s evident t h a t disturbance s i g n i f i c a n t l y a f f e c t s the engineering q u a n t i t i e s measured using t h e pressuremeter. Table I 1 1 sumnarizes q u a l i t a t i v e l y the r e s u l t s based on comparison w i t h t h e case o f a matched c u t t i n g shoe and without d e l i berate i n t r o d u c t i o n o f s o i l softening. I n t h e f o l l o w i n g each q u a n t i t y i s discussed separately.
T o t a l h o r i z o n t a l s t r e s s
The measured t o t a l h o r i z o n t a l s t r e s s oho i s reduced by s t r e s s release
i n v o l v e d i n an oversized hole and by mechanical softening. This view
i s c o n s i s t e n t w i t h several pieces o f work recorded i n t h e 1 i t e r a t u r e .
I-larsland and Randolph (1977) c a r r i e d o u t t e s t s i n s t i f f f i s s u r e d London c l a y by means o f t h e Menard pressuremeter. The borehole was oversized and mechanical s o f t e n i n g was i n e v i t a b l e . The r e s u l t i n g ah0 values from conventional i n t e r p r e t a t i o n a r e small er than those drawn from t h e v a s t experience on t h i s c l a y . Tavenas (1975) conducted Menard pressuremeter t e s t s i n a s o f t s e n s i t i v e c l a y and reported t h a t t h e r e s u l t s were s c a t t e r e d b u t appeared t o be 1 ower than t h e more re1 ia b l e
TABLE I 1 1
SUII,IARY OF QUALITATIVE EFFECTS OF SOIL DISTURBANCE DURING PRESSUREI,IETER TESTS ON MEASUREMENT OF ENGINEERING QUANTITIES
Engineering
q u a n t i t y I n s i t u Undrained
h o r i z o n t a l I n i t i a l shear
mechanism pressure modulus s t r e n g t h
Stress released by oversized h o l e Stress induced by undersized h o l e f4echanical s o f t e n i n g o f s o i l
Note: -1, 0, +1 denote r e s p e c t i v e l y values lower than, equal t o and higher than those from using a matched c u t t i n g shoe without d e l i berate i n t r o d u c t i o n o f softening.
r e s u l t s from t o t a l pressure c e l l s . B a g u e l i n e t a l . (1972) performed t e s t s u s i n g t h e French s e l f b o r i n g pressuremeter w i t h a c u t t i n g Shoe s l i g h t l y o v e r s i z e d by 1 / 1 0
mn.
The measured h o r i z o n t a l S t r e s s wasexpressed i n terms o f t h e - c o e f f i c i e n t o f e a r t h pressure a t r e s t which nas found t o be equal t o 0.35. This value i s low when compared w i t h
t r i a x i a l t e s t r e s u l t s and from J a k y ' s equation.
As disturbance leads t o a r e d u c t i o n o f h o r i z o n t a l s t r e s s . t h e v a l u e obtained f o r t h e case o f a matched shoe may a l s o be lower than t h e t r u e value p r i o r t o t e s t i n g . The reason f o r t h i s i s t h a t d i s t u r b a n c e cannot be t o t a l l y e l i m i n a t e d even i n t h i s case. V i b r a t i o n from t h e r o t a t i n g inner rod and c u t t e r , and shearing o f s o i l by t l w advance of t h e pressuremeter, a r e b o t h p o s s i b l e sources o f disturbance.
The use o f an undersized c u t t i n g siioe w i l l g i v e
a
h o r i z o n t a l s t r e s s higher than t h a t from t h e matched c u t t i n g shoe. A t t h i s p o i n t i t $ 5n o t c l e a r i f such a measured uho w i l l be h i g h e r than t h e t r u e i n s i t u value f o r t h r e e reasons. F i r s t , as e x p l a i n e d p r e v i o u s ) y, the h o r i z o n t a l Stress measured u s i n g t h e matched shoe may y i e l d a low value. Secondly, t h e s o i l on t h e w a l l o f t h e undersized borehole i s s u b j e c t t o shear and
hence disturbance t o accomnodate t h e probe. T h i s d i s t u r b a n c e tends t o reduce t h e measured ah0 value. T h i r d l y , h o r i z o n t a l s t r e s s r e l a x a t i o n may take p l a c e d u r i n g t h e w a i t i n g p e r i o d between p l a c i n g t h e probe and
i n f l a t i n g t h e membrane.
I f a choice has t o be made betwe?n t h e t h r e e s i z e s o f c u t t i n g shoe, i t appears t h a t t h e s l i g h t l y undersized one i s more promising. Tile reason i s t h a t f o r t h e o t h e r two s i z e s , t h e u n d e r e s t i m a t i o n o f J ~ O i s u n l i k e l y t o be removed o r counterbalanced. because o f unavoidable disturbance.
Based on the r e s u l t s i n Tables 1 and 11. the o v e r s i z e d borehole increases the modulus by 30 p e r c e n t b u t mechanical s o f t e n i n g may reduce i t by f o u r times. The n e t e f f e c t . t h e r e f o r e . w i l l be an underestimate of t h e modulus. This i s c o n s i s t e n t w i t h t h e experience o f comparing standard and s e l f b o r i n g pressuremeter t e s t r e s u l t s by Baguelin e t a l . (1972). Armar e t a l . (1975) and Tavenas (1975).
A s l i g h t undersized borehold \ t i l l a l s o produce a lower estimate o f
t h e modulus. For the two depths s t u d i e d here. the moduli a r e about 50 and 20 per cent l e s s than t h e values w i t h the matched c u t t i n g shoe. A s i m i l a r study on a very s o f t marine c l a y (Law and Eden 1980) showed t h a t t h e r e d u c t i o n i s s m a l l e r , i n t h e o r d e r o f 15 p e r c e n t . I t i s b e l i e v e d t h a t f o r c l a y w i t h lower s e n s i t i v i t y , t i l e u n d e r e s t i m a t i o n may be even less. Denby (1978). f o r instance, found a reasonable agreement between the pressuremeter ~nodulus ( f r o m undersized b o r e h o l e ) and t h a t deduced from the f i n i t e element a n a l y s i s o f excavation behaviour i n San Francisco Bay mud.
I d e a l l y , a matched c u t t i n g shoe should be used f o r the a c c u r a t e d e t e r m i n a t i o n o f modulus. I t i s . however, n o t always p r a c t i c a b l e t o o b t a i n a matched slioe as t h e membranes f r o ~ t ~ the ~ n a n u f a c t u r e r do have
K. T. Law and W. J. h e n 30 1
s l i g h t s i z e v a r i a t i o n . I n t h i s case, a s l i g h t undersized shoe may be considered.
i Undrained s t r e n g t h
Table 111 shows t h a t s t r e s s released i n an oversized borehole and mechanical s o f t e n i n g b o t h tend t o increase t h e measured undrained s t r e n g t h . The undersized borehole produced s l i g h t l y lower s t r e n g t h
than t h e matched borehole. This s l i g h t l y lower s t r e n g t h has been found
t o exceed t h a t o f o t h e r t e s t s (Wroth and Hughes 1973, Eden and Law 1980
and Lacasse e t a l . 1981). Factors such as s t r a i n r a t e , disturbance and
s t r e s s p a t h have been given as p a r t o f t h e explanation.
I t appears t h a t w i t h reasonable care when conducting t h e t e s t , t h e pressuremeter w i l l s t i l l overestimate t h e a v a i l a b l e undrained s t r e n g t h of t h e s o i l . More aggravating i s t h e f a c t t h a t w i t h l e s s care, t h e overestimation w i l l even be higher. Caution i s required, therefore, i n applying t h e undrained pressuremeter s t r e n g t h f o r design. A t t h i s p o i n t then i t i s advisable t o avoid an oversized h o l e and t o minimize mechanical s o f t e n i n g o f t h e s o i l t o be tested.
SUMIdARY AND CONCLUSIONS
There a r e two components o f disturbance i n v o l v e d i n pressuremeter t e s t i n g : 1 ) s t r e s s change due t o t h e probe i n s e r t i o n and 2 ) mechanical
softening o f s o i l around the probe. Each component has a s i g n i f i c a n t
i n f l u e n c e on t h e measured engineering q u a n t i t i e s . I n p r a c t i c e . both
components a r e o p e r a t i v e simultaneously. The i n f l u e n c e s have been
studied i n d i v i d u a l l y and together f o r Champlain Sea c l a y by means of t h e Cambridge pressuremeter and t h e r e s u l t s , which f o l l o w , are found t o be c o n s i s t e n t w i t h o14er studies i n t h e l i t e r a t u r e .
1 ) I n s t a l l a t i o n o f the probe causes a s t r e s s change i n the s o i l i r r e s p e c t i v e o f the c u t t i n g shoe. The s l i g h t l y undersized shoe appears t o produce a b e t t e r estimate f o r h o r i z o n t a l s t r e s s than o t h e r sizes.
2 ) The i n i t i a l modulus i s increased by about 30 per c e n t w i t h an oversized h o l e and decreased by about f o u r times w i t h the presence o f an annulus zone o f softened s o i l . A s l i g h t l y undersized borehold y i e l d s a s l i g h t l y lower estimate o f modulus. 3 ) Stress r e l e a s e and s o f t e n i n g cause an u n r e a l i s t i c a l l y h i g h
estimate o f undrained strength. The s l i g h t l y undersized
borehole y i e l d s t h e lowest s t r e n g t h which i s already higher than those from o t h e r t e s t s .
Based on t h i s study i t i s d e s i r a b l e , t h e r e f o r e , t o s e l e c t a s l i g h t l y undersized c u t t i n g shoe as i t i s n o t always possible t o have a matched one. The t e s t should be conducted c a r e f u l l y and t h e r e s u l t s t r e a t e d w i t h c a u t i o n . I t i s l i k e l y t h a t t h e measured l l o r i z o n t a l s t r e s s and i n i t i a l modulus w i l l be i n t h e r i g h t range w h i l e the undrained s t r e n g t h may s t i l l be overestimated.
302 UPDATING SUBSURFACE SAMPLINGS
I
The conscientious efforts of A. Laberge and T.J. Hoogeveen, Technical
Officers, in conducting these tests is gratefully acknowledged. This
paper is a contribution from the Division of Building Research,
National Research Council of Canada, and is pub1 ished with the approval
of the Director of the Division.
Ii
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T h i s p a p e r , w h i l e b e i n g d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g R e s e a r c h , remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h . N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a , O t t a w a . O n t a r i o , K I A 0R6. Ce document e s t d i s t r i b u 6 s o u s forme d e t i r 6 - a - p a r t p a r La D i v i s i o n d e s r e c h e r c h e s e n b a t i m e n t . Les d r o i t s d e r e p r o d u c t i o n s o n t t o u t e f o i s l a p r o p r i 6 t 6 d e 1 ' B d i t e u r o r i g i n a l . Ce d o c u m e n t n e p e u t S t r e r e p r o d u i t en t o t a l i t 6 ou e n p a r t i e s a n s l e c o n s e n t e m e n t d e 1 1 6 d i t e u r . Une l i s t e d e s p u b l i c a t i o n s d e l a D i v i s i o n p e u t s t r e o b t e n u e en e c r i v a n t 2 l a S e c t i o n d e s p u b l i c a t i o n s , D i v i s i o n d e s r e c h e r c h e s e n b a t i m e n t , C o n s e i l n a t i o n a l d e r e c h e r c h e s Canada, Ottawa, O n t a r i o , K1A 0R6.
