Static cone penetration tests

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Static cone penetration tests

STATIC CONE PENETRATION TESTS by

J.H. Schmertmann

PREFACE

Professor John H. Schmertmann, Department of Civil Engineering, University of Florida, was a visiting scientist with the Geotechnical Section, Division of Building Research, from September 1971 to

August 1972. During part of his stay he conducted investigations on three in situ test methods in sensitive clay: the static cone

penetrometer, the hydraulic fracture method and the Menard pressuremeter.

His work on the hydraulic fracture method has subsequently been incorporated in the publication, "Minor principal stress measurements in marine clay with hydraulic fracture testsff by M. Bozozuk,

Proceedings, Conference on Subsurface Exploration for Underground Excavation and Heavy Construction, Henniker, N.H., ASCE, 1974, pp. 333-349. Work with the modified Menard pressuremeter and the the static cone penetrometer is now presented in DBR Reports 450 and 451as a record for the benefit of ensuing work on these two test methods.

Ottawa May 1979

C

.

B. Crawford Director, DBR/NRC

'I'he purpose of the study was to investigate further the possibility of predicting undrained strengths from cone tests, i.e., determine the Nc factor in the equation qc = Nc x s U . It was hoped that during

penetration testing the newly developed Fugro friction cone would provide additional quantitative information on the friction ratio, or ratio of local sleeve friction to bearing capacity of the cone tip. The friction ratio might provide an indication of the sensitivity of the clay and hence allow a more reliable determination of Nc.

A Fugro friction-cone tip with especially high sensitivity

000 kg tip, 750 kg sleeve) was obtained. The cone was calibrated and .eld tests involving four continuous profiles within the research area north of the DBR building were carried out. The cone bearing values for these four tests and the friction ratios determined for one of them are presented in this report.

METHODS AND RESULTS

Suuulemental Theoretical Study

Ladanyi (2) has shown that the shape of the undrained stress-strain compression curve has a great influence on the theoretical derivation of the value of the Nc factor, the calculated value of Nc varying from 1 to 10. He simplified the stress-strain curve into a sequence of three straight-line portions, involving four unknowns. Thus, despite this simplification, an investigator must obtain four independent undrained strength parameters before Nc can be determined. Even the friction cone offers only two parameters - end bearing and local friction

(or friction ratio). In principle, therefore, friction-cone data provide insufficient information to determine Nc

.

If two of the four unknowns are of minor importance, however, it may be possible to use friction cone measurements to estimate Nc

.

To investigate this possibility a parametric study of Ladanyi's equation for Nc was carried out. The details of this study are

presented in Appendix A. Figures A-1 and A-2 in Appendix A present the results in graphical form and show that within certain broad limits for sensitivity (St) and the strain at complete rernoulding ( E ~ ) the Nc

factor depends almost entirely on the slopes of the first two increasing and decreasing stress lines of Ladanyi's three-line approximation.

Figure A-2 expresses this as the ( E + / E m ) ratio.

Thus, perhaps in many cases the Nc factor can be adequately estimated on the basis of only two unknowns. After the stress-strain simplification to the Ladanyi three-line case, it remains to be seen whether highly sensitive clays, such as the Leda clay, fall within the St and cr limitations and thus allow a two-parameter estimate of Nc

.

The results obtained in this study and reported in Internal Report 450 indicate that they do not.

Cone Bearine Loes

After appropriate calibration of the cone tip in the laboratory, including calibrations in a humid room at lower temperatures,* a series of four soundings, S1 to S4 were performed within the trial research area north of Building M-2 at the Montreal Road Laboratories of the National Research Council. For each sounding, continuous penetration loss of end-bearing and friction-sleeve resistance were obtained on a two-pen chart recorder. There was an interruption in the continuity of

penetration at 5-ft intervals to check the drill rods at the end of thrust of the hydraulic feed of the drilling. By regulating the feed valve to the hydraulic cylinder a nearly constant rate of penetration of

1.5 cm/s was achieved and used for soundings S1, S3 and S4. For S2, a rate of 0.4 cm/s was used to indicate the influence of the rate of

penetration. A comparison plot of the depth dependence of the cone-bearing values for these four tests is shown in Figure 1. In this figure, three of the profiles show bearing values determined at 1-ft intervals, without averaging over these intervals. The fourth log (sounding S4) shows bearing values obtained by graphically averaging over each successive 1-ft interval.

It may be noted in Figure 1 that about a fourfold decrease in

penetration rate appeared to produce about an 8 per cent reduction in cone bearing. This is based on only one test at the slower rate, however.

Figure 1 also shows the cone-bearing profile determined by Ladanyi (3) using the 10 cm2 Borros electrical tip. The straight-sided Fugro tip appears to yield qc values approximately 52 per cent greater than those from the Borros tip. The shaft diameter of the Rorros tip reduces to 3.20 cm only 1.5 cm above the 3.57 cm diameter of the base of the cone. It is suspected that the reduced diameter permits the escape of remoulded clay "fluid" from under the cone, which relieves pressure and reduces qc. Local Friction and Friction Ratios

With the use of the new Fugro friction-cone tip it soon became obvious, despite the special increased sensitivity of the sleeve, that local friction was of very small magnitude and near the limit of what could be sensed with precision. Thus, the zero position of the chart line for local friction became of special importance. It was found that this position drifted significantly and somewhat unpredictably as a result of ground temperature effects, the radial pressures on the tip due to insertion and perhaps the interaction between the cone and sleeve transducers. To avoid this problem, a procedure was devised of temporarily reversing the direction of movement at convenient depth intervals and then determining the zero friction

position by averaging the before and after downward movement values with the

*Calibration data are not included because the cone tip has been dismantled for new strain gauges and will require new calibrations.

i n t e r m e d i a t e upward movement v a l u e s . The upward movement was c o n d u c t e d a t t h e same r a t e a s t h e downward. T h i s p r o c e d u r e was u s e d f o r sounding S4. 'I'he method a p p e a r s t o b e s u c c e s s f u l i n t h a t i t gave r e a s o n a b l y w e l l

d e f i n e d p o s i t i o n s f o r z e r o f r i c t i o n which were c o n s i s t e n t w i t h d e p t h . A small c o r r e c t i o n f o r t h e e f f e c t o f t h e cone r e a d i n g on t h e s l e e v e r e s i s t a n c e r e a d i n g was a l s o made. T h i s e f f e c t was p a r t o f t h e i n i t i a l c a l i b r a t i o n and had t o b e i n c l u d e d t o e s t a b l i s h t h e z e r o s l e e v e f r i c t i o n p o s i t i o n .

A f t e r making t h e above c o r r e c t i o n s , it was p o s s i b l e t o compute l o c a l f r i c t i o n w i t h some p r e c i s i o n , d e s p i t e i t s v e r y s m a l l magnitude. D i v i d i n g t h e a v e r a g e l o c a l f r i c t i o n o v e r some d e p t h i n t e r v a l by t h e a v e r a g e cone b e a r i n g o v e r t h e same i n t e r v a l g i v e s t h e f r i c t i o n r a t i o (FR) f o r t h a t i n t e r v a l . F i g u r e 1 i n c l u d e s a p r o f i l e o f t h i s r a t i o a s d e t e r m i n e d from t h e s i n g l e r e c o r d a v a i l a b l e , S4, u s i n g c o m p u t a t i o n d e p t h i n t e r v a l s o f 1 . 0 f t . The f r i c t i o n r a t i o p r o f i l e i n F i g u r e 1 shows a c o n t i n u o u s d e c r e a s e w i t h d e p t h o v e r t h e d e p t h i n t e r v a l i n v e s t i g a t e d , r a n g i n g from a b o u t 1 . 0 p e r c e n t a t 20 f t t o a b o u t 0.20 p e r c e n t a t 45 f t . I t i s known from p r e v i o u s e x p e r i e n c e t h a t i f t h e c l a y were i n s e n s i t i v e t h e f r i c t i o n r a t i o would be a b o u t 5 t o 8 p e r c e n t . Thus, t h e s e n s i t i v i t y o f t h i s c l a y d e f i n i t e l y c a u s e d a d r a m a t i c r e d u c t i o n i n t h e o t h e r w i s e e x p e c t e d f r i c t i o n r a t i o . F u r t h e r m o r e , t h e r e i s a f i v e f o l d d e c r e a s e i n f r i c t i o n r a t i o w i t h d e p t h which c o r r e s p o n d s t o t h e known s i g n i f i c a n t i n c r e a s e o f s e n s i t i v i t y w i t h d e p t h f o r t h e c l a y a t t h e s i t e . Thus, a t t h i s s t a g e o f t h e r e s e a r c h t h e r e i s e v e r y r e a s o n t o b e l i e v e t h a t t h e f r i c t i o n r a t i o w i l l c o r r e l a t e w i t h s e n s i t i v i t y . S e n s i t i v i t i e s a t t h e s i t e , a s d e t e r m i n e d by t h e f i e l d v a n e , v a r i e d from a b o u t 30 a t 26 f t (FR = 0 . 7 3 p e r c e n t ) t o 50 a t 44 f t (FR = 0 . 2 0 p e r c e n t ) . The r a t i o o f ( s l e e v e f r i c t i o n ) / ( r e m o u l d e d vane s t r e n g t h ) v a r i e d from a b o u t 2 . 5 t o 4 .

There i s now some q u e s t i o n o f t h e i m p o r t a n c e o f s e n s i t i v i t y a s a s o i l p a r a m e t e r f o r t h e s e c l a y s . S e n s i t i v i t y i n t h e s e c l a y s h a s always been d i f f i c u l t t o measure and d e f i n e . With L a d a n y i ' s r e c e n t work ( 3 ) , t o g e t h e r w i t h t h e p r e s s u r e m e t e r r e s e a r c h r e p o r t e d i n a companion r e p o r t ( 4 ) , i t a p p e a r s t h a t t h e immediate p o s t - p e a k b e h a v i o u r i s o f more i m p o r t a n c e t h a n t h e c o m p l e t e l y remoulded s t r e n g t h a t v e r y h i g h s t r a i n s .

THE FUGRO TIP

From t h i s s t u d y i t a p p e a r s t h a t a s t r a i g h t - s i d e d e l e c t r i c p e n e t r o m e t e r , w i t h o u t d i a m e t e r r e d u c t i o n n e a r t h e t i p , i s r e q u i r e d t o e v a l u a t e b e a r i n g c a p a c i t y f a c t o r s o r u n d r a i n e d s h e a r s t r e n g t h from s t a t i c cone p e n e t r a t i o n t e s t s . The Fugro t i p p r e s e n t s some problems t h a t need t o b e r e s o l v e d , however. The i n t e r a c t i o n o f t h e cone w i t h t h e s l e e v e and t h e e f f e c t o f a l l - a r o u n d p r e s s u r e on t h e s l e e v e r e a d i n g s need f u r t h e r s t u d y . The w a t e r s e a l a p p e a r s t o b e i n e f f e c t i v e and t h e method o f bonding t h e s t r a i n gauges a p p e a r s s e n s i t i v e t o t h e p r e s e n c e o f w a t e r . Recent i n f o r m a t i o n r e c e i v e d

from people associated with the aerospace industry suggests that Fugro's present method of selecting strain gauges and bonding them is quite old fashioned and could be greatly improved. An alternative is to purchase the Fugro cones and select and mount improved gauges.

It is convenient to have a cable connection above the tip but the design of the present Fugro connection makes it very difficult to

waterproof adequately. After some electrical problems it was decided to remove this connection from the system rather than use it in its present form

.

REFERENCES

(1) Ladanyi, B. and Eden, W.J. Use of the deep penetration test in sensitive clays. Proceedings, Seventh International Conference on Soil Mechanics and Foundation Engineering, Mexico, Vol. 1, 1969, pp. 225-230.

(2) Ladanyi, B. Deep punching of sensitive clays. Proceedings, Third Panamerican Conference on Soil Mechanics and Foundation Engineering, Caracas, Vol. 1, 1967, pp. 533-546.

(3) Ladanyi, B. A study of deep penetration tests in sensitive clays. National Research Council of Canada,'Division of Building Research, Internal Report 360, 1968, Figure 14.

(4) Schmertmann, J.H. Pressuremeter tests in Leda clay. National Research Council of Canada, Division of Building Research, Internal Report 450, 1979.

FIGURE 1

CONE BEARING AND FRICTION RATIO PROFILES FROM FOUR PRELIMINARY SOUNDINGS BEHIND DBR BUILDING

A P P E N D I X A

Additional Theoretical Studies from Ladanyi's Equations for

6

_{d - P f - f ' * h :}

_{% ,}

G P )

5

e p )

$9

3

--

s,

20,

roo

%

5

a,

10, 5 0

-

8 . 0 2 , 0 . a 0 , 1.00 €Ip

-

The key question is whether the set of two measurements obtained by the friction cone has the theoretical potential to be an indicator of (correlate with) N

.

C If so, under what limitations of soil types or soil characteristics? This key question can be investigated by making a parametric study of the factors in eq. (15).

Conclusions

(assuming equation valid)

1. For St

2

_{10, the Nc factor is practically}

independent of St.

2 . At any given St > 10, the Nc factor is practically independent of the E, strain,

for E~

2

10%.

3. The factor of overwhelming importance in predicting Nc for clays with St

2

10 is the ratio (Er/cP)

.

Significance:

For a clay with known St

2

10 and cr

2

10% (most sensitive clays), predicting Nc involves predicting the ratio of the two unknowns (cr/cP). The static cone also determines a ratio, f/q = FRY and therefore has the potential to correlate

qc -

PA

with (E~/E~,), _{determine Nc, and then obtain su =}

N c

FIGURE A-1

[gl

[ l o g l 0 s c a l e

1

Appears o n e can u s e a p p r o x i m a t e e q u a t i o n w i t h e r r o r - <

l o % ,

compared t o Ladanyi (15) i f :

[g]

- 4 Comments : 1 . The s l o p e r a t i o ( E + / E - ) a l s o a p p e a r s t o b e a good i n d e x o f N c , a s a s u b s t i t u t e f o r ( E J E )

.

P

2 . The s l o p e r a t i o may even be s u p e r i o r t o t h e s t r a i n r a t i o i n t h a t : a . somewhat l e s s s e n s i t i v e t o o t h e r v a r i a b l e s i n t h e Nc p r e d i c t i o n ;

h . p e r h a p s e a s i e r t o g e t s l o p e r a t i o b e c a u s e n o t n e c e s s a r y t o go t o t h e E r s t r a i n .

FIGURE A-2

Implications of Previous Study

For certain clays, perhaps most sensitive ones, a single ratio of 2 parameters determines Nc ,

The friction cone determines 2 parameters, qc and fS f

S

Their ratio - - - FR should depend on

"

c

stress-strain curve, or

[F]

1

Should be possible to obtain correlations:

&a

rod &;st

This method (cone) will not permit getting St

,

unless

E +

-

E - and St somehow related.

E +

Would expect that St increase as -