Interpretation of the consolidation test

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Publisher’s version / Version de l'éditeur:

ASCE Journal of the Soil Mechanics and Foundation Division, 90, SM5, pp.

87-102, 1964-12-01

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Interpretation of the consolidation test

Crawford, C. B.

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4056 September, 1964 SM 5

Journal of the

SOIL MECHANICS AND FOUNDATIONS DIVISION

Proceedings of the American Society of Civil Engineers

INTERPRETATION O F THE CONSOLIDATION TESTa

By C a r l B. c r a w f o r d l

SYNOPSIS

Undisturbed specimens of a sensitive clay a r e loaded in increments of s h o r t and longduration and a t a constant r a t e of strain. The preconsolidation p r e s s u r e and the proportion of p r i m a r y and secondary consolidation, a s deter

-

mined by pore p r e s s u r e measurements, a r e shown t o be a function of t e s t procedure. Comparison with field r a t e s of loading shows that the laboratory r a t e s a r e quite unrealistic.

INTRODUCTION

Consolidation of s a t u r a t e d s o i l i s a p r o c e s s of volume reduction due to t h e expulsion of w a t e r f r o m the void space. If the p r o c e s s is slow, i t c a n o c c u r without the creation of significant p r e s s u r e s i n the pore water; if i t is rapid, the compression is inhibitedby the inability of the w a t e r t o drain away quickly

Note.-Discussion open until F e b r u a r y 1, 1965. Separate discussions should b e sub- mitted f o r the individual p a p e r s in t h i s symposium. To extend the closlng date one month, a w r ~ t t e n request must be filed with the Executive Secretary,ASCE. T h i s p a p e r

is p a r t of the copyrighted Journal of the Soil Mechanics and Foundations Division, P r o - ceedlngs of the American Society of Civil E n g ~ n e e ~ s , Vol. 90, No. SM5, September, 1964.

a P r e s e n t e d a t the J u n e 16-19, 1964, ASCE Sol1 Mechanics and Foundations Con- ference on Design of Foundations for Control of Settlement, a t Evanston, 111.

1 Head, Soil Mechanics Sect., Dlv. of Buildlng R e s e a r c h , Natl. Research Councll, Ottawa, Canada.

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enough, and t h i s r e s u l t s in a temporary i n c r e a s e in the p r e s s u r e of the p o r e water. The build-up and the dissipation of pore water p r e s s u r e s i s related t o the magnitude and r a t e of loading, the distance the draining w a t e r m u s t travel, t h e permeability of the soil, and the compressibility of the s o i l structure.

K. Terzaghiz recognized the important influence of hydrodynamic t i m e lag in the consolidation of relatively impermeable soils. Using c e r t a i n s i m - plifying assumptions, Terzaghi was able to develop a mathematical expression f o r the r a t e a t which consolidation could occur during the r e l e a s e of water under p r e s s u r e . This i s called "primary consolidation," in o r d e r to d i s - tinguish i t from a f u r t h e r compression, called Usecondary consolidation," which o c c u r s without appreciable p r e s s u r e in the p o r e water. Considerable laboratory work has been conducted in recent y e a r s to investigate t h e relative contributions of primary and secondary consolidation.3,4,5,6 This work h a s contributed considerably to the understanding of s o i l compressibility, especially of t e s t techniques.

It can be shown that both p r i m a r y and secondary consolidation a r e simple e m p i r i c a l divisions of a continuous compression process. There i s ample evidence that the relative contribution of each i s largely a function of t h e laboratory t e s t procedure, particularly the r a t e of loading.4,7 F u r t h e r m o r e , the prediction of actual consolidation in the field has not been generally satisfactory. ~ e r z a ~ h i , 8 drawing on his v a s t knowledge of c a s e r e c o r d s , stated, "These r e c o r d s disclose a r e m a r k a b l e variety of secondary t i m e effect." Terzaghi quoted c a s e s of m e a s u r e d secondary effect varying directly with time, and o t h e r s in which i t varied with the log of time. Terzaghi a l s o described a subsidence problem in which the predicted settlement w a s twenty t i m e s the actual value; he could only account f o r t h i s discrepancy a s a t i m e effect.

The t e s t s reported herein illustrate how completely unrealistic the r a t e s of compression a r e in the standard laboratory test, compared t o field com-

2 T e r z a g h i , K., " P r i n c i p l e s of Soil Mechanics-Settlement and Consolidation of Clay," Engineering News-Record, November, 1925, pp. 874-878.

3 Newland, P. L., and Allely, B. H., 'A Study of t h e Consolidation C h a r a c t e r i s t i c s of a Clay," Geotechnique, London, England, Vol. 10, No. 2, 1960, pp. 62-74.

4 L e o n a r d s , G. A., and Girault, P . , "A Study of t h e One-Dimensional Consolidation T e s t , " P r o c e e d i n g s , 5th I n t e r n a t l . Conf. on Soil Mechanics, London, Vol. 1 , 1961, pp. 213-218.

- -. - - - .

5 Lo, K. Y., "Secondary C o m p r e s s i o n of Clays," J o u r n a l of t h e Soil Mechanics and Foundations Division, ASCE, Vol. 87, No. SM4, P r o c . P a p e r 2885, August, 1961, pp. 61-87. - - - .

6 Wahls, H. E., YAnalysis of P r i m a r y and S e c o n d a ~ y Consolidation," J o u r n a l of t h e Soil Mechanics and Foundations Division, ASCE, Vol. 88, No. SM6, P r o c . P a p e r 3373, D e c e m b e r , 1962, pp. 207-231.

7 Hamilton, J. J., and C r a w f o r d , C . B., 'Improved Determination of Preconsolidation P r e s s u r e of a Sensitive Clay," Special T e c h n i c a l Publication No. 254, ASTM, 1959, pp. 254-271.

8 T e r z a g h i , K., 'Discussion," P r o c e e d i n g s , 3 r d Internatl. Conf. on Soil Mechanics, Z u r i c h , Switzerland, Vol. 3 , 1953, pp. 158-159.

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SM 5 CONSOLIDATION TEST 8 9 pression. It is suggested that this is a much m o r e influential factor than many of the other known e r r o r s of testing.

SOIL

The s o i l used f o r t h e s e t e s t s has been generally considered a m a r i n e deposit, but recent geologicalwork has c a s t doubt on e a r l i e r interpretations.g Its engineering properties a r e s i m i l a r to those of t h e extensive deposits of "Ledan clay in the St. Lawrence and Ottawa River valleys.10

Uniform block s a m p l e s of clay w e r e cut f r o m a s m a l l tunnel, in the city of Ottawa, a t a depth of 33 ft. The soil on which the t e s t s w e r e made had a natural w a t e r content averaging 58.4%, a liquid limit of 54%, and a plastic limit of 25%. It had approximately 65% clay sized p a r t i c l e s (< 0.002 mm), approximately 2 g r a m s p e r l i t e r of s a l t s in the pore water, and a field vane strength of 0.5 kg p e r s q cm; the sensitivity w a s approximately 50.

TEST PROCEDURE AND APPARATUS

Most of the t e s t s w e r e made with vertical p r e s s u r e s of 0.5, 1, 2, 4, 8, and 16 kg p e r s q cm. Departures f r o m t h i s loading schedule were slight and had no apparent effect on the results. Previous t e s t s on a s i m i l a r soil, i n which the loadincrement r a t i o was v a r i e d substantially, have been published.7 Variations in procedure included daily load increments (Series A), weekly increments (Series B), and i n c r e m e n t s a t the end of p r i m a r y consolidation (Series C). A fourth s e t of t e s t s (Series D) involved controlled r a t e of loading. All t e s t s except S e r i e s B w e r e conducted during winter a t room t e m - perature, which normally v a r i e s f r o m 72°F to 78°F. The long t e r m t e s t s (Series B) w e r e made in a controlled t e m p e r a t u r e r o o m a t 68 5 1/4"F.

All specimens w e r e tested in 'teflonn coated stainless s t e e l consolidation r i n g s (with a n a r e a of 20 s q cm and either 1 c m o r 2 c m high). All t e s t s except those of S e r i e s B w e r e done with the r i n g sealed to a b a s e plate, in t h e center of which a 1/2-in. diameter Alundum disc was flush mounted and con- nected to a no-flow p r e s s u r e measuring system. The top s u r f a c e of t h e specimen was covered with a free-draining Alundum disc. In the S e r i e s B t e s t s , pore p r e s s u r e s w e r e not measured; the consolidation ring w a s floated between two free-draining discs. Therefore, only in S e r i e s B was the maximum length of drainage path equal to one-half the height.

Incremental loading t e s t s w e r e conducted in the normal manner, with a n 11 t o 1 lever ratio consolidation p r e s s and a load increment ratio of ~ ~= 1. / p The controlled r a t e of loadingtests w e r e made in a standard gear-driven s o i l compression machine with a proving ring to m e a s u r e the vertical load.

P o r e p r e s s u r e s w e r e m e a s u r e d manually, using the N o r w e g i a n null- indicator device.11 All pore p r e s s u r e lines w e r e of copper tubing, a n d c a r e

9 Gadd, N. R., 'Surfisial Geology of 0 t t a w a Map-Area Ontario and Quebec," Ge? -

logical Survey of Canada, 1963, P a p e r 62-16.

1 U Crawford, C . B., "Engineering Studies of Leda Clay," Soils in Canada; S ~ ~ e c i a l Publication No. 3 , Royal Soc. of Canada, 1961, pp. 200-217.

11 Andresen, A., and Simons, N. E., 'Norwegian T r i a x i a l Equipment and Techniques," Proceedings, ASCE R e s e a r c h Conf. on S h e a r Strength of Cohesive Soils, Boulder, Colo.,

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9 0 September, 1964 SM 5 was taken to flush a i r out of the s y s t e m before each t e s t . Because of t h e flexibility of the pore p r e s s u r e measuring system, the maximum p o r e p r e s - s u r e was not recorded until approximately 1 / 2 mill after each load had been applied. This apparatus c h a r a c t e r i s t i c l i m i t s the accuracy of m e a s u r e d values of pore p r e s s u r e , especially the low p r e s s u r e s developed in the r e - compression range.

TEST RESULTS

A s in m o s t r e s e a r c h with undisturbed soils, the quantity of uniform mate- r i a l was limited, and preliminary t e s t s w e r e done in o r d e r to plan the testing

TABLE 1.-TEST RESULTS

I I I I I I I - - - - _L - - - I

- - -

L - - - A - - A at end of primary 2.65 - - - increments ~ g 6 - 1 - l g

T

1 H

r-;?-rL6?4

2.20

1

Test Series

a By chance, the pressure of 2 kg per sq cm was allowed to act for 4,300 min. The additional secondary compression deflected the pressure-void ratio curve more than normal, causing a lower interpretation of preconsolidation pressure.

program. In t h i s case, twelve t e s t s a r e available on almost identical soil specimens, and each type of t e s t proved to be satisfactorily reproducible. T e s t r e s u l t s a r e given in Table 1, in which the best e s t i m a t e of preconsolidation p r e s s u r e using the common e m p i r i c a l procedurela i s indicated. Details of typical t e s t r e s u l t s a r e shown in the illustrations presented subsequently.

'pecirnen

No.

NORMAL INCREMENTAL LOADING (TEST SERIES A)

Specimens w e r e loaded daily, p o r e p r e s s u r e s w e r e m e a s u r e d a t the lower surface. and f r e e drainage was allowed a t the ton. Tvnical time-comnression

Initial Height,

H, in centimeters

1 2 Casagrande, A

.,

"The Determination of the Preconsolidation Load and Its Practical Significance," Proceedings, 1st Internatl. Conf. on Soil Mechanics, Cambridge, Mass., Vol. 3, 1936, pp. 60-64. Length of Drainage Path Nat' water C ~ ~ $ , n t ~ Nat. Void Ratio Preconsol. Pressure, in kilograms per square centimeter ~ ~ ~ d i ~ ~

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SM 5 CONSOLIDATION T E S T 9 1

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9 2 September, 1964 SM 5

c u r v e s a r e shown in Fig. 1. In Fig. 2, the r a t i o of p o r e p r e s s u r e to applied p r e s s u r e is plotted in relation to deflection f o r the l a s t four loading i n c r e - ments on specimen 96-1-18. This proves to be a linear relationship over most of the range, and by extrapolating to z e r o pore p r e s s u r e , i t i s possible

0 0.02 0.04 0.06 0.08

Deflection, in inches

FIG. 2.-RELATIONSHIP BETWEEN P O R E PRESSURE AND D E F L E C T I O N UNDER INCREMENTAL LOADS: SPECIMEN 96-1-18

to establish a well-defined end of p r i m a r y consolidation. The end of p r i m a r y consolidation a r b i t r a r i l y obtained in t h i s manner i s shown a s a solid c i r c l e on each of the t i m e c u r v e s of Fig. 1. It o c c u r s somewhat e a r l i e r than that determined by the usual approximation.

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SM 5 CONSOLIDATION TEST 9 3

The linear relationship shown in Fig. 2 r e s e m b l e s that reported by D. W. ~ a ~ l o r , l 3 but r e c e n t t e s t s of a s i m i l a r s o i l (with p o r e p r e s s u r e m e a s u r e - m e n t s by t r a n s d u c e r ) indicate m o r e c u r v a t u r e and longer periods of p o r e p r e s s u r e dissipation. This leads to g r e a t e r difficulty in separating p r i m a r y and secondary consolidation, but i s not thought to affect the g e n e r a l conclusions resulting f r o m t h e work reported herein. The present equipment i s sufficient to suggest (by extrapolation) that 80% t o 85% of each applied s t r e s s increment above the preconsolidation p r e s s u r e i s t r a n s m i t t e d immediately t o

1 10

Pressure, in kilograms per square centimeter

FIG. 3.-COMPRESSION-LOG PRESSURE CURVES FOR NORMAL AND LONG TERlM INCRE- MENTAL LOADING

the pore water. This i s not an unreasonable value in view of the probability of the p r e s e n c e of g a s in the specimen and the influence of s i d e friction.

F o r comparison of t e s t s , t h e percentage compression (rather than the void ratio) i s plotted against the logarithm of p r e s s u r e in Fig. 3 . Two sepa- r a t e c u r v e s a r e shown f o r specimen 96-1-18. The upper c u r v e indicates the compression at the end of p r i m a r y consolidation (Fig. 2) f o r each p r e s s u r e

13 T a y l o r , D. W., " R e s e a r c h o n Consolidation of Clays," S e r i a l 82, ~ a s s a c h u s e t g I n s t . of T e c h . , 1942.

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SM 5 CONSOLLDATION TEST 9 5 increment. The lower c u r v e r e p r e s e n t s the compression a t t h e endof loading period, in t h i s c a s e a f t e r one day. The difference between the two (curves) r e p r e s e n t s t h e secondary consolidation under each increment.

SLOW INCREMENTAL LOADING (TEST SERIES B)

Specimens w e r e loaded in weekly increments. Because pore p r e s s u r e s w e r e not measured, t h e specimens w e r e allowedto d r a i n a t t h e top and bottom

FIG. 5.-COMPRESSION-LOG PRESSURE CURVES FOR SPECIMENS LOADED AT END O F PRIMARY CONSOLIDATION

of a floating ring. Time-compression c u r v e s f o r one of t h e t e s t s (96-1-20) a r e shown in Fig. 4. The o t h e r long t e r m t e s t (96-1-13) departed f r o m t h e usual loading schedule. It w a s c o m p r e s s e d under increments of 0.4, 0.8, 1.6, 3.3, 6.6, and 13.2 kg p e r s q c m , but t h e pressure-void ratio c u r v e w a s almost identical to that f o r 96-1-20, which is plotted in Fig. 3. Comparison with 96-1-18 i l l u s t r a t e s the increasing contribution of secondary consolidation with long increments of loading. T h i s probably h a s an exceptional influence on the interpretation of preconsolidation p r e s s u r e , because the 2 kg per s q

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c m load is n e a r t h e probable preconsolidation p r e s s u r e . Another feature of the S e r i e s B t e s t i s that it c o m p r e s s e d much more, even during the f i r s t day, than did the S e r i e s A t e s t under the s a m e p r e s s u r e of 2 kg p e r s q cm. The r e a s o n f o r t h i s is not evident, but it may be due t o t h e long duration of the preceding increment.

RAPLD INCREMENTAL LOADING (TEST SERIES C)

Specimens w e r e loaded in the usual increments, but, in these c a s e s , the pressure-deflection c u r v e s w e r e extrapolated (as in Fig. 21, and the next

Pressure, In kilograms per square centimeter

FIG. 6.-COMPRESSION-LOG PRESSURE CURVES FOR CONSTANT RATE O F LOADING

increment w a s applied a t the anticipated end of p r i m a r y consolidation. The p r e s s u r e - c o m p r e s s i o n c u r v e s a r e shown in Fig. 5 in which the durationof load application f o r each increment is noted. Below the preconsolidation load, the time required f o r pore p r e s s u r e dissipation s e e m s t o be unrelated t o thickness. F o r the f i r s t increment above the preconsolidation p r e s s u r e (i.e., 4 kg p e r s q cm), the t i m e r e q u i r e d i s approximately directly proportional

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SM 5 CONSOLIDATION TEST 9 7 to the thickness. F o r the l a s t two increments, the t i m e for p r i m a r y consolidation i s approximately proportional to the s q u a r e of the thickness.

CONTROLLED RATE O F LOADING (TEST SERIES D)

Test S e r i e s D w a s a s p e c i a l s e t of t e s t s in whichpore p r e s s u r e w a s m e a s - u r e d a t the bottom of each specimen while it w a s being consolidated by continuous v e r t i c a l loading. F o r a l l t e s t s , the loading r a t e was s e t for 0.0009 in. p e r min, but, due to deflection of the proving ring, the specimens w e r e com- p r e s s e d at approximately 4% p e r h r t o the preconsolidation load, and then a t a r a t e of 7% o r 14% p e r hr, depending on specimen thickness.

The t e s t s in this s e r i e s gave almost identical pressure-deflection curves; the curve f o r specimen 96-1-25 i s shown in Fig. 6. Maximum pore p r e s s u r e a t the base of the specimen w a s l e s s than 5% of the applied p r e s s u r e . If pore p r e s s u r e s a r e deducted f r o m applied p r e s s u r e s , the broken curve in Fig. 6 i s obtained. The two c u r v e s enclose the possible range of v e r t i c a l effective s t r e s s e s within the specimen. Specimen 96-1-27, which w a s twice a s thick a s 96-1-25, developed a maximum pore p r e s s u r e equal t o 8% of the applied p r e s s u r e , but even t h i s amount had little effect on the pressure-deflection curve.

In the range f r o m 0% to 15% compression, it took specimen 96-1-27 (2 c m thick) twice a s long a s it took specimen 96-1-25 (1 c m thick) t o c o m p r e s s t h e s a m e amount. In o t h e r words, the specimens w e r e forced t o c o m p r e s s a t a r a t e that v a r i e d directly with the thickness of specimen, and not with the square of the thickness, in accordance with c l a s s i c a l theory; only slightly higher pore p r e s s u r e s w e r e generated in the thicker specimen.

RATE O F COMPRESSION

It i s difficult to compare r a t e s of compression f o r specimens loaded in increments, because the r a t e v a r i e s greatly during the test. The virgin compression of specimen 96-1-20, f o r example, o c c u r r e d during a 28-day period, o r at an average r a t e of approximately 1% p e r day, but the actual r a t e varied f r o m approximately 1% p e r min, during the f i r s t minute of each load application, t o approximately 1% p e r week a t the end of each loading period. The r a t e a t the end of primary compression f o r t h i s m a t e r i a l averaged 1% p e r hr. The r a t e s of compression f o r the various types of t e s t during loading f r o m 2 t o 4 kg p e r s q c m a r e shown in Fig. 7. The 1-cm thick specimen (96-1-23) has the highest r a t e , averaging m o r e than 3% p e r min during the f i r s t minute of loading and approximately 40% p e r h r during the complete loading period. This h i g t f o r c e d - r a t e of consolidation probably leads to a n unusually high degree of disturbance. Specimen 96-1-27 w a s loaded a t the slowest r a t e , a constant 7.4% p e r hr.

It i s common practice to study the influence of compression r a t e by varying the s i z e of load increment. In t h i s manner, K. ~ a n ~ e 1 - 1 4 found that s o i l

-

l4 Langer, K., 'Influence of Speed of Loading Increment on the P r e s s u r e Void Ratio Diagram of Undisturbed Soil Samples," Proceedings 1 s t Internatl. Conf. on Soil Mechanics, Cambridge, Mass., Vol. 2 , 1936, pp. 116-118.

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was l e s s compressible under s m a l l load increments. Langer attributed t h i s to two factors: (1) Large increments caused a shock to the soil s t r u c t u r e ; and (2) the induced pore p r e s s u r e gradients caused internal rupture of the s t r u c t u r e . ~ a ~ l o i - 1 3 reported that consolidation proceeded a t a slower r a t e when load increments w e r e small, and a f t e r extensive study, he concluded that predictions of settlement a r e likely to be seriously in e r r o r when load

increments in the fieldand i n the t e s t a r e not s i m i l a r . Field loading is seldom applied i n increments, and a f u r t h e r study of this factor is warranted. It i s necessary, however, to keep i n mind that two properties, in particular, con- t r o l t h e r a t e a t which compression can occur; these are: (1) The permeability of the soil; and (2) the plastic r e s i s t a n c e of the s o i l s t r u c t u r e .

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SM 5 CONSOLLDATION TEST 99 According to the 'Terzaghi theory, the coefficient of consolidation v a r i e s directly with the permeability and inversely with the compressibility of the s t r u c t u r e . It follows that substantial pore p r e s s u r e s will be c r e a t e d by l a r g e load increments, and that hydrodynamic effects will control the r a t e of compression. When increments a r e s m a l l , however, the pore p r e s s u r e gradients will be small, and the plastic r e s i s t a n c e of the s o i l s t r u c t u r e will have a relatively g r e a t e r effect. G. A. Leonards and B. K. R a m i a h l 5 demonstrated t h i s experimentally and concluded that

".

. .

the coefficient of consolidation obtained f r o m any particular laboratory t e s t i s purely a r b i t r a r y and can vary significantly f r o m the value that would obtain in the field."

The compression index and the empiricalpreconsolidation p r e s s u r e do not depend on s o i l permeability; they depend only on the compressibility of t h e s o i l s t r u c t u r e . F o r most clays, the s t r u c t u r e continues to deform indefinitely under a constant effective s t r e s s , but a s long a s each load increment i s maintained f o r the s a n e t i m e interval, the compression index a p p e a r s to be relatively unaffected by t h i s characteristic. R. D. ~ o r t h e ~ l 6 found that specimens, reloaded a t the end of p r i m a r y consolidation, gave a satisfactory evaluation of t h e compression index, and s i m i l a r t e s t s reported herein (Series C) confirm this.

F o r the s o i l specimens described herein, the e m p i r i c a l preconsolidation p r e s s u r e i s greatly affected by the loading procedure. According to Table 1, the preconsolidation p r e s s u r e based on n o r m a l t e s t procedure (daily loading)

is approximately 1.8 kg p e r s q cm, but, by varying the procedure, values ranging f r o m 1.3 to 3.0 can be obtained. It is evident that the lower values a r e associated with l a r g e amounts of secondary consolidation. This may be due partly to s t r u c t u r a l damage caused by the shock of large load i n c r e m e n t s (as suggested by

anger),

but the r a t e of loading is undoubtedly an important factor. Insufficient field observations have been made to establish which of the laboratory t e s t s is c o r r e c t , but field evidence i s being gathered a t every opportunity in o r d e r t o a s s e s s these factors.

LABORATORY RATE COMPARED TO FIELD RATE

The laboratory r a t e s of compression a r e a l l considerably f a s t e r than known field r a t e s . The maximum f o r Leda clay, under the heavily loaded National Museum in Ottawa, f o r example, was only 1/2% p e r yr.17 Compression of

Leda clay under an e a r t h fill18 is occurring a t the maximum known r a t e , approximately 1% per yr. When specimen 96-1-23 w a s loaded f r o m 2 kg t o 4

kg per s q cm, it c o m p r e s s e d 1% in one tenth of a minute, o r approximately

1 5 L e o n a r d s , G. A.,and Ramiah, B. K., U T i m e Effects in the Consolidation of Clays," Special T e c h n i c a l Publication No. 254, ASTM, 1959, pp. 116-130.

16 Northey, R. D., 'Rapid Consolidation T e s t s f o r Routine Investigations," P r o c c e d -

ings, 2nd Australia-New Zealand Conf. on Soil Mechanics, C h r i s t c h u r c h , N. Z., 1956,

PP. 20-24.

. .

1 7 C r a w f o r d , C. B., 'Settlement Studies on t h e National Museum Building, Ottawa, Canada," P r o c e e d i n g s , 3 r d Internatl. Conf. on Soil Mechanics, Z u r i c h , Vol. 1, 1953,

PP. 338-345.

. .

18 Eden, W. J., 'Field Studies on the Consolidation P r o p e r t i e s of Leda Clay,"

Pro-

c e e d i n g s , 14th Canadian Soil Mechanics Conf., Assoc. C o m m i t t e e on Soil and Snow Mechanics, Natl. R e s e a r c h Council, Ottawa, Canada, 1960, No. T M 69, pp. 107-118.

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100 September, 1964 SM 5 five million t i m e s a s f a s t a s the maximum field r a t e . The maximum field r a t e is only approxin~ately 1/50 of the r a t e of secondary compression after a load increment has been maintained f o r a week. Obviously, i f r a t e of compression affects the compressibility of the soil, the r i s k of extrapolating laboratory t e s t r e s u l t s to actualproblems is great. The estimation of p e r m e - ability f r o m consolidation testing is particularly suspect, because of t h e rapidity of loading required t o produce pore water p r e s s u r e i n a s m a l l e l e - ment of soil.

The confusion that can r e s u l t f r o m the concept that p r i m a r y and secondary consolidation a r e s e p a r a t e components of total compression is demonstrated by Figs. 5 and 6. The specimens shown in Fig. 5 have experienced p r i m a r y consolidation only (positive pore pressurethroughout the t e s t ) ; the t e s t shown in Fig. 6 has, according t o the usual definition, experienced secondary consolidation only (pore p r e s s u r e virtually z e r o throughout). The two types of t e s t w e r e conducted at approximately the s a m e average r a t e of strain, but the S e r i e s C t e s t s (Fig. 5) w e r e subjected to a wide variation in r a t e , and t h e compression curve a p p e a r s t o be l e s s reproducible.

These observations lead t o the belief that f u r t h e r development of consolidation theory is being inhibited by preoccupation with an unrealistic p r i m a r y consolidation, which is artificially induced in the laboratory and unrelated t o that in nature. A change i n experimental technique may lead t o m o r e fruitful r e s e a r c h results.

SUGGESTED REVISION TO CONSOLIDATION ANALYSIS

Fundamentally, a saturated, undisturbed clay soil is a complex a r r a n g e - ment of solid particles, held together by interparticle forces, with i t s void s p a c e s full of water. The s t r u c t u r e s o formed r e s i s t s compression and deformation. According to ~ a ~ l o r , l 3 i t has a plastic r e s i s t a n c e that depends, in magnitude, on the r a t e of compression. In the ordinary laboratory consolidation test, the relationship between deformation and applied load is masked by unknown p o r e p r e s s u r e s , which depend on certain t e s t conditions.

The suggestion is made herein that the laboratory consolidation t e s t b e conducted at a steady r a t e of compression, sufficiently slow to prevent development of significant p o r e p r e s s u r e s . This would be s i m i l a r to a drained t r i a x i a l compression t e s t , except that l a t e r a l s t r a i n is prevented. Such a t e s t should provide a m o r e r e a l i s t i c evaluation of soil compressibility by avoiding the impact and the extremely rapid r a t e s of s t r a i n caused by incremental loading. R e s e a r c h could then be extended into a m o r e r e a l i s t i c range of loading r a t e s .

This method will not p e r m i t direct estimations of field r a t e s of consolidation, but the b e s t predictions could probably b e made by estimating the field pore p r e s s u r e s i n the manner described by A. W. Skempton and A. W. ~ i s h o ~ l ~ and by T. W. ~ a m b e 2 0 o r by the method of Skempton and L.

- 1 9 Skempton, A. W., and Bishop, A. W., "The Gain in Stability Due to P o r e P r e s s u r e Dissipation in a Soft Clay Foundation," P r o c e e d i n g s , c i n q u i 6 m e ~ o n g r e s d e s G r a n d s B a r r a g e s , No. R . 57, P a r i s , F r a n c e , 1955.

20 L a m b e , T . W., " P o r e P r e s s u r e s in a Foundation Clay," J o u r n a l of t h e Soil ;Me- chanics and Foundations Division, ASCE, Vol. 88, No. SM2, P r o c . P a p e r 3097, A p r i l , 1962, pp. 19-47.

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SM 5 CONSOLIDATION TEST 101 ~ j e r r u m , 2 1 and then estimating the r a t e of dissipation under field conditions. In t h i s manner, the changes in effective s t r e s s could be computed and r e l a t e d to corresponding changes in volume and s h e a r i n g resistance.

CONCLUSIONS

1. T h e r e i s substantial field evidence that the prediction of consolidation settlement f r o m laboratory t e s t s i s not always satisfactory.

2. Much of the difficulty in predictingconsolidation settlement f r o m labo- r a t o r y t e s t s may be due t o the g r e a t differences between r a t e s of compression in the laboratory and in the field. It i s shown that, in o r d e r t o c r e a t e a hydrodynamic effect, the laboratory r a t e may be a s much a s s e v e r a l million t i m e s a s f a s t a s the field r a t e .

3. The measurement of the preconsolidation p r e s s u r e of uniform undisturbed specimens of Leda clay i s satisfactorily reproducible

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10%) by each of s e v e r a l specific test methods.

4. The variation in the preconsolidation p r e s s u r e , however, i s f r o m 1.3 k g to 3.0 kg p e r s q cm, depending on the loadingprocedure in e a c h t e s t . Much of the difference can be attributed to differences in secondary consolidation, according to the u s u a l definition.

5. P o r e p r e s s u r e measurements, extrapolated to z e r o deflection, suggest that the maximum p r e s s u r e developed in the specimen i s 80% to 85% of the applied load, and that this p r e s s u r e dissipates a s a direct function of deflection. P r i m a r y consolidation, by direct measurement, i s found to be com- pleted a little e a r l i e r than suggested by e m p i r i c a l methods.

6. Ordinary laboratory specimens of t h i s clay can be c o m p r e s s e d by a s much a s 25% in a few hours, e i t h e r with o r without t h e development of substantial pore p r e s s u r e s . P r e s s u r e - v o i d ratio c u r v e s in the two c a s e s a r e s i m i l a r , but, by the u s u a l definitions, the compression in one c a s e i s a l l primary, and, in the other, it i s a l l secondary.

7. The division of consolidation into p r i m a r y and secondary components i s shown to be an a r b i t r a r y separation of a continuous compression p r o c e s s , and the relative contribution of each depends on the method of loading.

8. It i s suggested that m o r e attention be given to measurement of t h e p r e s s u r e - c o m p r e s s i o n c h a r a c t e r i s t i c s of a s o i l independent of hydrodynamic time lag. T h e s e c h a r a c t e r i s t i c s should be investigated a t r a t e s m o r e com- patible with field c a s e s .

The t e s t r e s u l t s presented herein illustrate the substantial influence of r a t e of testing on the confined compression c h a r a c t e r i s t i c s of an undisturbed clay. The g r e a t difference between laboratory and field r a t e s of consolidation suggests the need f o r f u r t h e r study of t h i s obviously important t e s t

2 1 Skempton, A. W., and Bjerrum, L., "A Contribution to the SettIement Analysis of

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variable. However, any consolidation theory developed in the laboratory must be evaluated by field observations.

ACKNOWLEDGMENTS

The w r i t e r is grateful f o r the invaluable a s s i s t a n c e of B. J. Bordeleau in conducting the tests.

This paper is a contribution f r o m the Division of Building Research, NationalResearch Council, Canada, and is published with the approval of t h e Director of the Division.

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K E Y WORDS: clay (materials); consolidation; settlement; soil mechanics; s t r a i n s ; testing -

ABSTRACT: To c r e a t e significant pore water p r e s s u r e in a tiny t e s t specimen of clay, i t must be loaded with large increments. This causes the specimen to compress a t r a t e s to s e v e r a l million t i m e s the r a t e for an equivalent element of soil in the field. A laboratory investigation of compression r a t e s , under incremental and continuous loading, showed that the compressibility of an undisturbed sensitive clay was greatly influenced by the r a t e of compression. These rapid t e s t s gave pressure-compression curves similar to those f o r identical specimens, and demonstrated that the relative contributions of primary and secondary consolidation i s a function of test procedure. When compared with long duratioil incremental loading tests, however, they revealed a much higher preconsolidation p r e s s u r e and l e s s compressibility. It was therefore concluded that the influence of compression r a t e may be important in the interpretation of consolidation t e s t s and warrants more attention and research.

REFERENCE: Crawford, C a r l B., "Interpretation of the Consolidation Test," Journal of the Soil Mechanics and Foundation Division, ASCE, Vol. 90, No. SM5, ~ r 0 c . X 4056, September, 1964, pp. 87-102.