INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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The Effect of Overconsolidation on the Development of Pore Pressure in Saturated Clays

Influence de la surconsolidation sur l’évolution de la pression interstitielle dans les argiles saturées

E. TOGROL, dr.eng., Technical University, Istanbul, Turkey

SUMMARY SOMMAIRE The effect of the initial consolidation process on pore-pressure L’influence de la consolidation initiale, sur l’évolution de la development at the critical state is analyzed. An expression is pression de l’eau interstitielle à l’état critique est étudiée. On a obtained for pore pressure developed at the critical stale in terms établi une expression formulant la pression interstitielle à l’état of overconsolidation ratio and water content at the critical state. critique en fonction du rapport de la surconsolidation et de It is shown that in slightly overconsolidated soils where this la teneur en eau à l’état critique. En effectuant des essais triaxi­ expression is valid, initial consolidation pressures can be pre­ aux à teneur en eau constante sur des argiles légèrement surcon­ dicted through an undrained triaxial compression test. Undrained solidées, on confirme la validité de cette expression. Des essais triaxial test results for Bentler kaolin with varying initial consoli­ réalisés sur des échantillons de kaolin Benller préparés selon dation processes are given in support of the proposed expression. les procédures initiales de consolidation variées, sont en accord avec l’expression proposée.

to measure pore-pressure variations during undrained containing states with higher water contents, all m easured triaxial compression tests with different initial consolidation pore pressures under standard test conditions are positive processes is an equivocal task. However, data obtained for and in this portion overconsolidation is not strong enough homogenous saturated remoulded clays are suitable fo r to cause serious inconsistencies in the measurements. analytical treatment. Satisfactory results may be obtained Furthermore, the experimental evidence for this portion particularly for slightly overconsolidated clays in w hich largely supports the above-mentioned assumptions. little or no negative pore pressure develops when they are shared. PROPOSED RELATIONSHIP Hvorslev (1937) and Peynircioglu (1939) in their classical In an undrained triaxial test at each constant voids ratio works studied the shear-strength properties of overconsoli­ plane, as in Fig. 1, the horizontal difference between stress dated clays extensively. Our knowledge of these properties is further advanced by the use of triaxial tests and the refine­ <7 ment of experimental techniques. Yet it does not seem that a general agreement has been reached, especially fo r strongly overconsolidated samples (Hvorslev, 1960). In this paper, by considering only slightly overconsolidated clays, a con­ venient method of correlating the amount of overconsolida­ tion and pore pressure at failure for saturated soils is suggested. p p. p p =1/3 (oj'+2o- 3 ') CRITICAL STATE OF SLIGHTLY OVERCONSOLIDATED SOIL

In a strain-controlled triaxial test step by step m easure­ fig . 1. A typical stress path of an ment of effective stresses is possible and a plot of these undrained lest with an initial con­ stresses in an appropriate fashion together with th e voids solidation pressure p. ratio would give a stress path as suggested by Roscoe, Schofield, and Wroth (1958). Deviator stress ( q)t m ean effective normal stress (p ), and voids ratio (e), or water content (w) for saturated samples, are selected as co­ ordinates. In this space, stress paths that a particular soil can ex­ perience will be in a domain enveloped by the state boundary surface, and emin and q = 0 planes. All these paths theo­ retically “end” in a critical state line on the state boundary surface. Projection of the critical state line with an angle of tan-13 is named as the critical overconsolidation line. lo g p Critical state and critical overconsolidation lines define a surface which cuts the domain into two. In the portion fig . 2. Consolidation lines.

382 path and applied stress path would give the pore pressure at it, a refinement is brought to the critical state concept with this instant. Therefore, when there is no volume change the immediate practical ends. An important application o f this magnitude and variation of pore pressure as it is m easured expression would be a fast and correct assessment o f over­ in an undrained test could be associated with the effective consolidation ratio of a saturated soil. Once the relevant soil normal stress, deviator stress, and initial consolidation pres­ constants are estimated it would only be necessary to deter­ sure at a given voids ratio. mine the water content and pore pressure of the sam ple at It is interesting to consider the ratio of mean norm al stresses of two points on virgin and overconsolidation lines with the same voids ratio, which is p/p' in Fig. 2. Overcon­ solidation ratio (n) being p'0/p the aforesaid ratio could be written as:

An undrained stress path starts from an initial consolida­ tion pressure p (Fig. 1). In the portion of the domain that we are considering, p could take any value between the virgin consolidation pressure (/?') for this voids ratio and the p'n on the critical overconsolidation line. The values related to the critical state line will be subscripted with f. The excess pore water pressure when stress path reaches the critical state line can be written in two parts.

Uf = ab + be

= % qt + p'n- a- pt. (2 ) q(, p', and p( all could be expressed in terms of the voids ratio and five fundamental soil constants. These constants are slopes of virgin and overconsolidation lines (X,«), voids ratios corresponding to unit mean normal pressures on virgin and critical state lines, and the slope of critical state line (A/). Hence, Equation 2 can be written as i/f = F(w f, n) (3 ) w here F specifies a unique function for the soil considered.

TEST RESULTS 30 35 40 45 A series of strain-controlled triaxial compression tests on Water content at failure, wf (% ) virgin and slightly overconsolidated remoulded saturated fig. 3. Results of undrained tests with different initial samples of Bentler kaolin was carried out. The experim ental consolidation processes plotted with n = constant lines material has the following properties: w, = 63 per cent; drawn from predicted relationship. wP = 30 per cent; activity 0.61; 50 per cent < 2 G = 2.63. In general, standard test procedure and equipment w as used both of which are amply described by Bishop and Henkel (1 9 5 7 ). Experimental evidence was obtained in support of the existence of a unique relationship between the maxim um deviator stress at failure and mean effective normal stress and the water content, i.e., the critical state line (Togrol, 1962). Making use of the experimentally determined soil constants Equation 3 is written for kaolin

( ( 40.5 «" = V -° 'I+ « °V eXPV ' 7.5 - )■ (4) Experimental evidence appears to confirm this predicted relationship (Fig. 3). In Fig. 4 the typical stress path of an undrained test with overconsolidation ratio 3 is show n to­ gether with the stress path of a virgin consolidated sam ple at the same voids ratio. A nomogram is also prepared fo r the prediction of u{ from overconsolidation ratio and vice versa for a given critical state water content (Fig. 5).

DISCUSSION AND CONCLUSIONS Equation 3 illustrates the possibility of expressing pore pressures at critical state in terms of overconsolidation ratio fig. 4. Results of an undrained test on a sample with n — 3; and voids ratio under the assumptions made above. T hrough Bentler kaolin.

383 Water content the critical state in an undrained test. At present, it may not be justified to carry this prediction further than for slightly

Pore pressure overconsolidated clays.

(kg/cm 2) ACKNOWLEDGMENT The author gratefully acknowledges the kind support given to his work by Ord. Professor Dr.- Ing. A. H. Peynir cioglu.

REFERENCES

B ish op , A. W., and D. J. H enkel (1957). The measurement of soil properties in the tr ¡axial test. London, Edward Arnold. H vorslev , M. J. (1937). Über die Festigkeitseigenschaften gestörter bindiger Böden. Ingeni0rvidenskabelige Skrifter, A Nr. 45, K0benhavn. ----- (1960). Physical components of the shear strength of saturated clays. Proc. A.S.C.E. Research Conference on Shear Strength of Cohesive Soils. P eynircioglu , A. H. (1939). Über die Scherfestigkeit bindiger Bodenarten. Berlin, Julius Springer. R oscoe , K. H., A. N. S chofield , and C. P. W rot h (1958). On

f ig . 5. Nomogram drawn from proposed the yielding of soils. Geotechnique, Vol. 8, pp. 22-53. relationship between pore water pressure T ogrol , E. (1962). Kohezyonlu zeminlerde kayma gerihnesi, at failure, overconsolidation ratio, and efektif basmc, ve su muhtevasi arasmda baglanti. Istanbul, water content at failure. I.T.Ü. Insaat Fakültesi.

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