Normally consolidated clay

If we perform undrained triaxial tests (namely, void ratio e=constant) on isotropically and normally consolidated clay at several confining pressures p, the stress-strain behavior is schematically shown in Fig. 6.2. It is observed that at the final stage of loading (namely, the failure state) the stress ratio rj = q/p becomes constant qf = q/p )f = M), which is referred to as the critical state, which means that the deformation is developed under a constant volumetric plastic strain and a constant shear stress at the critical state q = Mp.  

This result is similar to drained triaxial tests (namely, CTj = constant, dp = — (l/3)rfa(, dq= — da, dq/dp = 3). As shown in Fig. 6.2d, if an undrained stress path B-B under a consolidation pressure p g intersects with a drained stress path A-A under a consolidation pressure a point C, the void ratio obtained from the undrained test is the same as the void ratio obtained from the drained test. We can draw these states in a space p, q, e) as illustrated in Fig.6.2f, which shows that the critical state is reached after travelling the surface referred to as the state boundary surface or Roscoe surface in both the undrained and drained tests. The line of failure is known as the critical state line (CSL). It is noted that the projection of CLS in the space p, q, e) onto the space (p, q) is given as = Mp, and the surface formed by CLS and its projection onto q, e) is referred to as Hvoslev surface, which is a failure surface found by Hvoslev in 1937 through a series of direct shear tests conducted on Vienna clay. 

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For normally consolidated clay deposits in the Gulf of Mexico and many other regions of the world. 

Schmertmann 1. ICUtriax. i e Conduct consolidation test and Assumes that strength versus water content relationships and consolidation curves for normally consolidated clays will always be parallel. In addition appreciable errors in strer th may result from a small error in fixing the slope of the field strength curve. Other problems are based on predicting Schmertmann (1956)... 

Togrol and Guler (1984) suggested an empirical relation for normally consolidated clay that related the deviatoric stress at failure to excess pore pressure developed during cyclic loading ... 

Most offshore piles in normally consolidated clay have a 713 value in the range of 10-100. A plot of data for various pile types versus 713 on a log scale is presented in Figure 10.9. Focht Jr. and Kraft (1983) believe that lambda versus log 7 3 is the best single correlation applicable to all clay profiles however, for normally consolidated clays both lambda and beta with log 7I3 provide good correlations. 

For a normally consolidated clay where v(/ is constant and generally less than 0.4, the range of a lies between 0.8 and 1.0. For overconsolidated clays the v / ratio varies and therefore a will also vary. 

At the same cyclic shear stress level x y, normally consolidated clay will be more resistant to cyclic loading than overconsolidated clay. This means that on a total stress basis a foundation on normally consolidated clay may be designed with lower safety factor for cyclic loading than one with overconsolidated clay. 

Shear modulus and failure shear stress in undrained static loading are reduced by undrained cyclic loading. Effective shear stress parameters C and ( ) are not influenced by undrained cyclic loading for overconsolidated clay. For normally consolidated clay C seems to increase. For both normally consolidated and overconsolidated clays, cyclic loading will influence the pore pressure response to undrained static loading. 

The most frequently used gradients in many clay soils vary between 30 and 45°. In some clays, however, in order to achieve stability, the slope angle may have to be less than 20°. The stability of slopes in clay depends not only on its strength and the angle of the slope but also on the depth to which the excavation is taken and on the depth of a firm stratum, if one exists, not far below the base level of the excavation. Slope failure in a uniform clay soil takes place along a near-circular surface of slippage. For example, the critical height, H, to which a face of an open excavation in normally consolidated clay can stand vertically without support can be obtained from ... 

For most buildings, it is the relative deflections that occur after completion that cause damage. Therefore, the ratio between the immediate and total settlement is important. In overconsolidated clays, this averages about 0.6, whereas it usually is less than 0.2 for normally consolidated clay. This low value coupled with larger total settlement makes the problems of design for normally consolidated clays much more demanding than for overconsolidated clays. 

Unfortunately, the geoteehnical community tends to rely on a number of old empirieal eorrelations that were derived from a small and early data set that are not at all applieable to the situations for whieh they are now applied. Case in point A rather reeent textbook (circa 2008) indicates the following two correlations (cited back-to-back) for use in estimating the undrained shear strength of soft normally-consolidated clays ... 

Within the simplified CSSM, the undrained shear strength ratio for normally-consolidated clays in DSS can be evaluated as (Wroth 1984) ... 

The soft clays were modelled using effective stress parameters, assuming linear elastie behaviour bounded by a Mohr-Coulomb envelope t/s = sin

shear stress/normal stress diagram for plane strain, in which the Mohr-Coulomb envelope is marked, together with a typical effective stress path for undrained behaviour of a normally consolidated clay. This path reaches failure at point F on the Mohr-Coulomb envelope with undrained strength c . 

This effective stress approaeh was known in the inquiry as Method A. An alternative approaeh for undrained behaviour. Method B, is simply to speeify the undrained strength of the material, c , rather than the effective stress parameter pore pressures are still wrong. This becomes more problematic if consolidation is to be modelled following an undrained stage. 

A tangent to the curve at these stress levels would produce quite different values of Cc- Except for soft normally consolidated clay, the compression index is both of uncertain value and irrelevant to engineering situations. The meaning of Cc for a residual soil has not been seriously addressed. As originally conceived Cc was the slope of the virgin consolidation line. No such line exists for a residual soil, and Cc seems to be taken as the slope of the tangent to the end of an e-log(p) graph. As such it is of arbitrary value and of no practical use. 

The shearing behavior of a lightly over-consolidated clay is as follows initially it swells from a point A on the NCL to a point D, as shown in Fig. 6.3d, e. Then, after performing an undrained triaxial test, the shear stress q directly attains the point C on the CSL. Thus, the behavior of a lightly over-consolidated clay is different from that of a heavily over-consolidated clay, and is similar to the normally consolidated clay. On the other hand, if we perform a drained test, the stress q reaches the CSL at a point F, after which the clay experiences plastic flow under a constant volumetric plastic strain. 

Similarly, the undrained shear strength of a normally consolidated clay can be hnked to its consistency, expressed as the hquidity index as shown in Figure 8.11. (see also box soil consistency in section 9.1.3.2). 

Fi gure 8.10 Effective friction angle for normally consolidated clays (Terza i et al., 1996). 

Figure 8.11 Shear strength of soft normally consolidated clay as a function of liquidity index ( Wasti and Bezirci, 1986 after Skempton and Northey, 1953).

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