Over-consolidated clay

If the current stress state of volumetric effective stress is lower than the stress state experienced in the past, the clay is said to be over-consolidated, and we define the over-consolidated ratio as OCR= py p[ where p is the preload pressure and p l 

6 Classical Theory of Consolidation for Saturated Porous Media 

The response of an over-consolidated clay depends on the initial state, namely from the position in relation to the critical state line (CSL). As shown in Fig. 6.3d the clay, which is located at the left side of the CSL is heavily over-consolidated, referred to as the dry side or drier than critical, while the clay that is located at the right side of the CSL is lightly over-consolidated, referred to as the wet side or wetter than critical. 

We observe a softening response, as shown Fig. 6.3a, b for a drained test, and the stress reaches a residual state described as R in Fig. 6.3a after a peak point Q the volumetric strain changes at Q from compression to dilation. 

Let us consider the shearing behavior of a heavily over-consolidated clay. We start to make it swell from a point A on the normally consolidated line (NCL) to a point B shown in Fig. 6.3d, e. When we perform an undrained triaxial test, the shear stress q attains the yield point P on the Hvoslev surface (namely the maximum point for the stress ratio rj = q/p ), and moves on the Hvoslev surface to reach the point C on the CSL. On the other hand, if a drained triaxial test is performed from the point B, the shear stress q attains the peak strength Q, then the stress reaches a residual state R through a softening process. Note that in practical experiments a shearing slip surface is commonly observed, and the whole specimen cannot reach the residual state. 

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Butler, F. G. (1975), Heavily over consolidated clays. Review paper Session in. Settlement of Structures, Proceedings of a Conference Organized by the British Geotechnical Society, Cambridge, Pentech Press, London, pp. 531-572. 

Figure 7. An oedometer test on an over-consolidated clay.

The sensitivity of clays is defined as the ratio of their undisturbed and remoulded strength, and varies from about 1.0 for heavily over-consolidated clays to values... 

Till consists of a variable assortment of rock debris that ranges in size from fine rock flour to boulders and is characteristically unsorted (Bell, 2002). The compactness of a till varies according to the degree of consolidation undergone, the amount of cementation and size of the grains. Tills that contain less than 10% clay fraction are usually friable, whereas those with over 10% clay tend to be massive and compact. 

For this second deformation the settlement will be based on the thickness of the consolidated clays over the softened clays and on the diameter of these. 

The determination of the settlement of a uniform charge of width B over the consolidated clays of Hj power, situated over non-deformable layer is a widely studied problem. 

The high rim of this beam is the covering of consolidated clays over the softened area, whereas the span of the beam is in accordance with the diameter of the softened area. This calculation model is shown is the following figure 5. 

Lower alameda formation Pleistocene-age, dense to very dense sand and gravel interbedded with hard over-consolidated low plasticity clay. 

Comparing these figures with the pre-consolidation stresses as determined in the laboratory suggests that the soft clay is normally consohdated, while the firm clay is slightly over-consolidated. 

Above the water table, groundwater can also occur in perched aquifer conditions. In these instances, groundwater occurs in relatively permeable soil that is suspended over a relatively low permeability layer of limited lateral extent and thickness at some elevation above the water table. Perched groundwater occurrences are common within the vadose zone high-permeability zones overlie low-permeability zones of limited lateral extent in unconsolidated deposits. However, perched conditions can also occur within low-permeability units overlying zones of higher permeability in both unconsolidated and consolidated deposits. In the latter case, for example, a siltstone or clay stone overlies jointed and fractured bedrock such that groundwater presence reflects the inability of the water to drain at a rate that exceeds replenishment from above. 

First the round wall is constructed and then small pieces of wood are laid down all over the floor these pieces are mainly wooden branches. The raw wood which is cut to a suitable length is taken into the oven piece by piece and stocked vertically and closely on the floor. The raw wood is then covered by many small pieces of wood to form, a spherical surface on top. It is covered with straw-mats or rice straws and then with moist clay, which is consolidated by beating it this completes the oven. Then wood is burnt for a long time ( about one day ) at the ignition hole to ignite the raw wood. When a large amount of white... 

As shown in Figure 8 (41), the tailings solids sediment rapidly over a period of months up to approximately 20% solids (Stokes law sedimentation up to 5% solids, hindered sedimentation after that). The fine tails then consolidate over the next 5 years (or 3-5 years for some ponds reference 44) to approximately 30% solids, at which point consolidation slows dramatically. Scott and Dusseault (40) postulated that this drop in the consolidation rate is due to the high concentration of unrecovered bitumen in the MFT (which leads to a hydraulic conductivity 0.1-0.01 of that of equivalent material with no bitumen) and the low effective density of the MFT solids (5, 41, 45-48). As discussed earlier, the interaction of mineral components has also been postulated to explain the lack of consolidation. This implies a floe structure determined by the clay mineral interactions that are, in turn, a function of the water chemistry. Syncrude MFT (fines content > 30 wt%) have a low permeability of 1-0.01 nm s 1 (46, 47). The dewatering behavior is completely dominated by this low permeability and the long drainage paths that result when the MFT are stored in deep ponds (49). 

Figure 2.1 illustrates the results of cumulative electrical charge versus cumulative volume of flow for various different clay and clay mixtures when the major contributing ion in the soil matrix was cadmium. In these experiments, the initial concentration of Cd" in each medium was determined after mixing clay slurry with a concentrated solution of cadmium salt, which was left to consolidate to a constant effective stress over a few weeks, and measuring the total cadmium retained in the compressed clay matrix. This procedure allowed uniform distribution of the ion in the clay, the time necessary for potential ion exchanges to take place, and more importantly, the full development of the electric double layer at the ion concentration retained within the clay bulk fluid. 

Waterfront area consisted of approximately 15 ft of loose (N = 5) hydraulic fill (fine sand with silt and clay) overlying 5 ft of medium dense (N - 30) fine to coarse sand 15 ft of gravel, followed by 16 ft of consolidated soft material with coarse sand... 

Consolidation of silt is influenced by grain size, particularly the size of the clay fraction, porosity and natural moisture content. Primary consolidation may account for over 75% of total consolidation. In addition, settlement may continue for several months after construction is completed because the rate at which water can drain from the voids under the influence of applied stress is slow. 

The compressibility and consolidation of tills are determined principally by the clay content, as is the shear strength (Table 5.17). For example, the value of compressibility index tends to increase linearly with increasing clay content, whereas for tills of very low clay content (less than 2%), this index remains about constant (C = 0.01). The shear strength of till can range from 150 kPa to over 1.5 MPa. 

The settlement due to a uniform load on the consolidated unit clays situated over a non-deformable stratum. 

In order to understand better the proposed approach, the example of the oedometer test is again explored. Traditional practice is to structure the laboratory session so that a student group undertakes the experiment, in essence, several times to develop a consolidation curve. Each point on the curve is obtained by applying a load to a soil specimen and recording the settlement over a period of around 30 minutes. Many clays often require a much longer period of time to consolidate and specially selected or amended clay is needed to achieve primary consolidation within 30 minutes. Usually, 6 to 8 points are needed to generate a representative consolidation curve. Hence, the time needed in the laboratory can be quite extensive and the measurement process itself is extremely dull and tedious. 

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