APPLICATION OF CSM TO CEMENTED SOILS
We can adopt the basic tenets of CSM to provide a framework for understanding the stress–strain and failure responses of cemented soils. In most cases, natural soils are cemented with various degrees of cementation. One cementing agent is calcium carbonate that is commonly found in groundwater. Under favorable conditions, calcium carbonate crystallizes; fi lls up some void spaces, reducing the void ratio of the uncemented soil; and bonds soil particles. Let us consider an isotropically and normally consolidated, uncemented soil with a current mean effective stress, pro (Figure ). Recall that for normally consolidated soils, prc 5 pro . The initial yield surface for the normally consolidated, uncemented soil is ABO. Now, assume that the soil becomes cemented, with the cementing agent filling a portion of the void and the soil remaining saturated. Let Cm be the ratio of the volume of the cementing agent to the total initial soil volume. Then the change in porosity is Cm and the change (decrease) in void ratio is Cm (1 1 eo ), where eo is the initial void ratio of the uncemented soil. This decrease in void ratio occurs without any change in mean effective stress, so point O on the NCL for the uncemented soil (Figure 11.43b) moves vertically downward to point O9.
The cementation has the following effects.
1. It causes a change in the state of the soil, converting it from a normally consolidated state to an overconsolidated state. Point O9 must then be on an unloading/reloading line for the cemented soil.
2. The slopes of the NCL, lcm, and the URL, kcm, of the cemented soil are lower than those of the uncemented soil.
3. The cemented soil is now associated with a new, expanded RSW surface, FC . The preconsolidation stress for the cemented soil, prcm, is found as follows. From the compression line for the uncemented soil,
Application of CSM to cemented soils.
while from the unloading/reloading line for the cemented soil,
Subtracting Equation we get
ButTherefore, solving for prcm, we obtain
The CSL of the cemented and uncemented soil is the same provided the volume of soil particles is greater than the volume of the particles of the cementing agent. The degree of overconsolidation depends on the type and amount of the cementing agent. The theoretical expected stress–strain responses of cemented soils would be similar to those of the heavily and very heavily overconsolidated uncemented soils shown in Figure . The difference between the limiting stress surface, DF, for cemented soils and Hvorslev’s surface for the uncemented soils is that there is no tension cutoff. The bonding of the particles by the cementing agent confers a tensile resistance, qcm, to the soil. Failure of cemented soils generally occurs in a diffused failure mode. One or more bifurcations would initiate instability when the ESP approaches or reaches the surface, DF. The stress–strain response of cemented soils cannot be simulated using classical continuum mechanics when bifurcation occurs. Three possible stress–strain responses are shown in Figure . The shear strength of cemented soil, tcm, on the limiting stress surface is
where mcm is the slope of the limiting stress surface. Neither mcm nor qcm is a fundamental soil parameter. They both depend on the type, amount, and distribution of the cementing agent within the soil mass. The value of mcm is found from shear tests by plotting the peak deviatoric stress versus the mean effective stress.