Question

In a Consolidated-Drained (CD) triaxial test, why is pore water pressure allowed to dissipate completely during both the consolidation and shearing stages?

Accepted Answer

In a Consolidated-Drained (CD) triaxial test, pore water pressure is allowed to dissipate completely during both the consolidation and shearing stages to ensure that the effective stress within the soil sample is always known and that the measured shear strength parameters are expressed in terms of effective stresses. Pore water pressure is the pressure exerted by the water within the voids (pores) of the soil, and its dissipation means that this pressure is allowed to equalize with the ambient pressure, typically atmospheric, by permitting water to flow into or out of the soil sample. This process is facilitated by permeable end plattens and a drainage system connected to the sample. The effective stress principle states that the total stress applied to a soil is carried partly by the solid soil particles (effective stress) and partly by the pore water (pore water pressure), with the effective stress primarily governing the soil&#x27;s strength and deformation behavior.  During the consolidation stage, the soil sample is subjected to an all-around confining pressure (total stress). When this pressure is applied, it initially increases the pore water pressure within the sample. By allowing complete dissipation, the excess pore water pressure is forced out of the sample, leading to a reduction in its volume as the soil particles rearrange and come closer together. This process continues until the pore water pressure returns to its initial value (often zero gauge or atmospheric), and the entire applied confining pressure is then borne by the soil skeleton as effective confining stress. This ensures that the soil sample is fully consolidated under the desired effective stress state, mimicking the long-term, drained conditions that often occur in the field where slow loading allows pore pressure to dissipate. If dissipation were not complete, the effective confining stress would be lower than intended, and the sample would not be properly prepared for testing. During the subsequent shearing stage, an axial load (deviator stress) is applied to the consolidated sample at a very slow rate. The slow loading rate is critical because it provides sufficient time for any tendency for pore water pressure to change (either build up or decrease) to be entirely offset by water flowing into or out of the sample. Instead of pore water pressure changing, the sample undergoes a measurable volume change (either compression or dilation). This condition is referred to as &#x27;drained&#x27; because water is allowed to drain freely. By maintaining a constant pore water pressure (typically zero gauge) throughout shearing, the effective stress on the failure plane is directly equivalent to the total applied stresses, making it straightforward to determine the effective cohesion (c&#x27;) and effective angle of internal friction (φ&#x27;) directly from the Mohr-Coulomb failure envelope. This approach ensures that the fundamental shear strength parameters, which depend only on the interaction between soil particles, are accurately measured without the influence of transient pore water pressure changes.

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In a Consolidated-Drained (CD) triaxial test, why is pore water pressure allowed to dissipate completely during both the consolidation and shearing stages?