What is the difference between drained and undrained shear strength

In general, we normally treat coarse-grained soils such as sands and gravels as drained materials bcoz their permeability is high and therefore water can flow freely through the large and continuous void spaces. You can consider water flow in fine-grained soils is a little like a game of snakes and ladders whether water advances to a certain pt but then as to backtrack as the void spaces come to an abrupt end.

Thus, the porous nature of soil has a direct influence on soil strength. We can illustrate this by again calling on Mohr circle for 2D stress as many practical problems can be treated by analysis in 2D. Consider a representative element within the bulb of soil influenced by the rafts floating. The Mohr circle for these stresses looks like so,. This is bcoz the raft loading increases both the horizontal and vertical normal stresses.

These increases take place in unison as the load is transferred directly into greater intergranular stresses. Its dist away being a measure of the foundations FOS. The K 0 or in-situ stresses remain essentially the same as before. This time however that the foundation load is applied, the Mohr circle will again shift to the right but its dia remains constant. Hence the soil grains are prevented from reconfiguring into a denser stronger structure. Then the application of an increasing confining pressure in each test will simply mean that the pore water pressure in each specimen is increased by the same amount.

Also note that the characteristics of all three specimens in terms of eff stress is represented by the same circle. This is an imp concept to understand. The lesson here is under undrained loading, saturated fine-grained soils will have a strength limited by their eff stress prior to loading. But in time, assuming the soil has not failed under the loading its strength will increase with transfer of load from the pore water to the soil grains.

For overconsolidated clays, c' is not very large and is typically within the range of 0 to psf. By dissipation of pore water pressure, i. It can be derived from an unconfined compression test UU triaxial , vane shear test or simply using a pocket penetrometer. On the other hand, drained parameters are derived from more expensive consolidated drained CD or undrained CU triaxial tests.

Figure 2 presents one of the well-established correlations. Figure 2. If the groundwater level is way too deep from the ground surface metres , for clay soils, can undrained parametres be used? Constant Cohesion is constant throughout the material. F Depth from Top of Layer Cohesion is a function of depth, where depth is measured from the top of the material layer that is l ocal to the slice , to the center of a slice base. Cu Top is the undrained Cohesion at the top of the material layer local to the slice.

Cu Change is the rate of change of undrained Cohesion with depth. If you wish to specify a maximum undrained soil strength, select the Cutoff checkbox and enter a maximum value for Cohesion. If the rate of Cu Change is negative, then the Cutoff value represents the minimum soil strength. F Depth from Horizontal Datum Cohesion is a function of depth, where depth is measured from a user specified Datum y-coordinate to the center of a slice base.

Cu Datum is the undrained Cohesion at the Datum elevation. However, we are not sure if it was really undrained or partially drained. Also we do not know how much the viscous effect contributed in the penetration resistance. When we plot the results of the CPT example from the previous slide on Schnider et al. Can we use the results of this CPT in the design of drained or undrained cases? Where v is the penetration rate, d is the diameter of the penetrometer and cv is the coefficient of consolidation.

Randolph and hope and Schneider et al. Therefore, for the same soil penetration resistance of a spudcan can be lower than the recorded resistance by a CPT. Range of uncertainty: Depends on the application. Sometimes very high. All third party information featured within the presentation remains the intellectual property of their respective originators.

David White, Dr. Fraser Bransby and other colleagues at Fugro. Total views 11, On Slideshare 0. From embeds 0. Number of embeds 3. Downloads Shares 0. Comments 0.