Interpretation of results Theoretical approach to obtaining mechanical soil parameters

Figure 1.34 shows a typical applied pressure-settlement diagram. As can be seen, two loading and unloading cycles are usually performed. The first, lower-amplitude cycle is intended to ensure close contact between the plate and the surface of the ground, since this, despite being previously leveled, may not be completely flat.

Typical pressure-settlement diagram of a plate load test

Figure 1.34 Typical pressure-settlement diagram of a plate load test.

If the test is conducted until failure of the foundation soil, Equations 6.16 and 6.15 in Chapter 6 may be applied for the evaluation of the undrained shear strength, c,„ or the effective strength parameters, c' or ', according to the type of loading in undrained or drained conditions, respectively.[1]

This is not, however, the most usual way of interpreting the results of plate load tests. Indeed, under current conditions, the test is conducted involving much lower stresses than those corresponding to soil failure, allowing the estimation of the deformation modulus, by means of the interpretation of results in the light of the theory of elasticity. As will be seen in Chapter 6, the settlement, s, of a rigid shallow foundation, of circular area of diameter B, for an applied loading pressure Aqs, on a linear and homogeneous elastic half-space, with a Young’s modulus E and Poisson’s ratio v, corresponds to (according to Section 6.3.2):

This equation allows to evaluate the stiffness modulus of the soil, taking the coordinates s and Aq$ from a point on the test diagram and adopting a value for v. In the case of clayey soils, the loading of the test occurs in undrained conditions, for which a Poisson’s ratio equal to 0.5 should be adopted. For granular soils, the stiffness modulus is estimated under drained conditions, and it is reasonable to adopt values of 0.2 to 0.3 for Poisson’s ratio. For these soils, intended for loading under drained conditions, there is a need to wait for the settlements to stabilize at each loading step. In certain cases, when the soil has a significant fines content, this may require some time, leading to a rather time-consuming test.

There is no single criterion for choosing the point in the diagram at which to proceed with the calculation presented above. If the objective is to predict stiffness for the loaded soil at a given value of Aqs, it seems reasonable to select the point where the ordinate corresponds to that value."

When these tests are performed to characterize foundation soils or embankments for road or railway works, it is common to designate the stiffness moduli obtained for the 1st and 2nd loading cycles by £V, and EV2, respectively. In view of what was previously mentioned regarding the two loading cycles, it should be understood that the £V2 modulus is considered the more representative, although the £V,/£V, ratio should also be taken into account (AFNOR, 2000).

It should also be noted that Equation 1.31 is valid when the test is performed on the ground surface or at the base of a preliminary excavation (which should be at least five times wider than the plate diameter). Otherwise, the ground stresses above the test platform will tend to influence the results, reducing the settlement, and thus leading to an overestimation of the modulus. In these cases, the use of a correction factor is recommended (see Section Direct use of test results

When the test is conducted to predict settlements of a shallow foundation, the previously exposed approach may not be the most advantageous because the deformation modulus obtained is representative of a very superficial ground horizon, the thickness of which is of the same order of magnitude as the plate diameter. That modulus may be considerably lower than that affecting the behavior of the actual foundation because, even in a homogeneous soil mass, stiffness typically increases with depth, due to the increase of the effective stresses.

Considering, as shown in Figure 1.35a, a footing and a plate with the same geometric shape and under the same applied pressure, the ratio of the respective settlements, s/sp, would increase linearly with the ratio of the respective diameters, B/Bp, in cases where the modulus is constant in depth (see Equation 1.31). As will be discussed in detail in Chapter 6, the increase of the modulus with depth reduces the influence of the size of the foundation on the settlement (see Section This is reflected in Figure 1.35b, which illustrates a proposal for the evaluation of the footing settlement, based on the plate settlement, for the same pressure, in sands (Bergdahl et al., 1993). This proposal is valid if the sand layer has a thickness of at least twice the diameter of the foundation.

Example of direct use of plate load test results

Figure 1.35 Example of direct use of plate load test results: a) plate and footing of the same geometry, under the same pressure, on sandy soil; b) the ratio of the footing and plate settlements versus the ratio of the respective diameters (Bergdahl et al., 1993).

Final remark

As will be seen in Chapter 10, plate load tests are of great importance in characterizing the quality of compacted fills for road and railway works.

In addition, when the test is performed with the largest plates of the aforementioned range, allowing for a much larger volume of soil than most other field tests, it is especially suited for the characterization of the stiffness of natural soil masses or embankments composed of coarse gravels or other large particles. One aspect that severely limits a broader application of the test is that it requires direct access to the layer to be characterized, so that its implementation becomes difficult - and therefore even more costly and time consuming - at significant depths.

  • [1] In this case, with the plate at the ground surface or at the base of an excavation (much larger than the platediameter): (i) in a sandy soil, the vertical bearing capacity,
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