PART 5 RECONSTRUCTING THE PRESSUREMETER CURVE
Given po, a , b , cu and the measured maximum pressure and cavity strain at the end of the test, equations [17], [23], [26] and [27] permit the total pressure:cavity strain curve to be calculated.
In principle only po is in doubt, so overlaying the calculated curve on the measured curve allows po to be iteratively adjusted until the two curves co-incide. In practice po being doubtful also implies the origin for strain is not correct. By taking advantage of the fact that the origin for the contraction event is fixed and hence the shear strength derived from the unloading must be right, the strain origin is adjusted until loading and unloading shear strengths agree. Adjusting po is the final step. Figure 20 is an example from a test on this contract.
Fig.20 Curve comparison
Of course the procedure only works if the solution is an appropriate one for the material under test. In the example given the adjustments to the strain origin are small and the derived value for po is close to that from inspection (lift-off). The usual discovery in stiff clay is that the origin need to be moved in a negative direction a small amount, consistent with a view of the test as under-drilled.
Given that there is only one degree of freedom in the procedure it is a fairly rigorous example of the curve modelling approach.