Necessity for in situ tests

Liquefaction assessment methods have therefore developed with a heavy reliance on in situ testing. Initially this was the SPT, but that test has poor repeatability and the variable contact area with the soil makes precise evaluation of the data difficult. It also does not measure excess pore water pressure, and important factor with finer grained soils. The modern electronic piezocone or CPTu test, with its accuracy and resolution of even thin layers in the soil, has effectively displaced the SPT. 

The CPT is an acceptable test under every methodology for liquefaction assessment. The CPT is a necessary test for any of the advanced methods of liquefaction assessment, particularly when moving into 2D cases of soil structure interaction or if looking to offshore situations. The CPT will even be necessary if taking undisturbed samples for cyclic simple shear testing, both as a means to extrapolate the test data obtained across the area of the site (only a few of these laboratory tests will normally be carried out) as well as provide data to estimate the sample disturbance from in situ to as-tested conditions. For a description of the CPT, see Appendix B-2.3.2. 

If there is no access to a seismic CPT, shear wave velocity can also be measured by lowering geophones down a PVC cased borehole and measuring shear wave arrival times at regular depth intervals down the hole. The shear wave source can either be at the ground surface (vertical shear wave velocity profiling) or in an adjacent borehole (cross hole shear wave velocity measurement). 

More recently (in the last 25 years or so), surface wave methods have become popular for measurement of shear wave velocity. These include SASW (spectral analysis of surface waves, e.g. Stokoe et al, 1994), see Appendix B 5.3.6 and MASW (or multichannel analysis of surface waves, e.g. www.masw.com by Park Seismic LLC). The basis of the technique is the dispersive property of Rayleigh-type surface waves when propagating in a layered system. No subsurface drilling or equipment is needed, but more sophisticated data processing is required to determine the shear wave velocity. 

Although the CPTu, SCPT, and geophysical methods are standard and necessary for characterizing hydraulic fills, they are not the only applicable tests. In some circumstances the self-bored pressure meter can provide data on the degradation of soil modulus with strain (useful for evaluation of lateral pile capacity in fills, especially under earthquake loading). The cone pressure meter , a combination of CPT and full-displacement pressure meter may also be useful for the same purpose. 

---