Assessment of the performance of soft ground improvement based on CPTu data

There have been many ways to assess the performance of ground improvement, and in many cases the performance of the soft ground may not be as expected or similar to the design. Most projects base their assessment on the monitoring of geotechnical instrumentations such as settlement plate, extensometer, piezometer, inclinometer and so on. However, the data cannot really satisfy the real objective of the ground improvement, which is to measure the trends of parameters like settlement and the dissipation of excess pore pressures. Therefore, other methods should be employed to fulfil this objective.

The main purposes of ground improvement in soft soils are to increase (a) shear strength and (b) stiffness of the ground, (c) to speed up the dissipation of excess pore pressure and (d) to ascertain stability of the embankment. Geotechnical instrumentation may be useful to answer points (c) and (d); however, the data resulting from the instrumentation do not measure the shear strength nor the stiffness of the soils after ground improvement.

To some extent, knowledge of shear strength is necessary to determine bearing capacity of the ground on application of buildings. The use of CPTu has become more popular to derive the required parameters, and the author has developed some important methods to assess the results of ground improvement. The power of CPTu is because it can be used to generate soil stratification, determine the shear strength and stiffness of the soil prior to and after ground improvement, to determine degree of consolidation, estimate bearing capacity and settlement for footing and/or pile foundations.

A number of case histories are discussed in this chapter, including evaluation of the degree of consolidation at a project site using CPTu (Rahardjo et al., 2008, 2016a), the shear strength increases and the remaining excess pore pressure (Rahardjo et al., 2008). The use of CPTu parameters Bq and Bq* have been very useful to derive the degree of consolidation (Rahardjo et al., 2016b, 2017) and to determine the cone factor (Nkt) which is used to estimate shear strength, as well as some other features of the implementation of CPTu for assessment of ground improvement. In summary, CPTu has been proved to provide important data in addition to the geotechnical instrumentation to achieve the main goals of assessment of ground improvement.