The results of penetration and dissipation tests performed using piezocone and piezoball at a research site in Western Australia are compared in this paper. It was observed that the pore pressure measured at the mid-height of the piezoball is consistently lower than that measured at the shoulder (u2 position) in the piezocone tests, but the reverse is true during extraction. It was also found that the deduced values of in situ horizontal coefficient of consolidation, ch, are about four times the values of vertical coefficient of consolidation, cv, measured at the yield stress during constant rate of strain consolidation tests on high quality Sherbrooke samples. It is also interesting to find that the rate of excess pore pressure dissipation around the piezoball was, on average, about 2.5 times faster than the rate of excess pore pressure dissipation around the piezocone when the two penetrometers have the same diameter.
The determination of consolidation or permeability characteristics of soil is necessary in a wide range of geotechnical design problems, particularly ones that involve partial consolidation and in which construction time scale is of primary concern. For instance, consolidation parameters are important in estimating the increase in pile or suction caisson capacity after installation before peak design loads occur, since partial consolidation could render the design inadequate. Erbrich1 also showed that knowing the coefficient of consolidation in intermediate soils (such as silts) is vital in the interpretation of in situ tests and predicting spudcan foundation behaviour. For pipeline design, consolidation parameters are also important for assessing the extent to which pipeline motions may be considered drained or undrained and the time scale for which consolidation and recovery of strength occurs following remoulding of the seabed due to cyclic pipeline movement. In view of the importance of the consolidation characteristics of soil in geotechnical design problems, there is strong incentive for reliable and cost effective methods of estimating consolidation parameters from in situ tests.
In practice, consolidation parameters are normally estimated from laboratory consolidation tests on (nominally) undisturbed samples. However, it is difficult and expensive to recover high quality samples from offshore sediments, and also laboratory consolidation tests are generally restricted to purely vertical consolidation. In situ tests can provide a relatively cost and time effective alternative for estimating the coefficient of consolidation of offshore sediments, avoiding concerns over the quality of the recovered samples and also modelling more three-dimensional consolidation processes.
One of the most common in situ tests that are used for the estimation of in situ coefficient of consolidation is the piezocone dissipation test. The coefficient of consolidation is estimated at discrete depths of interest by monitoring the decay of excess pore pressure with time, and comparing the time for a given degree of dissipation with theoretical solutions. The pore pressure can be measured at different locations on the piezocone i.e. at the cone tip, on the mid-face of the cone, just behind the cone shoulder and immediately behind the friction sleeve2. However, published experience3, 4 suggests that dissipation tests with the pore pressure measured