Project Title: Use of Full-Flow Penetrometers for Measuring Shear Strength and Consolidation Characteristics of Soft Ground
Student Name: Cathal Colreavy
Supervisor: Dr. Conleth O’Loughlin
Funding Body: Strand 1 to Masters Level; IRCSET Embark to PhD
The extensive road infrastructure program currently underway in Ireland presents significant geotechnical design challenges as many of these routes traverse soft ground. Furthermore, the recent emphasis on offshore renewable energy requires devices to be located in deep water where seabed sediments are typically soft. This presents significant geotechnical design challenges that require accurate measurement of the undrained shear strength (su) and the rate at which su increases during loading. Difficulty in obtaining undisturbed samples has led to increased reliance on in-situ penetration testing in which a probe (Figure 1) at the end of a series of rods is pushed vertically into the ground at a constant rate. su is derived from the penetration resistance (q) of the probe through a constant factor (N); su=q/N. The probe is pushed into the ground using a specially designed rig (Figure 2). The most common type of probe is the cone. However this device suffers from a number of drawbacks such as the unequal balance of stress around the cone and the difficulty in establishing an appropriate N factor. Recently full-flow probes have emerged, such as the T-bar or ball, which have a number of advantages over the cone. The main advantage is the availability of accurate plasticity solutions that provide computed N values which have been shown to be much better behaved than the corresponding empirical determined N values for the cone. However these plasticity solutions do not take account of real soil behaviour, namely strain rate and strain softening effects.
Figure 1 Various penetrometer probes
Figure 2 Truck mounted CPT rig (In Situ Site Investigation Ltd.)
In recent years, pore pressure measurement has been introduced into ball penetrometers with pore pressure measurement at different locations (piezoball). This has advanced the capability of these devices for the assessment of soil parameters. To date, piezoball penetrometers have been used with pore pressure elements at various locations. However there is not yet any clear guidance on the optimum location to measure pore pressure on the ball. The piezoball used in this study (Figure 3) measures pore pressure at three locations (the tip, mid-face and equator).
The primary aims of the proposed research are to:
- Establish a database of reduced scale laboratory full-flow penetrometer tests to examine the combined effects of viscous strain rates and strain softening.
- Conduct full-flow penetrometer field tests to verify adjusted N values established from the laboratory tests (in objective 1).
- Assess the relative merit of the piezoball dissipation test in determining the coefficient of consolidation in the field and centrifuge.
- Establish the optimum design for the piezoball which can be used in industry.
It is expected that meeting these objectives would represent a significant contribution to the advancement of this technology, and that realisation of these objectives would provide the experimental evidence and confidence their use as a standard site investigation tool for soft ground.
Figure 3 Piezoball showing tip, mid-face and equator filter positions
- Colreavy, C., O’Loughlin, C.D., Long, M., Boylan, N. & Ward, D. (2010) Field experience of the piezoball in soft clay. Proc. 2nd Int. Sym. On Cone Penetration Testing, CPT10, Huntington Beach, May.
- 7th International Conference on Physical Modelling in Geotechnics (ICPMG 2010), June 2010, Zurich.