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Chapter 4
Part 1: Special cones: geotechnical cones
Flow penetrometers
| FLOW PENETROMETERS 4.8 Undrained shear strength Undrained shear strength profiling is an important part of many ground investigations, especially in soft clay. This is normally achieved by insitu vane testing or laboratory strength measurements on undisturbed soil samples. This approach suffers the disadvantage that such tests can only be taken at discrete, well spaced depths in the profile and the presence of stiffer materials can affect the results obtained. In contrast, the CPT provides a continuous measurement of undrained shear strength, but, regrettably, the CPT is not very accurate in soft clay deposits due to the low tip resistances being measured. Also the deformation mechanism around the cone during penetration is asymmetric in the vertical plane; consequently a correction for overburden pressure and porewater pressure is also required. The empirical and theoretical solutions relating undrained shear strength to cone tip resistance are difficult to apply objectively and resulting estimates can therefore be erroneous. |
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| Composition Many of the limitations outlined above can be overcome by varying the shape of the penetrometer such that it causes symmetrical flow during penetration. Two such devices, the T-Bar and the spherical ball are shown in Figure 27. The T-bar consists of a short cylindrical bar measuring 250 mm in length and 40 mm in diameter, while the ball cone is 163 mm in diameter. Both devices are attached at right angles to penetrometer rods, just below a calibrated load cell. Also included in the shaft is an inclinometer to indicate any deviation from the vertical during insertion. The device can also incorporate porewater pressure transducers. Advantages Flow penetrometers have two major advantages over the conventional electric cone. Firstly, for both flow penetrometers (T-bar and spherical ball), the soil deformation mechanism is symmetrical in the plane perpendicular to the axis of the penetrometer, and the load cell measures what is essentially a differential force (or net pressure) on the bar with no adjustment being made for the overburden stress and ambient pressure. Secondly, the correlation between net pressure on the flow penetrometer and the shear strength of the soil is made via an exact plasticity solution; this operates within a potential range of flow penetrometer factor of less than ± 10% (due to different roughness of the bar surface) compared with cone factors which may vary from as low as 7 in sensitive clays to over 15 – that is a range of ± 35%. Remoulded strength An additional use of the flow penetrometer is to assess the remoulded strength of the soil by monitoring the bearing resistance during extraction along the same path as insertion. |