Chapter 9
Near-shore marine testing
Overwater cantilever frames
OVERWATER CANTILEVER FRAMES 9.5 | |
For cone penetration tests in harbours, often a cantilever frame can be used. This is much cheaper than deploying a platform. The tests must be carried out using a casing (Figure 86); without a casing, the 36 mm thrust rods for a CPT will buckle. Figure 86 The standard casing is 55 mm diameter which provides additional lateral support to the CPT rods. Generally these 55 mm casings are themselves supported by a drill casing to resist the lateral forces of tide, current etc. It is essential that, prior to a marine investigation, the requirements are properly assessed. If the purpose of the investigation is to assess the near-surface sediments of the harbour or other bed, then typically the tests should be performed in two parts. The first test is performed from the bed surface with no support casing to enable the surface deposits to be tested without disturbance. The thrust is very limited on this first push due to the lateral stability of the rods. On completion of the first push the casing can be set to a depth in the initially tested material; then once properly supported, the full thrust of the equipment can be used to advance the test to the required depth. Figure 87 (below) shows a plot of a typical CPT test in a harbour. |
Figure 87
On the left side of the chart (Figure 87), the plot shows the cone resistance and the sleeve friction. In the middle is the friction ratio which allows the soil type to be estimated, as shown on the right. The harbour bed consists of a soft clay layer of approximately 6 m overlaying silty sand (possibly highly weathered mudstone). The cantilever platform can be used as a standalone item of equipment (Figure 86) or can be mounted on a variety of land-based or marine equipment thereby providing a very flexible, cost effective alternative to conventional equipment. |