Cone Penetration Testing

Cones & penetrometers

Cone Penetration Testing

Cones

There are three fundamental properties being measured using piezocones – cone resistance, sleeve friction and porewater pressure. The typical piezocone houses load cells and associated strain gauges to measure these properties as well as an accelerometer to measure slope or inclination of the cone. It’s worth noting that depth is measured independently from a reference point in the rig. Cones are typically one of two types – subtraction or compression. Compression cones measure cone resistance and local sleeve friction independently on two separate bridges. The subtraction cone measures cone and local sleeve friction in a combined fashion through one load cell – the upper strain gauge bridge measures cone resistance plus sleeve friction and the lower strain gauge bridge measures cone resistance only, therefore the local sleeve friction is the difference between the two – hence the name. Compression cones are more sensitive and accurate but can only operate over a pre-defined lower stress range. They are therefore suited to very soft sensitive soils in a fairly homogeneous stratigraphy. Subtraction cones are less accurate (particularly in respect of sleeve friction) but operate over a much wider stress range overall being governed overall by the capacity of the upper load cell which measures both cone and sleeve friction. In mixed soils both are equally accurate hence we generally use subtraction cones for robustness. There are also tension cones but these are less commonplace – a versus of the compression cone where the strain is measured in tension rather than compression.

Compression cone

Subtraction cone

The strain gauges, which are thin conductive strips mounted on the load cells, measure electrical signal through a terminal – this is based on Pouillet’s Law of electrical conductance. The cone and sleeve resistance are nominally measured in one plane whereas the porewater pressure is measured uniformly via a strain gauge mounted on a diaphragm over a hollow chamber. This chamber is saturated with a fluid and pressure changes propagate through a filter ring.

The change in resistance is measured in millivolts and is then digitised via a converter. This may be within the cone or separately within a, A/D converter. The former is more common place nowadays. These digitised signals are then channelled into binary data that can be computed via interface console. This will normally result in a compressed standardised raw data arrangement.

The change in resistance is measured in millivolts and is then digitised via a converter. This may be within the cone or separately within a, A/D converter. The former is more common place nowadays. These digitised signals are then channelled into binary data that can be computed via interface console. This will normally result in a compressed standardised raw data arrangement.

The positioning of the porewater filter along the cone can be important in particular soil types. Positioning the porewater filter at the shoulder of the cone (the u2 position) is the typical approach for normally consolidated soils. However, in strongly dilative and heavily overconsolidated soils, placing the porewater filter in the tip of the cone can improve porewater pressure traces yielding better profiling. 

Full flow penetrometers are a specialist form of penetrometer undertaken to provide estimates of peak shear strength and residual shear strength in soft and very soft soils, sediments or tailings. The larger surface area of the full-flow penetrometer and the behaviour of soft soils as it shears around the penetrometer can help provide a more uniform interpretation of these parameters compared with traditional cones, which have a smaller surface area and promote shearing rather than flow.

Cone Penetration Testing

Cones

There are three fundamental properties being measured using piezocones – cone resistance, sleeve friction and porewater pressure. The typical piezocone houses load cells and associated strain gauges to measure these properties as well as an accelerometer to measure slope or inclination of the cone. It’s worth noting that depth is measured independently from a reference point in the rig. Cones are typically one of two types – subtraction or compression. Compression cones measure cone resistance and local sleeve friction independently on two separate bridges. The subtraction cone measures cone and local sleeve friction in a combined fashion through one load cell – the upper strain gauge bridge measures

The strain gauges, which are thin conductive strips mounted on the load cells, measure electrical signal through a terminal – this is based on Pouillet’s Law of electrical conductance. The cone and sleeve resistance are nominally measured in one plane whereas the porewater pressure is measured uniformly via a strain gauge mounted on a diaphragm over a hollow chamber. This chamber is saturated with a fluid and pressure changes propagate through a filter ring.

The change in resistance is measured in millivolts and is then digitised via a converter. This may be within the cone or separately within a, A/D converter. The former is more common place nowadays. These digitised signals are then channelled into binary data that can be computed via interface console. This will normally result in a compressed standardised raw data arrangement.

cone resistance plus sleeve friction and the lower strain gauge bridge measures cone resistance only, therefore the local sleeve friction is the difference between the two – hence the name. Compression cones are more sensitive and accurate but can only operate over a pre-defined lower stress range. They are therefore suited to very soft sensitive soils in a fairly homogeneous stratigraphy. Subtraction cones are less accurate (particularly in respect of sleeve friction) but operate over a much wider stress range overall being governed overall by the capacity of the upper load cell which measures both cone and sleeve friction. In mixed soils both are equally accurate hence we generally use subtraction cones for robustness. There are also tension cones but these are less commonplace – a versus of the compression cone where the strain is measured in tension rather than compression.