A cone penetration test is used to determine geotechnical
properties of soils The testing procedures for CPT are outlined in ASTM D-5778, The cone penetration test has become internationally one of the most widely used and accepted test methods for determining geotechnical soil properties.
The cone penetration test can be completed from the ground surface. Cone penetration test rigs vary in size - from small portable rigs to large truck-mounted rigs. Each rig has benefits and limitations but they all conduct the same test. A cone penetration test rig pushes a steel cone (about 32mm
wide) into the ground, generally up to 20m below the surface or until the cone reaches a hard layer. The steel cone contains an electronic measuring system that records tip resistance and sleeve friction.
As the cone is pushed into the ground, the soil responds with differing degrees of resistance. This resistance is recorded using force sensors in the tip.
At the same time as the sensors are recording resistance at the cone tip, sensors in the friction sleeve are recording sleeve friction along a 100mm length. Some cones also have a pore water transducer, which records water
pressure in the soil. These readings can be used to determine ground water responses as the cone is pushed through the soils.
A cone penetration test typically takes between 30 minutes and three hours. As the cone goes into the ground, measurements are constantly sent back to the rig and recorded on computer.
Cone penetration test results are used by geotechnical engineering specialists to understand the soil properties (the relative density of the soil and the soil behaviour type, both of which are calculated from the cone penetration test cone tip resistance and sleeve friction) and how the ground is likely to behave under different levels of earthquake shaking. This information can help in the design of foundations and ground improvements.
By doing a test before and after ground improvement works, cone penetration test results can also be used to determine how much strength a soil has gained following ground improvement works.
CPT soundings can be very effective in site characterization, especially sites with discrete stratigraphic horizons or discontinuous lenses. It is a valuable method of assessing subsurface stratigraphy associated with soft materials, discontinuous lenses, organic materials (peat), potentially liquefiable materials (silt, sands and granule gravel), and landslides.
The tip resistance is theoretically related to undrained shear strength of a saturated cohesive material, while the sleeve friction is theoretically related to the friction of the horizon being penetrated.
The tip of the cone penetrometer senses out ahead of itself as it induces a local bearing failure of the soil it passes through. The tip resistance recorded by the instrument is an average across this tip influence zone.
The friction ratio is given in percent. It is the ratio of skin friction divided by the tip resistance. It is used to classify the soil, by its behavior, or reaction to the cone being forced through the soil. High ratios generally indicate clayey materials (high c, low Ø) while lower ratios are typical of sandy materials (or dry desiccated clays).
Piezocones also measure in-situ pore pressure , in either dynamic (while advancing the cone) or static (holding the cone stationary) modes. The Differential Pore Pressure Ratio is used to aid in soil classification according to the Unified Soil Classification System (USCS).
#Geotechnical_ Engineering