We work with a geotechnical drilling truck (25 Ton) that has a 200 kN press by which we can offer Cone Penetration Tests (CPT) and complementary geotechnical investigation in any region of Bolivia.
The CPT (Cone Penetration Test) is a method to identify the layers and physical properties of the subsoil. The CPT measurements are performed continuously and electronically as the depth is increasing.
The CPT probe is a Dutch innovation that was developed over time, being an economic method of soil research without producing a direct disturbance of the terrain that has been investigated.
The CPT probe provides accurate information of the different subsoil layers including their parameters and resistance.
The CPT consists on pushing a conical tip (cone) in to the existing ground by means of tubes that have a length of 1 meter. The pipes are pushed into the ground and joined one to each other by a threaded union that is extended one after the other, in this way depths greater than 30 meters can be achieved depending on the subsoil found underneath the terrain and the capacity of the pushing equipment. The cone measures and registers electronically the pressure at the tip (qc), lateral friction at the shaft (fs), and additionally the water pressure and inclination of the cone.
Results and benefits obtained from CPT
The friction ratio which is defined by the ratio of absolute lateral friction and the resistance at the tip of the cone defines the type of soil. Each type of soil has a unique friction value within a given range. The resistance at the tip of the cone gives a good impression of soil compaction.
With this data the stratification of the soil and its geotechnical parameters will be obtained:
- Soil unit weight (gt)
- Relative density (Dr)
- Internal friction angle (j)
- Cohesion (c')
- Undrained shear strength (cu)
- Modulus of elasticity (E)
- Deformation parameters (settlements).
It is also important to note that a CPT probe is like performing a test pile. The data obtained by the CPT probe, cone resistance and lateral friction on the shaft, are the exactly forces that determine the bearing capacity of a pile. As a consequence of the previous said, CPT probes gives a good perception of the bearing capacity of a pile.
The benefit of the CPT probe is also extended thanks to the continuous data recording every two (2) centimeters through all layers, whether hard, soft or intermediate. Taking for example a particular case of pile foundations, the CPT probe records information continuously through the subsoil layers every two (2) centimeters so it can detect all strata and even the most irrelevant layer for a correct foundation design of the pile. With this information it is also known if there are some intermediate soft layers that a detected hard layer for founding the pile-tip may contain. In this way with the data obtained from the CPT probe the approximate deformations of the pile can be determined, considering that the CPT probe must be at least 5 meters deeper than the base level of the pile.
Comparison between CPT and SPT
In Bolivia the SPT (Standard Penetration Test) is the most applied test for investigation of soil. It is by means of this probe and the count of the number of blows that are necessary to push a sampling spoon with which will be predicted a future ground behavior.
This method is discontinuous in which the data can be taken at intervals of several meters trying to measure resistance at the tip of the spoon (without being able to measure the lateral friction of the shaft neither the insitu water pressure). The advantages of the CPT over the SPT are:
The CPT probe can measure resistance at the tip of the cone plus lateral friction and pore pressure. It is on the basis of these parameters that the soil types can be obtained as well as the geotechnical parameters of the different layers of the terrain under study.
The CPT probe obtains continuous data every two (2) centimeters as the cone penetrates the soil, while the SPT probe obtains its data discontinuously and at predetermined intervals.
The SPT probe requires more laboratory tests to complement the study.
With the CPT probe, reliable correlations between the pressure at the tip (qc) and the lateral friction at the shaft (fs) have been developed as well as with the geotechnical parameters.