A realistic determination of mechanical parameters of soils which could represent the natural condition has always been a concern in geotechnical engineering. Therefore, in addition to sampling and laboratory tests, in-situ tests have been introduced and recommended in various technical references. The Cone Penetration Test (CPT) is one of the most accurate field tests in determining the parameters of fine-grained (very soft to hard) and granular (loose to medium-dense) soils.
CPT probe with force and water presure sensors
The tool consists of a cylindrical rod with a conical tip that penetrates into the soil under a force of static pressure at a constant rate of 2cm per second. During the penetration, various electrical sensors measure and record the forces required for the penetration of the tip of the cone (qc), the frictional shaft resistance (fs) and the pore water pressure (u) separately and continuously. A cone penetration test system that is equipped with a pore water pressure sensor is called a CPTu. In fact, without any need for sampling and only using these three parameters of qc, fs and u, the soil type and subsurface layering can be determined with very good accuracy. Also, various physical and mechanical soil parameters (for bearing capacity and settlements calculations) can be extracted.
In order to improve the quality of services and increase the ability to provide special services in the field of geotechnical engineering, Pars GeoEnviro Inc has accuired a CPTu test system. The device is SCPTu type, which in addition to the ability to measure tip resistance, shaft resistance and pore water pressure, also has the ability to measure the propagation speed of shear and compression waves (VS, VP) (Seismic CPTu).
Using the technical capabilities and experiences of several domestic and international projects, the R&D department decided to install the device on a chain wheel system (Track Mounted), This way, in addition to no need to supply and transfer any kind of surcharche, the ability to move in all types of soft and wet lands has been secured. Due to the limited availibility of this kind of in-situ tests in the country, alongside with providing faster and more accurate services to clients, Pars GeoEnviro Inc hopes to be able to take a considerable step forward by utilising this instrument as an advanced geotechnical technology within the country.
Main advantages:
- Rapid test hence time saving (possibility of testing several points with target depthes of 20 to 30 meters each one in one day)
- Provides a continuous profile of soil material and layering
- Suitable for some soils which are difficult to acquire undisturbed samples
- Ability to measure pore water pressure during penetration
- Minimum operator related errors and high repeatability
- Ability to interpret results based on strong theoretical foundations
- Evaluation of liquefaction potential of soft and loose saturated layers
- Direct calculation of bearing capacity of shallow and deep foundations
- Suitable for identifying the properties of soft and loose marine sediments
- Options to install a variety of sensors with various functionalities
Cone Penetration Test (CPTu) Description
The Cone Penetration Test (CPT) was first used in 1932 by Barentsen in the Netherlands. For this reason, this experiment is also known as the Dutch Cone. The basis of this experiment is based on soil resistance to cone penetration. The cone penetration test is performed in two ways: "mechanical" and "electrical with measurement of pore water pressure".
A schematic veiw of a CPT test
The advanced version of this test is performed according to ASTM-D-5778 standard and during the test, three independent parameters such as tip resistance (qc), wall friction (fs) and pore water pressure (u) are read and recorded as continuous profiles in depth. The diameter of the cone is about 35.7mm and the cross section of the horizontal plate is equivalent to 10cm square. The penetration rate of the cone in the test is 20mm/s and the penetration is done by a hydraulic system. Using this experiment, many soil parameters can be estimated with an exceptional accuracy without any need for sampling. It is even possible to detect very thin layers of sands or clays within the soil layering
A schematic veiw of a Seismic CPTu