The Cone Penetration Test (CPTu) is a widely used in-situ testing method in geotechnical engineering for subsurface soil exploration and characterization. It involves pushing a cone-shaped probe into the ground at a controlled rate and measuring various parameters during the penetration process. The test provides continuous or semi-continuous data with depth, which is valuable for soil profiling and determining soil properties.

The CPTu probe consists of a cylindrical steel rod with a cone tip and a friction sleeve. The cone tip has a 60-degree apex angle and a projected area of 10 or 15 square centimeters. The friction sleeve is a cylindrical surface above the cone tip, which measures the friction between the soil and the probe during penetration.

During the CPTu, the following measurements are typically recorded:

1. Cone resistance (qc): This is the force acting on the cone tip divided by the projected area of the cone. It provides an indication of the soil's strength and density.

2. Sleeve friction (fs): This is the force acting on the friction sleeve divided by its surface area. It helps in evaluating the soil's friction and drainage characteristics.

3. Pore water pressure (u): The probe is equipped with a porous filter to measure the pore water pressure generated during penetration. This parameter is valuable for assessing soil drainage conditions and consolidation characteristics.

4. Inclination: Some CPTu probes also measure the inclination of the probe during penetration, which helps in correcting the data for potential deviations from verticality.

The CPTu data, along with empirical and theoretical correlations, can be used to estimate various soil properties, such as:

The CPTu is particularly useful for profiling and characterizing soil deposits with high spatial resolution, especially in areas with complex subsurface conditions or interbedded soil layers. It is an economical and efficient method compared to traditional soil sampling and laboratory testing. However, the interpretation of CPTu data relies on site-specific correlations and local experience, and it may need to be complemented by other investigative methods in certain cases.

Cone Penetration Test with Pore Water Pressure Measurement (CPTu)

The Cone Penetration Test (CPTu) is one of the most important in-situ tests, which is used to identify subsurface layers condition: classify soil, detect layers and measure strength, determine deformation characteristics and the permeability of foundation soils. The view of this apparatus is shown in Figure 1. As can be seen, the main equipment of this device includes a penetration cone and a cylindrical casing. Due to its high speed, accuracy and repeatability as well as the continuity of output results, this test greatly helps estimate the characteristics of subsurface layers. In recent years, the use CPTu results has gained special attentions in geotechnical studies all around the world. The basis of this test is the penetration of the cone into the ground at a constant speed. The penetration cone is connected to the lower end of the cylindrical part of the device and by connecting to the drill rods and with the aid of a hydraulic jack, it penetrates into the ground depths at a constant speed (according to Standard). During penetration, the resistance of the cone tip and casing is continuously or intermittently read using embedded sensors. A schematic view of this test is observed in Figure 2.

This test can be performed in soft soils, to depths of more than 100 meters from the ground surface (provided that penetration remains vertical), and the presence of sandy layers, rock slabs, cemented zones and dense sand can limit cone penetration and damage and deflect the cone and rods. In general, the CPTu test has three main applications in the site assessment process, which are:

The cone penetration test is performed based on ASTM-D5778. It is necessary to measure the resistance of the pointed probe with pore water pressure (qc) and, if appropriate, the local friction sleeve on the probe (between the friction sleeve and the surrounding soil). In addition to friction resistance and friction on the sleeve, it is possible to measure the water pressure generated during penetration. In this device, the resistant force applied on the cone tip and the frictional force applied on the casing are measured by two load cells located inside the penetrometer. According to ASTM standard, it is possible to use independent load cells or differential type. Schematics of these two types of load cells are shown in Figure 3. As can be seen in the figure on the right, two separate and independent load cells are placed inside the device to measure cone tip pressure and casing friction. In the figure on the left, one of the load cells measures the compressive force on the cone tip and the other load cell measures the sum of the compressive force on the cone tip and the frictional force on the casing. By subtracting these two values, the casing frictional force is obtained. Regardless of the type of load cell used, the load cell used to measure casing friction should only be sensitive to the shear stresses applied on the casing.