CPTu-Tests
Cone/Piezocone Penetration Test (CPT/CPTu)
Cone Penetration Test (CPT) is one of the most important and widely used field tests for identification subsurface layers. CPT test was first conducted in the Netherlands in 1930, after which amendments were made to the primary device and used as one of the most important field tests by geotechnical engineers. This experiment, due to its high speed, accuracy and repeatability, as well as the continuity of the output results, has helped to estimate the sub-surface layers profile. In recent years, the use of this experiment has gained attention in geotechnical investigations all around the world.
The above test is performed in accordance with ASTM-D5778 standard based on the penetration of a steel cone attached to a cylindrical wall at a constant speed into the soil. The cone is connected to the drilling rods and with the aid of a hydraulic actuator at a constant speed of 2 cm⁄s penetrates deep into the soil and the resistance of the tip(qc) and friction wall(fs) is recorded continuously using sensors embedded inside the device during the penetration of cone. In soft soils, cone penetration can be carried out from the surface to a depth of more than 100 meters, if penetration is advanced vertically. In general, this test is suitable on fine soils (very soft to hard) and loose to semi-dense grain soils, and the presence of sand and sand lens layers, cement zones and dense sand can limit the penetration of the cone and cause damage to the cone and deviation of the cone and rods. If the device is capable of measuring and logging the pore water pressure(u) during its penetration, the test is called a Piezocone Penetration Test or CPTu.
Figure 1. CPTu Cone
The pore water pressure sensor can be placed either inside the cone or the cylinder or behind the friction wall. According to ASTM standard, the sensor is generally placed at the junction of the wall and cone, as shown in Figure 2 in its position with u2.
The penetration cone generally has an internal angle of 60 degrees and a cross-sectional area of 10 cm2. In case more space is needed to install sensors inside the cone or more stiffness for the cone, a cone with a cross-sectional area of 15 cm2 shall be used. The height of the friction wall for these two common cones is 134 and 145 mm, respectively. The schematic of the two common dimensions for this device is shown in Figure 2.
Figure 2. Schematic view of the CPTu cone
CPTu Advantages
This test has many advantages compared to the common drilling and sampling methods, including:
- Direct and continuous data logging of soil layers and a more accurate soil stratigraphy,
- Repeatability of the test within the same area,
- Cost effective method, compare to drilling,
- Measuring the pore water pressure and the possibility of measuring the water level in field,
- The CPTu test causes minimal disturbance to the soil, maintaining the integrity of in situ conditions which is crucial for accurate geotechnical analysis (especially in soft or sensitive clay and loose sand),
- Capability in developing a profile of liquefaction potential with depth, identifying critical layers that may be prone to liquefaction during seismic events
- The results can be used readily to perform the design of foundations, and implement necessary ground improvement or mitigation measures,
- The CPTu test is faster compared to many other methods, allowing for quicker data acquisition and reduced field time. CPTu can be automated and used with advanced data logging systems, improving the efficiency and reducing the possibility of human error.
Limitations of CPTu Method
- CPTu testing requires specialized equipment and trained personnel. The availability and cost of such equipment can be a limitation. The test shall be stopped instantly if the cone reaches very dense/rock layers, to avoid damage to the cone.
- The sensors and equipment must be regularly maintained and calibrated to ensure accuracy, adding to operational costs and complexity.
- CPTu can have difficulty penetrating very dense soils, cemented layers, or rock. In such cases, the equipment might reach its capacity, leading to refusal without sufficient depth penetration.
- In coarse, gravelly soils, the cone tip can be deflected or damaged, and the data may become unreliable. The presence of large particles can obstruct the penetration process.
- Unlike borehole sampling methods, CPTu does not provide physical soil samples for laboratory testing. This limits the ability to perform detailed analyses such as grain size distribution, Atterberg limits, or organic content determination, which require actual soil specimens.
- Interpreting CPTu data requires expertise and can be complex. Misinterpretation of data can lead to inaccurate conclusions about soil properties and behavior. Empirical correlations used to derive soil properties from CPTu measurements may not always be accurate for all soil types and conditions.
Baran Khak va Pey CPTu Device: Description
Baran Khak va Pey Co. uses the state of the art equipment from of the Dutch company A.P. van den berg to perform CPTu tests onshore, nearshore and offshore. So far, several projects have been completed using this device to investigate the physical and mechanical characteristics of the subsurface layers. In offshore operations, CPTu and hydraulic actuator equipment are placed on the jack-up barge and CPTu testing is performed at the desired locations at sea. To perform this test onshore, in order to provide the required overhead and ease of movement, the equipment is installed on the rubber track platform or chain wheel system and transferred to the desired position. The CPTu testing equipment of Baran Khak va Pey Company are shown in the following figures.
We, Baran Khak va Pey Company, offer innovative solutions to our clients through specialized technical projects. Our unwavering commitment to excellence, empowers us to navigate even the most challenging problems.
Figure 3. Baran Khak Va Pey CPTu equipment
Figure 4. Onshore CPTu Testing
Data Interpretation by Baran Khak va Pey Co.
Interpreting CPTu (Cone Penetration Test with pore pressure measurement) data involves analyzing the recorded measurements of cone resistance, sleeve friction, and pore pressure to derive meaningful geotechnical information about the subsurface conditions. The process can be broken down into several key steps, each of which contributes to a comprehensive understanding of soil properties and behavior.
1. Initial Data Collection and Calibration
The first step in interpreting CPTu data is ensuring that the collected data is accurate and reliable. This involves calibrating the equipment before the test and checking the baseline readings. During the test, the cone penetrometer is pushed into the soil at a constant rate, and continuous measurements of cone resistance (qc), sleeve friction (fs), and pore pressure (u) are recorded. Ensuring the equipment is properly calibrated and functioning correctly is crucial for obtaining high-quality data.
2. Soil Profiling and Stratigraphy
One of the primary outputs of CPTu data interpretation is the creation of a soil profile. The continuous nature of CPTu data allows for detailed stratigraphic logging. By analyzing variations in cone resistance, sleeve friction, and pore pressure with depth, different soil layers can be identified. High cone resistance typically indicates denser or more competent soil layers, such as sands or gravels, while lower resistance values might suggest softer soils, like clays or silts. The ratio of sleeve friction to cone resistance (friction ratio) helps in distinguishing between different soil types and their characteristics.
Figure 5. CPTu Results: Variation of Physical Properties in Depth
3. Soil Behavior Type Classification
CPTu data is often used to classify soil behavior types (SBT) using empirical charts and correlations. Different soil behavior types can be identified, such as clean sands, silty sands, clayey silts, and clays. This classification aids in understanding the mechanical properties and potential responses of the soil under loading conditions.
4. Pore Pressure Analysis
The pore pressure readings from CPTu provide critical insights into soil conditions, especially in saturated soils. Pore pressure measurements help in identifying the presence of cohesive soils and assessing their consolidation characteristics. In over-consolidated clays, for example, negative pore pressure readings might be observed. Conversely, in normally consolidated or under-consolidated clays, positive pore pressure readings are typical. Analyzing the dissipation of pore pressure over time can provide information about the soil's permeability and consolidation properties.
Figure 6. CPTu Results: Variation of Mechanical Properties in Depth
5. Deriving Geotechnical Parameters
From the raw CPTu data, various geotechnical parameters can be derived using established correlations and empirical formulas. For instance, the undrained shear strength of clays can be estimated using cone resistance values and empirical correlations specific to clayey soils. For sandy soils, relative density and friction angle can be inferred. Additionally, the modulus of elasticity and over consolidation ratio can be estimated, providing valuable input for geotechnical design and analysis.
6. Liquefaction Potential Assessment
CPTu data is also instrumental in assessing the potential for soil liquefaction, especially in seismic regions. By analyzing the pore pressure response and soil behavior type, the susceptibility of certain soil layers to liquefaction can be evaluated. Empirical methods, such as the Robertson and Wride method, use CPTu data to estimate the cyclic resistance ratio (CRR) and compare it with the cyclic stress ratio (CSR) induced by an earthquake to predict the likelihood of liquefaction.
Figure 7. CPTu Results: Variation of Safety Factor Against Liquefaction in Depth