In-situ testing refers to testing methods that are carried out directly on the ground, rock, or soil at a particular site. These tests are conducted to gather information about the geotechnical properties of the site, such as strength, stiffness, permeability, and deformation characteristics. In-situ testing is an important component of site investigation, as it allows engineers and geologists to evaluate the subsurface conditions and the suitability of the site for various types of construction projects.
In-situ testing methods are typically divided into two broad categories: (1) penetration tests, which involve driving a tool or instrument into the ground and measuring the resistance or penetration rate, and (2) non-penetration tests, which involve measuring various physical properties of the ground or soil without penetrating it. Some of the most commonly used in-situ testing methods include the Standard Penetration Test (SPT), the Cone Penetration Test (CPT), the Pressuremeter Test, and the Dilatometer Test. Each of these tests has its own advantages and limitations, and the choice of test method will depend on the specific objectives of the investigation and the properties of the site being investigated.
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Importance in site investigation
In-situ testing is an important part of site investigation as it helps to obtain a better understanding of the characteristics and behavior of soil and rock at the actual site conditions. The results obtained from in-situ testing are used to assess the geotechnical properties of soil and rock, which in turn are used for design and construction of various civil engineering projects such as buildings, roads, bridges, dams, and tunnels.
In-situ testing can also provide information on the presence of natural hazards such as landslides, liquefaction, and subsidence. By understanding the geotechnical properties of the site, engineers can design appropriate foundations, earthworks, and other geotechnical structures to minimize the risk of failure and ensure the safety and stability of the structures over their design life.
Types of in-situ tests
There are various types of in-situ tests used in geotechnical site investigation. Some of the commonly used in-situ tests are:
- Standard Penetration Test (SPT): This test involves driving a split spoon sampler into the ground using a standard weight and fall height. The number of blows required to drive the sampler over a set distance is recorded and used to determine the soil resistance.
- Cone Penetration Test (CPT): This test involves pushing a cone-shaped penetrometer into the ground at a constant rate of penetration. The resistance of the soil to penetration is measured and used to determine soil properties.
- Pressuremeter Test: This test involves inserting a cylindrical probe into a borehole and then expanding it using water pressure. The pressure required to expand the probe is measured and used to determine soil stiffness and strength.
- Vane Shear Test: This test involves inserting a vane into the soil and then rotating it to measure the torque required to cause failure. The test is used to determine the shear strength of soft to stiff clays.
- Plate Load Test: This test involves placing a steel plate on the ground surface and applying a load. The settlement of the plate under load is measured and used to determine the bearing capacity of the soil.
- Cross-Hole Test: This test involves drilling two or more boreholes and placing seismic sensors in them. An acoustic wave is generated in one borehole and the time it takes to travel to the other boreholes is measured. The test is used to determine the shear wave velocity and stiffness of the soil between the boreholes.
- Downhole Test: This test involves inserting a probe into a borehole and measuring the properties of the soil at different depths. The test is used to determine soil stratigraphy and stiffness properties at different depths.
Standard Penetration Test (SPT)
The Standard Penetration Test (SPT) is a widely used in-situ test to determine the geotechnical properties of soil at a site. The test involves driving a standard sampler into the ground using a hammer with a standard weight and falling distance. The number of blows required to drive the sampler a standard distance into the soil is recorded as the SPT “N-value.” The SPT is used to estimate the soil’s shear strength, density, and other properties.
The SPT test is performed by drilling a borehole into the soil, usually using a hollow stem auger, and inserting a sampler tube into the borehole. A split-spoon sampler is then driven into the soil at the bottom of the borehole using a standard hammer of 63.5 kg weight and falling through a standard distance of 75 cm. The number of blows required to drive the sampler the first 15 cm of penetration is not counted, and the number of blows required for the next three 15 cm increments is recorded as the SPT N-value.
The SPT is a relatively quick and inexpensive test, making it a popular choice for site investigations. However, the test results can be affected by factors such as soil type, sampler size, and hammer energy, which must be taken into account when interpreting the results.
Cone Penetration Test (CPT)
The Cone Penetration Test (CPT) is a widely used in-situ testing method in geotechnical engineering. It involves pushing a cone penetrometer into the ground at a constant rate of penetration and measuring the resistance to penetration and pore water pressure at different depths. The cone penetrometer typically consists of a steel cone tip, a series of friction sleeve segments, and a pore water pressure transducer. As the cone penetrometer is pushed into the ground, the resistance to penetration and pore water pressure measurements are recorded continuously, providing a profile of the soil or rock properties with depth.
The CPT is often used to obtain information about soil type, strength, and compressibility, as well as groundwater conditions. The test results can be used to estimate soil bearing capacity, foundation settlement, liquefaction potential, and soil slope stability. The CPT is considered to be a reliable and cost-effective method for site investigation, particularly for large and complex projects.
Pressuremeter Test
The pressuremeter test is an in-situ test used to measure the in situ stress-strain characteristics of soils and rocks. In this test, a cylindrical probe is inserted into the ground, and the probe is then expanded by applying pressure to the inside of the probe. The pressure is increased incrementally while the volume of the probe is monitored. The resulting pressure-volume data is used to determine the modulus of deformation and shear strength parameters of the soil or rock being tested. The pressuremeter test is often used in soft soils where standard penetration testing or cone penetration testing may not be suitable. It is also commonly used in geotechnical engineering design for foundations and retaining structures.
Vane Shear Test
Vane shear test is an in-situ geotechnical test used to determine the undrained shear strength of soft to stiff cohesive soils. The test involves pushing a four-bladed vane vertically into the soil, rotating it at a constant rate of shear, and measuring the torque required to maintain the rotation. The torque measurement is used to determine the maximum shear resistance, which is used to calculate the undrained shear strength of the soil.
The vane shear test is particularly useful in the assessment of the strength of clays and silts, and is often used to estimate the undrained shear strength of soils for the design of shallow foundations, retaining walls, and embankments. The test is quick, inexpensive, and can be performed in boreholes or test pits without the need for soil excavation.
Plate Load Test
The Plate Load Test (PLT) is an in-situ test used to determine the ultimate bearing capacity of soil and the likely settlement of a structure on that soil. The test involves placing a steel plate on the ground surface and loading it with a hydraulic jack until the plate starts to sink into the soil. The amount of settlement and the pressure applied to the plate are measured at different stages of loading to establish a load-settlement curve.
From the load-settlement curve, the ultimate bearing capacity of the soil can be estimated, along with the deformation modulus and other soil properties. This information is useful in designing shallow foundations for structures, such as buildings and bridges, and in assessing the stability of embankments and slopes.
PLT is typically used in conjunction with other site investigation techniques, such as borehole drilling and sampling, and geophysical surveys, to provide a comprehensive understanding of the soil and rock conditions at a site.
Cross-Hole Test
Cross-hole testing is an in-situ geophysical technique used to determine the seismic properties of soil or rock materials between two or more boreholes. The method involves generating a seismic wave in one borehole using an energy source such as a hammer or explosive and recording the response to the wave in a nearby borehole using a geophone or accelerometer. By analyzing the arrival time, amplitude, and frequency content of the wave at the receiver, geophysicists can determine the properties of the material between the boreholes, such as seismic velocity, shear modulus, and Poisson’s ratio. Cross-hole testing is commonly used for geotechnical and geophysical investigations of the subsurface, such as in foundation design, seismic hazard assessment, and groundwater characterization.
Downhole Test
Abdelrahman, Kamal & AL Otaibi, Naif & Ibrahim, ElKhedr. (2021). Assessment of land subsidence as an environmental threat facing Dammam city, eastern Saudi Arabia based on soil geotechnical parameters using downhole seismic approach. Journal of King Saud University – Science. 33. 101233. 10.1016/j.jksus.2020.101233.
Downhole testing is a type of in-situ testing that involves measuring geotechnical parameters in a borehole. This type of testing is commonly used in mineral exploration, but can also be used in geotechnical engineering and environmental investigations.
There are several types of downhole tests, including:
- Downhole seismic testing: This involves measuring the velocity and attenuation of seismic waves in the rock layers surrounding a borehole. This information can be used to infer the elastic properties of the rock, which can be important in geotechnical engineering applications.
- Downhole logging: This involves lowering a logging tool down a borehole to measure various properties of the rock, such as density, porosity, and electrical conductivity. This information can be used to characterize the geology and hydrology of the subsurface, and can be important in mineral exploration, geotechnical engineering, and environmental investigations.
- Borehole permeability testing: This involves injecting or pumping water into a borehole and measuring the pressure response. This information can be used to estimate the permeability of the surrounding rock, which can be important in hydrogeological and geotechnical engineering applications.
Overall, downhole testing can provide valuable information about the subsurface geology and properties, which can be important in a wide range of applications.
