SPT Soil Test
Last updated
Last updated
The yield strength of a floor is the maximum load per unit area that supports the floor or foundation without displacement. Of course, this depends on the type of soil. Soil types can be categorized as very loose, loose, moderate, dense and very dense, depending on whether the soil is sticky, non-sticky or rocky. initially.
Soil mass densities are calculated in kg / m3 and vary from very loose soils of 1600 kg / m3 to dense soils of 2100 kg / m3. Beyond this bulk density value, the soil is classified as very dense.
Safe Capacity (SBC) is the allowable load that can withstand the ground and will not be damaged by shifts or exceed the allowable settlement limit. Ultimate Bearing Capacity (UBC) is the absolute maximum pressure at which the ground begins to break.
This is also known as the total carrying capacity of the soil. Sharing by factor of safety gives you a safe capacity (SBC). It is proposed that the factor of safety for normal buildings is 2, and the factor of safety for heavy structures is 2, 5, or 3.
Various tests are performed to determine the characteristics of the soil, especially for deep foundations. The correlation of soil properties from the soil survey report and the corresponding load tests of the actually constructed project is essential to determine what type of soil test should be performed.
Standard penetration test (SPT) is a common field dynamic test method used to determine the geoengineering properties of subsoil. Estimating the relative density and approximate shear strength parameters of the soil is a simple and inexpensive test. The test is done in a well.
The results can be used to determine the relative density, bearing capacity and settlement of granular soil. The results can also be used to correlate the approximate strength of sticky soil.
This test requires drilling a well to the desired sampling depth. This test uses a thick sample tube with an outer diameter of 50.8 mm, an inner diameter of 35 mm, and a length of approximately 650 mm.
A split spoon sample holder attached to the drill rod is placed at the test point. Repeatedly drop a 63.5 kg (140 lb) hammer from a height of 76 cm (30 inches) to drive the sampler into the ground to a depth of 15 cm (6 inches). The number of strokes required is recorded. This procedure is repeated two more times until a total penetration of 45 cm (18 inches) is reached.
Pros & Cons of SPT
The number of strokes required to penetrate the first 15 cm is called the "drop impulse" and the total number of strokes required to penetrate the remaining 30 cm depth is called the "standard penetration resistance" or "value N".
Will be. If the value exceeds N50, the test is interrupted and is called "deny". The interpreted results are used to estimate the geoengineering properties of the soil with various modifications. .
SPT is a low-cost, direct test procedure for determining the relative density and shear strength angles of non-stick soil, and the strength of hard, sticky soil. This test requires only a sample tube and a hammer, which mainly offers some notable advantages that it is easy and cheap to perform.
The SPT results are most useful not only for fine-grained sand, but also for coarse and sludge sand. However, for clay and gravel soils, the results are generally unreliable. SPT is used to provide results for empirically determining the susceptibility of sand layers to soil liquefaction.
Because SPT is an in-situ test method, the sample does not change while the test is running, unlike laboratory test methods where the sample must be disturbed. Therefore, SPT is widely used in such applications.
However, the SPT test has the disadvantage of being inaccurate compared to more advanced methods, especially when taking samples of coarse sand or clay. Soil must undergo additional tests to measure soil structure properties on the fly, such as density, strength, and consolidation properties.
SPT is generally not recommended because samples must be taken for this purpose and the collected samples have changed significantly. To overcome this limitation, tests are often performed using larger samplers with slightly different point shapes, which minimizes sample turbulence. However, it is difficult to convert hit counts to N values in such applications.
SPT only provides a value of N. This is not a direct representation of soil properties unless the value of N correlates with certain properties under test, such as density. You may need to use multiple correlations, which can result in poor quality results.
In addition to using rough correlations, you should normalize your data taking into account surge pressures, sampling techniques, and other factors. There are some correlations available in the literature to predict the properties and behavior of sticky soil (clay / sludge) using standard penetration tests, but the SPT value for predicting the properties of sticky soil.
The use of is not considered very reliable. Also, for loose and soft clay soils, the hit counts are very low and the samples change significantly during SPT, so results are obtained in turbulent conditions rather than in the original intact soil conditions.
Various methods have been proposed to compensate for the shortcomings of standard penetration tests, such as cone penetration tests (CPTs), field shift tests, and shear wave velocity measurements. Since this is the most common field soil test method, SPTs are frequently compared, correlated and converted into CPT test results.
Several methods have been developed to do this. For example, the Robertson and Campanella method is used to correlate SPT and CPT data by the ratio of normalized point resistance to atmospheric pressure to the value of N60.
Soil behavior is categorized according to specific proportions. The Jefferies and Davies method is another robust method for converting N values from SPT to CPT data and vice versa.
Penetration resistance testing and sampling with open-ended pipes began in the early 1900s. SPT was developed by the Reymond Concrete and Pile Company in the United States, and the barrel sample was split in 1927.
The American Society for Testing and Materials standardized testing in the 1950s. Since then, SPT has been adopted worldwide as the primary method for collecting geoengineering design data. It is estimated that 80-90% of ground surveys consist of SPT.
SPT is not available when continuous samples are required. Test results cannot be reproduced because results can vary significantly due to drilling perturbations, mechanical variables, and operator variables.
In addition, the use of the SPT method requires very careful observation and monitoring with detailed records during surgery. For this reason, old recorded data should be used with great care.
SPT results are also affected by many factors, including overload stress, soil density, soil type, particle size, soil age, and soil sediment stress history. This makes the choice of SPT for soil testing a little more selective for a particular application.
espite its setbacks, SPTs are widely used to predict the allowable bearing capacity of pedestals on sand by overloading pressure and correlating the position of the water table. SPT data are commonly used to predict liquefaction caused by seismic loads and to estimate post-earthquake shear strength.
The popularity of SPT may be due to its well-developed procedure, simplicity, ease of operation, robustness for use in a wide range of soil conditions, and its flexibility, but progress during operation is others. Slower than the on-site test of.