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The evaluation of the liquefaction potential of soils in seismic-prone region is essential for site selection, planning, and construction of structures. One can assess liquefaction potential through standard blasting tests, ground response analysis, laboratory tests, and expertise.
The use of laboratory tests is not desirable due to difficulties in taking undisturbed samples and reconsolidation processes. However, field tests need small samples with relatively low disturbance and can represent liquefaction potential satisfactorily.
How to Evaluate Liquefaction Potential of Soils in the Field?
1. Apply Experience
Experience about circumstances under which soil suffered from liquefaction during earthquakes is substantially valuable. Engineers can use the experience as guidance to expect the probable performance of the soil on the construction site.
However, one can use the expertise to evaluate liquefaction potential with a small number of well-defined case studies. Therefore, use the data from past experiences with suitable tests and analysis.
2. Standard Blasting Test
The standard blasting test is another technique to assess liquefaction potential. A specific quantity of explosive materials is placed at a certain depth based on explosive geometry, spacing, and soil type.
The explosion, after that, causes soil settlement. The settlement is then determined to find out whether the soil layer is susceptible to liquefaction during earthquakes of not.
For instance, in a particular project, a sand layer with a thickness of 7.5-10.5 m compacted with 5 kg ammonite explosives was detonated underground at a depth of 4.5 m.
It was observed that the resultant settlement ranged from 8-10 cm within a diameter of 5 m, the explosive point being the center point. The ratios of settlement from successive shorts were 0.6; hence there was no need to take liquefaction into account.
3. Use of Ground Response Analysis and Laboratory Test Procedure
It includes ground response analysis to specify stresses generated in the soil layer during an earthquake. Then, compare the determined stresses with those noticed to cause failure in cyclic load tests in the lab.
This technique provides a means for considering the effects of the amplitude and time history of the earthquake ground motions, the in-situ properties of the soils, variation of overburden pressure with depth, and the position of the water table. The procedure of ground response analysis and laboratory test is as follows:
- Evaluate the magnitude of ground motion that may develop in the base rock at the site under consideration.
- Specify the response of the overlying soil to the base motion. Assume the deformation of the soils is mainly due to the vertical progression of shear waves as a result of the base motion.
- Idealize shear stress history at various depth to find out the significant number of stress cycles and the equivalent uniform cyclic shear stress developed at each layer.
- Determine the cyclic shear stress required to cause liquefaction of the sand in the significant number of stress cycles.; the cyclic shear stress can be found using cyclic load tests on a representative sample of sand from the site.
- Compare the magnitude of equivalent cyclic stress developed at any depth with cyclic stress that causes liquefaction for the condition existing at that depth to specify whether liquefaction would occur or not.
FAQs on Evaluation of Liquefaction Potential of Soil
Soil liquefaction is the phenomenon in which the stiffness and the strength of the soil are lost under the action of earthquake force or due to rapid loading conditions. Soil liquefaction occurs in a fully saturated soil. To read more, please click here.
The cause of liquefaction is an applied force such as shaking due to earthquakes under which the saturated or partially saturated soil loses its strength and stiffness.
Sand is a type of soil that is highly susceptible to liquefaction. Gravel and silt may also suffer from liquefaction, in addition to fine grained soil in which strain softening can create effects similar to that of liquefaction.
The most suitable technique is replacing the soil susceptible to liquefaction or considering suitable soil improvement techniques to enhance soil property and decrease the possibility of soil liquefaction.
The evaluation of the liquefaction potential of soils in seismic-prone region is essential for site selection, planning, and construction of structures.
One can assess liquefaction potential through standard blasting tests, ground response analysis, laboratory tests, and expertise.