In Situ Test on Earth
Geotechnical testing is critical to understanding the properties of the earth in which structures, foundations and construction are built upon. An in-situ test, or a test conducted in the place the structures are intended to stand, can further enhance the accuracy of the realization of the structure’s strength and characteristics. Two common in situ tests in geotechnical engineering are the field vane shear test (FVST) and the plate load test (PLT). The purpose of this paper is to explain each test in detail, and to compare and contrast the merits and drawbacks of each in order to determine which method is most appropriate for certain soil profiles and geotechnical conditions.
Field Vane Shear Test (FVST)
The FVST is the most conservative in situ test used to determine the in situ strength of clays, silts, soft organic soils and fine-grained silty sands. Common in coastal and shallow marine soils, the test provides an estimate of the shear strength parameters, such as undrained shear strength, unconfined compressive strength, and friction angle. The test typically involves two steps, the first consisting of a thin sampling cone driven into the ground to a depth of forty-five centimeters, or higher in soil layers determined to be especially dense. The second step is the vane test itself, which involves the insertion of an aluminum vane with a curved blade into the formed hole left by the sampling cone. It is then rotated into the ground until the percentage of rotation is sufficient to produce shear failure.
The data obtained from the FVST can be used to predict undrained shear strength and unconfined compressive strength for a certain soil profile. A major advantage of the FVST is that it does not require soil disturbance or destruction. Without excavation, the test is able to accurately predict the soil’s strengths, with minimal disturbance and much less risk of damaging the soil. However, a disadvantage of this method is that the depth of the cone penetration is a limitation and cannot penetrate deeper than the meter range. This can, in turn, provide weak measurements of the deeper soil response due to irregular cone penetration.
Plate Load Test (PLT)
Regarded by some as being a lesser accurate in situ test to the FVST, the PLT is utilized to determine the safe bearing capacity of shallow foundations and to measure the settlement/strain characteristics of soils. Commonly used in road and pavement design, and in ground improvements, the test can determine the maximum load, or bearing capacity, of a plated platform of either a field plate or a trial block resting on the surface of a soil layer.
The test involves first inserting a steel plate of standard dimensions into the soil and then subjecting it to a sequential increasing loading system, monitored by a load cell, until reaching the failure point load. As a load is applied, the settlement or strain of the soil can be measured by electronic gauges and can provide an indication of the overall stiffness profile of a soil layer. The data collected can then be used to estimate the bearing capacity of a shallow footing.
A major advantage of the PLT is that it can measure strains with a high accuracy, which is favorable for predicting settlement and settlement profiles over deeper soil layers. This can be critical to understanding the performance of structures laid down on soil layers. A limitation of the PLT is that, like the FVST, it does not penetrate deep and is limited to detecting surface soil layer characteristics.
Conclusion
In situ testing is beneficial for geotechnical engineering, especially when designing and building structures. Depending on the type of soil and the conditions of the site, the FVST or the PLT, or a combination of the two can provide a comprehensive understanding of the profile of a given soil layer. The FVST is a useful test because it is non-destructive and can be used to measure the properties of clays and other fine-grain soils. The PLT is advantageous in that it can provide reliable results when calculating the maximum load, or bearing capacity of a plated platform. Although the depth of testing of each method is limited, they both can provide valuable insight into the conditions of particular soil layers. As such, both tests should be employed to gain a comprehensive understanding of the soil conditions at any given site.
