Field test using plate load gives actual estimation of bearing capacity (BC) for soils for shallow foundations.
Satish Barmade
Plate Load Test is one of the field test used for determining the bearing capacity of the soil and corresponding settlement under a given load. Plate load test consists of loading a steel plate placed at, where footing is to be rest and recording the settlements corresponding to each load increment with the help of minimum three(3) displacement dial gauges. The load typically increased gradually until the plate starts to sink. Rate of sinking is strictly recorded to understand the settling characteristics.
Typically, bearing capacity found is recorded as the ultimate bearing capacity of the stratum. The ultimate bearing capacity of soil reduced to safe bearing capacity by adopting a suitable factor of safety. Typical recommendation for factor of safety is 2 to 3.
The characteristic feature identified through a plot as specified in IS 1888-1982 (see fig. 1). The load intensity and settlement observations of the plate load test are plotted in the form of load settlement curves.
Fig 1. Load settlement curve for various soils (IS 1888-1982)
Safe bearing capacity (SBC) based on permissible settlement as indicated in codal provisions. As shown in fig 1, the settlement of footing is also related to the SBC of the soil.
The value of permissible settlement (Sf) for the different types of footings (isolated or raft) for various types structures are specified in the IS 1888-1982. code. The corresponding settlement of test plate (Sp) obtained using the following formula,
Where,
B = width of footing in mm
Bp = width of test plate in mm
Sp= settlement of the test plate in mm
Sf = settlement of footing in mm
Although, plate load test provides field safe bearing capacity of soil, it has certain limitations as follows,
- The test results reflect only the characteristics of the soil located within a depth of less than twice the width of the bearing plate. Typically the foundations are large than the test plate, the settlement and shear resistance of the soil against shear failure depends on the properties of stratum such as thickness. Thus, the results of the test could be misleading, if the characteristics of the soil changes at shallow depths.
- Plate load test is for a short duration, so it does not give the ultimate settlements, particularly, in case of cohesive soils.
General testing arrangement for plate load test performed in Maharastra by geoenvirovision for agencies like public works department (PWD), Maharashtra jeewan pradhikaran (MJP), Irrigation department, National highways Authority of India (NHAI), Maharashtra Industrial Development corporations (MIDC), Muncipal corporation (Nashik Municipal corporation), Builders, Infrastrastucture developers and contractors.
Apart from the plate load test other tests used for the field bearing capacity determinations includes,
- Standard penetration test (SPT)
- Cone penetration test (CPT)
Author: Satish Barmade
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