Plate Load Test Procedure & Calculation

What is a plate load test?

The plate load test is a field test performed to determine the ultimate bearing capacity of the soil and its corresponding settlement under a given load.

The plate load test is for the design of shallow foundations. This test is also to find out the modulus of the sub-grade reaction, also called the K-value of soil, for designing raft foundations and pavements.

IS codes that govern the method for conducting plate load tests

IS 1888 lays down the method for conducting load tests to estimate the bearing capacity of soils and their settlement.

How to carry out the plate load test?

The plate load test is carried out by placing a plate at the desired test location and depth. Initial seating pressure of 70 Kg/Sq.cm is applied and removed. This initial load ensures the location and verticality of the applied load. Re-apply the load. It is essential to maintain the alignment of the hydraulic jack so that the load is transferred vertically.

After the initial load, further load is applied in small increments. The corresponding settlement is recorded for each load increment with the help of a minimum of three(3) displacement dial gauges.

Load increments shall be 1 kg/Sq.cm or one-fifth of the estimated ultimate bearing capacity, whichever is less. The initial load and each load increment shall be kept for not less than one hour. The settlement is observed for each load increment at intervals of 1, 2.25, 4, 6.25, 9, 16, 25 & 60 minutes.

Continue the test till a settlement of 25 mm under normal circumstances or 50 mm in special cases, such as dense gravel, gravel and sand mixture, is obtained or till failure occurs, whichever is earlier. As the load applied is increased, a point will come when the settlement occurs at a rapid rate which is defined as failure.

The plate load test is terminated at this point; the load up to that point is divided by the area of the plate to get the ultimate bearing capacity of soil at that depth.

The safe bearing capacity of the soil is calculated by dividing the ultimate bearing capacity of the soil by a factor of safety. Typically the recommended factor of safety shall be 2 to 3. Where settlement does not reach 25 mm, the test shall continue till at least two times the estimated design pressure.

The characteristics of the various soils where load increments and correlated settlement observed during plate load test is plotted as settlement curves in IS 1888. The same is reproduced below:

The calculation for permissible settlement

Permissible settlement (Sf) for the different types of footings for various types of structures are specified in IS 1888. The corresponding settlement of the test plate (Sp) shall be calculated through the following formula,

Sf= Sp {[B(Bp+0.30)]/[Bp(B+0.300]}2

Where;

B = Width of footing in mm

Bp = Width of test plate in mm

Sp = settlement of test plate in mm

Sf = Permissible settlement of footing in mm

Sample calculations for carrying out plate load test based on anticipated bearing capacity

Anticipated ultimate bearing capacity of soil at test area = 375 kN/Sq.Mtr

Plate size used = 0.45 Mtr x 0.45 Mtr = 0.2025 Sq.Mtr

Therefore, the ultimate load on the plate = 375 kN/Sq.Mtr x 0.2025 Sq.Mtr = 75.94 KN

Initial load to be applied as per IS 1888 = 7 kN/Sq.Mtr

Therefore, initial load on plate = 7 kN/Sq.Mtr x 0.2025 Sq.Mtr = 1.42 kN

Load increments @ 20 % of ultimate load on plate = 75.94 kN x 0.20 = 15.188 kN, Say 15 kN

Note: Stop the test if settlement reaches 25 mm or failure happens before applying the anticipated ultimate load on the plate. If settlement does not reach 25 mm and/or failure does not happen on reaching the anticipated ultimate load on the plate, continue further till failure occurs or till settlement reaches 25 mm, whichever is earlier.

Sample calculations for safe bearing capacity based on test data

Plate size used = 0.45 Mtr x 0.45 Mtr = 0.2025 Sq.Mtr

Ultimate load on plate before failure = 81.00 kN

Ultimate bearing capacity per Sq.Mtr = 81kN / 0.2025 Sq.Mtr = 400 kN/Sq.Mtr

Settlement of the plate before failure = 20 mm

Safety factor considered = 2.50

Safe bearing capacity of soil = 400 kN/Sq.Mtr / 2.50 = 160 kN/Sq.Mtr

Sample calculation for permissible settlement (Sf) of footing based of test data

Width of footing at site (B) = 2600 mm

Width of plate used (Bp) = 450 mm

Settlement of test plate before failure (Sp) = 20 mm

Permissible settlement (Sf) = 20 {[2600 (450+.30)] / [450 (2600+0.30]}2 = 20 (1170780 / 1170135)2

= 20 x 1.001 = 20.02 mm

Advantages

The plate load test is a very useful method for gathering quick and reliable information for the design of shallow foundations.
It is cost-efficient and saves time.

Limitations

The plate load test has the following limitations:

The test result predicts the behaviour of soil located within a depth of more than twice the width of the bearing plate. In actual site conditions, the influence zone of the foundation is up to much greater depths. Therefore the result could be misleading if the soil characteristics change at shallow depths.
The period for carrying out a plate load test is short. Hence it cannot predict the settlement and ultimate bearing capacity for a longer period, particularly in cohesive soils.
The bearing capacity of a clayey soil is similar to the bearing capacity obtained by plate load test, but for soils like dense sandy soil, the actual capacity is usually more than that obtained by plate load test.
The settlement of loose sandy soil is usually more than the settlement obtained by the plate load test.

Apart from the plate load test, other field tests used for finding out the bearing capacity of the soil are as follows;