How to Conduct Plate Load Tests on Soils

Plate load tests is a short-term model loading test used to obtain information about the bearing capacity and settlement characteristics of soil. It is often adjudged the most direct approach to obtain this information and it is used as replacement for tests on prototype foundation which is practically impossible.

To conduct this test, a pit of the size 5 B_p x 5 B_p, where B_p is the size of the plate is excavated to a depth equal to the foundation depth (D_f). The plate is usually square or circular in shape and range from size of 300 mm to 750 mm. The plates are usually made of steel and comes in minimum thickness of 25 mm to provide adequate rigidity.

Other procedures for plate load tests include;

1. Excavate a central hole in the pit of size Bp x Bp and depth of the central hole (Dp) obtained from the relation below;

D_p/B_p = D_f/B_f, which implies that, D_p = [(B_p x D_f)/ B_f]

Where,
B_f = width of the pit, determined previously as 5 B_p

2. Place the plate in the central hole and apply load by means of hydraulic jack (hydraulic jack often used because it is usually more accurate and offers uniform loading), while increasing the load which may be gravity or dead weights in arbitrary increments. Initially, a seating load of 7 kN/m² is first applied which is released after sometime before actual loading is commenced.

3. The loading is applied in increments of about 20 % of the estimated safe load or (1/10) of the ultimate load.

4. The reaction of the jack is provided by means of reaction cross beam or steel trusses anchored at both ends. If the loading set up is a platform with dead weights, a loaded platform (kentledge) which may be in the form of sand bag, scrap iron, ingots or any other convenient heavy metal may be used to provide reaction.

5. Record the settlements due to load after 1, 5, 10, 20, 40, and 60 minutes and later at an hourly interval using at least 2-3 dial gauges with a least count of 0.02 mm until the rate of settlement is less than 0.02 mm/hour or least for 24 hours.

6. Continue the test until a load of about 1.5 times the anticipated ultimate load is applied, or until a settlement at which failure occurs or until at least the settlement of about 25 mm has occurred.

7. Plot the load-settlement or pressure-settlement curve in a log-log plot. The ultimate load for the plate, q_u (p) is indicated by a break on the plot between the load intensity, q and the settlement. If the break is well-defined, the ultimate load is taken as that corresponding to settlement of 1/5 of the plate width (B_p). On the natural plot, the ultimate load is obtained from the interaction of the tangent made where there is change of steepness in the curve.

8. The ultimate bearing capacity of the proposed foundation, q_u (f) can be obtained from the following equations;

            For clay soils, q_u (f) = q_u (p)

            For sandy soils, q_u (f) = q_u (p) x (B_f/B_p)

9. The settlement of the proposed foundation can be obtained from the following equations;

            For clay soils, s_f = s_p x (B_f/B_p)

            For sandy soils, s_f = s_p [(B_f (B_p + 0.3))/ (B_p (B_f + 0.3))]²

Where,
S_p is obtained from the load intensity-settlement curve for q_o.