How to Determine the Bearing Capacity of Foundation? | Soil Engineering

Plate load test (PLT) is an in situ test to determine the bearing capacity and probable settlement of a foundation under any given loading. The test essentially consists of loading a rigid plate in increments at the foundation level and determining the settlements corresponding to each load increment. The settlement of foundation can be obtained from the load-settlement relationship obtained from the plate load test.

Test Plate and Test Pit:

The test plate is a square or circular steel plate and 30, 45, 60, or 75 cm in size. The square plate is more commonly used. The plate is machined on the sides to have sharp edges and should have a minimum thickness of 25 mm.

The test pit is excavated up to a depth equal to the depth of foundation. The width of the test pit is five times the width of the test plate (B_p). At the center of the pit, a square hole is dug whose size is equal to the size of the test plate and the bottom level of which corresponds to the level of the actual foundation. The depth of the hole (D_p) should be such that –

Method of Load Application:

The load is applied on the test plate by a hydraulic jack.

The reaction of the hydraulic jack is borne by any of the following two methods:

1. Gravity Loading Platform Method:

In this method, a platform is constructed and supported on masonry walls as shown in Fig. 19.12. To give the required reaction to the hydraulic jack, the platform is loaded with an adequate number of sand bags, stones, or concrete blocks. The general arrangement in the method is shown in Fig. 19.12.

2. Reaction Truss Method:

Figure 19.13 shows the test setup in reaction truss method (the test plate, hydraulic jack, and the dial gauges not shown). In this method, the reaction to the hydraulic jack is obtained from the truss, which in turn is supported by anchor piles. The truss is usually of mild steel sections. Guy ropes are used for the lateral stability of the truss. The use of reaction truss is more advantageous than the gravity loading method since the former is simple, quick, and less difficult for erection and dismantling and is also more economical.

Test Procedure for Conducting Plate Load Test:

The test plate is firmly seated in the hole. If the ground is slightly uneven, a thin layer of sand is spread underneath the plate. A seating load of 70 g/cm² (7 kN/m²) is applied on the test plate, which is released before the actual test is conducted.

The load is applied by hydraulic jack in increments of one-fifth expected safe bearing capacity or one-tenth expected ultimate bearing capacity. The settlement of the test plate is recorded by dial gauges fixed at diametrically opposite ends with sensitivity of 0.02 mm. Settlement should be observed at each load increment at time intervals of 1, 4, 10, 20, 40, and 60 min, and, thereafter, at hourly intervals, until the rate of settlement becomes less than about 0.02 mm/h. After this, the next load increment is applied. The test is continued until the maximum load is equal to one-and-a-half times the estimated ultimate load or three times the expected allowable load.

Load-Settlement Curve:

The maximum settlement under each load increment is plotted against the load intensity as shown in Fig. 19.14. The failure load for the test plate (q_up) may be obtained without difficulty for dense cohesionless soils and cohesive soils as the load intensity where the settlement increases at a rapid rate with a little increase in load intensity.

In case of partially cohesive soils and loose- to medium-dense sands, the yield point may not be well-defined. In this case, the load intensity and the settlements are plotted on a log-log scale. The ultimate load for the plate q_up is indicated by a break on the log-log plot between the load intensity and the settlements as shown in Fig. 19.15.

Interpretation of Test Results:

Determination of bearing capacity and settlement is done as follows:

1. The ultimate bearing capacity of the proposed foundation q_uf can be obtained from the following relations:

i. For pure clayey soils, q_uf = q_up …(19.12)

ii. For sandy soils, q_ui = q_up x (B_f/B_p) …(19.13)

where B_f is the width of the foundation and B_p is the width of test plate.

2. The ultimate settlement of the footing can be determined for any loading (q_u) from the following relations:

i. For clayey soils, S_f = S_p x (B_f/B_p) …(19.14)

ii. For sandy soils, S_f = S_p {[B_f/(B_p + 0.3)]/[B_p(B_f + 0.3)]²} …(19.15)

where S_p is the settlement of the test plate at the given load intensity q_u, and B_f and B_p are the widths of footing and test plate in meters.

Limitations of PLT:

The following factors show the limitations of PLT:

1. Size Effect:

The results of PLT reflect the strength and settlement characteristics of the soil within the pressure bulb of the test plate, extending to a small depth of about two times the width of the test plate. As the pressure bulb of actual foundation extends to a much greater depth, PLT does not truly represent the actual site conditions up to the required depth.

2. Scale Effect:

The ultimate bearing capacity of cohesionless soils increases with the size of foundation. The set­tlement of footings in both cohesive and cohesionless soils is a function of width of footing. Thus, the bearing capacity and settlement of the actual foundation differs from those obtained from PLT using a small test plate. To reduce the scale effect, it is desirable to repeat PLT with plates of two or three different sizes and extrapolate the bearing capacity and settlement of the actual foundation.

3. Time Effect:

PLT is essentially a test of short duration. For clayey soils, it does not give the ultimate settlement.

4. Interpretation of Failure Load:

The failure load in PLT is not well-defined except in the case of general shear failure. An error of personal interpretation may be involved in another types of failure.

5. Reaction Load:

It is not practicable to provide reaction load of more than 250 kN. Hence, the test on plate of width larger than 60 cm is difficult.

6. Effect of Water Table:

Results of PLT conducted above the water table are not applicable for predicting founda­tion behavior, whose presser bulb is below the water table. This needs correction for the difference of submerged and un-submerged soil conditions.

7. Site and Season:

It is difficult to obtain results that represent the whole site and the whole season. In summer, the load test gives better results compared to those in wet season.

8. Shape of Foundation:

The test ignores the effect of shape of footing on bearing capacity and settlement. As such the results of PLT obtained using a square test plate cannot be applied for a circular, rectangular, or strip footing.

A PLT is valuable for predicting the settlement characteristics of cohesionless soils and gravels, which are difficult to sample. But the test must be conducted properly and results should be interpreted correctly.