Plate Load Test At Site

PLATE LOAD TEST :-

Use:

· This test is method of conducting load test on the soil i.e., evaluation of Ultimate bearing capacity and settlement under a given load.

· This method assumes that down to the depth of influence of stresses, the soil strata are reasonably uniform.

· This is verified by boring or sounding.

Plate load test is done to determine ultimate bearing capacity of the soil. The plate load test consisted of the following components:-

· Two rigid square plate of size 75cm×75cm and 45cm×45cm.

· Hydraulic jack to apply load on plate.

· Steel joist.

· 2 dial gauges ,G₁ & G₂.

· 1900 Sand bags each of approximately 35 kg. Approximately total load = 60 tonnes.

Procedure carried out at site to conduct Plate load test:-

Step1: Test Setup:-

i. A test pit* is dug at site up to the depth at which the foundation is proposed to be laid i.e. 3m.

ii. At the centre of the pit a small square depression or hole is made whose size is equal to the size of the test plate (75cm x 75cm) and bottom level of which corresponds to the level of actual foundation.

iii. The central hole of the size 75cm x 75cm is excavated in the pit.

iv. The mild steel plate (also known as bearing plate) used in the test should not be less than 25 mm in thickness and usually two square rigid plates are used of sizes:-

· 75 cm x 75 cm.

· 45 cm x 45 cm.

The 45 cm x 45 cm plate is kept above the 75cm x 75 cm plate as shown in fig a.

The plates are machined on side and edges.

Fig-a. Square bearing plates of different sizes.

Step 2: A loading platform is constructed over the column placed on the test plate and test load is applied by placing dead weight in the form of sand bags (1900 Sand bags of approximately 35 kg) .Whole weight of bags is on steel joist as shown in fig b.

Fig-b. Loading by sand bags.

Step3: The test is prepared and loose material so that 75 cm x 75 cm plate resists horizontally in full contact with the soil sub-grade.

Step4: Hydraulic jack is placed for applying the load to the test plate. Load is applied to the test plate through a centrally placed column. When the load is applied to the test plate, it starts sinking slowly.

Step5: Settlement of the plate is observed by 2 dial gauges,G₁and G₂ fixed at diametrically opposite ends. The dial gauges have a sensitivity of 0.02 mm.

Step6: Settlement should be observed for each increment of loads at 5,10,15, 20,……….60 tonnes.

Let me show you the clear Diagrammatical representation of plate load test at site:

*The width of the pit should be at least 5 times the width of the test plate.

The readings noted during test:

LOAD	TIME (min)	G₁(mm)	G₂(mm)
5 tones	1	5.28	3.7
	2.15	5.3	3.73
	4	5.1	3.73
	6.25	5.31	3.74
	9	5.31	3.75
	16	5.23	3.75
	25	5.36	3.79

LOAD	TIME (min)	G₁(mm)	G₂(mm)
10 tones	1	4.12	2.5
	2.15	4.21	2.52
	4	4.24	2.55
	6.25	4.3	2.55
	9	4.37	2.567
	16	4.37	2.57
	25	4.38	2.59

Settlements with load and time

Similarly, readings are noted than for 15, 20…., 60 tonnes of loads.

Result recorded:-

The net allowable bearing capacity for design of shallow foundations may be considered as 25t/㎡with minimum depth of foundation at 3.0 m with respect to the existing ground levels.

The gross bearing capacity shall be limited to 30 t/㎡.

Conclusion of Plate load test :-

The conclusion is based on the detail of test conducted at site. It determines its advantages and disadvantages:-

The advantages of Plate Load Test are:-

It provides the allowable bearing pressure at the location considering both

shear failure and settlement.

Being a field test, there is no requirement of extracting soil samples.

The loading techniques and other arrangements for field testing are identical to the actual conditions in the field.

It is a fast method of estimating behaviour of ground and foundation.

The disadvantages of Plate Load Test are:-

The test results reflect the behaviour of soil below the plate, not that of actual footing which is generally very large.

It is essentially a short duration test. Hence, it does not reflect the long term consolidation settlement of soil.