Method of Plate Load Test on soil for Bearing capacity and Settlement IS 1888 : 1982
Load test for estimation of bearing capacity of soils and its settlement.
Method of Plate Load Test on soil for Bearing capacity and Settlement IS 1888 : 1982
Plate load test
Test Standard :IS 1888: 1982
Apparatus :
Loading platform truss or Other Loading device.
Hydraulic jack with pressure gauge, electronic load cell, or proving ring.
Bearing Plates – Circular or square bearing plates of mild steel, minimum. thickness 25 mm and from 300 to 750 mm.
Dial gauges with 25 mm travel capacity.
Datum Beam or Rod.
Selection of Location
The locations for load test shall be based on exploratory borings, and unless otherwise desired, shall be conducted at an elevation of the proposed foundation level under the worst estimated conditions. In case the water table is within the depth equal to the width of the test plate, the test shall be conducted at water table level. In case water table is higher than the test level, it shall be lowered to the test level and maintained by pumping through a sump, away from the test plate, however, for the soils like cohesion less silt and fine sand which cannot be drained by pumping from the sump, the test level shall also be water table level.
Typical view of truss loading
Preparation of Test Pit
The pits, usually at the foundation level, having in general normally of width equal to five times the test plate or block, shall have a carefully levelled and cleaned bottom at the foundation level, protected against disturbance or changes in natural formation.
Dead Load
The dead load of all equipment used, such as ball and socket, steel plate, loading column, jack, etc, shall be recorded prior to application of load increment.
Slection of Size and Shape of Plate
In case of road problems and circular footing, square plates may be use. For clay and silty soils and for loose to medium dense sandy soils with N < 15, a 450 mm square plate or concrete blocks shall be used. In the case of dense sandy or gravelly soils ( 15 < N < 30 ) three plates of sizes 300 mm to 750 mm shall be used depending upon practical considerations of reaction loading and maximum grain size. The side of the plate shall be at least four times the maximum size of the soil particles present at the test location.
Test Procedure
Setup the Loading Device , If truss using as loading platform ,The loading platform shall be supported by suitable means at least 2.5 m from the test area with a height of 1 m or more above the bottom of the pit to provide sufficient working space. No support of loading platform should be located within a distance of 3.5 times size of test plate from its centre.
The test plate shall be placed over a fine sand layer of maximum thickness 5 mm and level horizontally by a spirit level to avoid eccentric loading. The hydraulic jack should be centrally placed over the plate.A minimum seating pressure of 70 g/cm2 shall be applied and removed before starting the load test.
Place two beam or datum bar over the ground and fixed with minimum two dial gauges resting at diametrically opposite ends of the plates. The dial gauges shall be so arranged that settlement is measured continuously without any resetting in between.
Load Increments
Apply the load to soil in cumulative equal increments up to 1 kg/cm2 or one-fifth of the estimated ultimate bearing capacity, whichever is less. The load is applied without impact, fluctuation or eccentricity .
Settlement and Test result Observation
Settlements should be observed for each increment of load after an interval of 1, 2.25, 4, 6.25, 9, 16 and 25 min and thereafter at hourly intervals to the nearest 0.02 mm. In case of clayey soils the ‘time settlement’ curve shall be plotted at each load stage and load shall be increased to the next stage either when the curve indicates that the settlement has exceeded 70 to 80 percent of the probable ultimate settlement at that stage or at the end of 24 hour period.
For soils other than clayey soils each load increment shall be kept for not less than one hour or up to a time when the rate of settlement gets appreciably reduced to a value of 0.02 mm/min.
The next increment of load shall then be applied and the observations repeated. The test shall be continued 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.
Alternatively where settlement does not reach 25 mm, the test should be continued to at least two times the estimated design pressure. If needed, rebound observations may be taken while releasing the load.
DETERMINATION OF ULTIMATE BEARING CAPACITY OR SETTLEMENT
Draw Shape of the Load/Settlement Curve
A load settlement curve shall be plotted out to arithmetic scale. The typical curves are shown in Figure
Curve A is typical for loose to medium cohesionless soil, it is a straight line in the earlier stages but flattens out after some time, but there is no clear point of failure.
Curve B is for cohesive soil, it may not be quite straight in the early part and leans towards settlement axis as the settlement increases.
curve C is for partially cohesive soils
curve D is purely for dense cohesionless soils.
Bearing capacity test of soil
Bearing Capacity for Clayey Soils
Ultimate bearing capacity = ultimate load for plate qu(f) = qu(p)Bearing Capacity for Sandy Soils Ultimate bearing capacity = ultimate load for plate x {Width of pit (Bf) / Size of Plate (Bp)} qu(f) = qu(p) x Bf / Bp
Method of Plate Load Test on soil for Bearing capacity and Settlement IS 1888 : 1982
Calculation of Foundation Settlement from Plate Load Test as per IS : 1904-1978 Foundation Settlement for Clayey Soils Settlement (sf) = sp x Bf/Bp Foundation Settlement for Sandy Soils Settlement (sf) = sp [{Bf(Bp + 0.3)}/{Bp(Bf + 0.3)}]2Where Bf and Bp are widths of foundation and plate
Method of Plate Load Test on soil for Bearing capacity and Settlement IS 1888 : 1982
