Plate Load Test

Aim : To determine ultimate bearing capacity and settlement of soil mass.

Apparatus:

1. Typical set up used for gravity loading OR Loading platform truss

2. Hydraulic jack of required capacity

3. Properly calibrated load measuring device, such as pressure gauge, electronic load cell, or proving ring

4. Circular or square bearing plates of mild steel

5. Dial gauges with 25 mm travel, capable of measuring settlement to an accuracy of 0.01 mm.

6. Datum Beam or Rod

7. A ball and socket arrangement

8. Loading columns

9. Steel beams

10. Wooden blocks

11. Collar

12. Reaction girder with cradles

Theory:

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. 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.

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.

Except in case of road problems and circular footing square plates may be adopted. For clayey 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.

 

Procedure:

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 center.

The test plate shall be placed over a fine sand layer of maximum thickness 5 mm, so that the center of plate coincides with the center of reaction girder/beam, with the help of a plumb and bob and horizontally levelled by a spirit level to avoid eccentric loading. The hydraulic jack should be centrally placed over the plate with the loading column in between the jack and reaction beam so as to transfer load to the plate. A ball and socket arrangement shall be inserted to keep the direction of the load vertical throughout the test. A minimum seating pressure of 70 g/cm' shall be applied and removed before starting the load test.

The two supports of the reference beam or datum rod shall be placed over firm ground, 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.

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 and in case of hydraulic jack load is measured over the pressure gauge, attached to the pumping unit kept over the pit, away from the testing plate through extending pressure pipes.

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.

A load settlement curve shall be plotted out to arithmetic scale. From this load settlement curve the zero correction which is given by the inter-section of the early straight lines or the nearly straight line part of the curves with zero deadlines shall be determined and subtracted from the settlement readings to allow for the perfect seating of the bearing plate and other causes.

Results & Calculations:

The safe bearing pressure for medium and dense sands could be read, corresponding to a settlement (`S_{p}`), which shall be calculated as under (`S_{t}`) taken as permissible settlement of footing.

`S_{t}=S_{p}\left[\frac{B\left(B_{P}+0.3\right)}{B_{p}(B+0.3)}\right]^{2}`

 

Where,

B = the size of footing in m,

`B_{p}`= size of test plate in m,

`S_{p}`= settlement of test plate in m, and

`S_{t}`= settlement of footing in m.

 

Conclusions:

As per IS-1888 (1982). The value of settlement must be in Range up to 25 mm & according to practical the obtained value is ok.( The Conclusions would be different.)