Plate Load Test:
Plate load test of soil is conducted to evaluate the safe bearing capacity and settlement of foundation soil, basically for clayey and sandy soils. This method believes that clown to the depth of influence of stresses the subsoil strata is relatively uniform and should be confirmed by boring. So, the plate load test of soil is helpful for the selection and design of the foundation.
Significance of Plate load Test of Soil:
- Determination of allowable bearing capacity of the open foundation for uniform foundation soil or rock deposits.
- It is suitable for gravel or boulder strata when SPT and DCPT does not give clear results.
- Used to find out the modulus of subgrade reaction (K) required for raft foundation and Rigid Pavement.
Apparatus for Plate Load Test on Soil:
The load plate load test apparatus includes MS bearing plates, loading equipment, and instruments to record the applied loads and resulting settlement.
Circular or square bearing plates of mild steel, not less than 25 mm in thickness and 750mm diameter with few other smaller diameters (usually 600, 450 and 300mm) plates as stiffeners of the same thickness are used for plate load test.
The loading arrangement can be a reaction truss with a hydraulic jack or kentledge. The reaction frame may suitably be loaded to give the required reaction load on the test plate. The applied load may be recorded either by a proving ring and dial gauge assembly or a pressure gauge connected to the output end of the hydraulic jack.
It can be done using three or four dial gauges(0.01 – 25.00 mm) set on the periphery of the bearing plate from an independent datum bar. The datum bar should be supported far from influence area.
Selection of Test Plate for Plate load Test:
- For circular footings and road pavement, circular plates are used for plate load test of soil.
- For standard penetration resistance value < 15: a 450mm square plate/concrete blocks shall be used.
- 15 < standard penetration resistance value < 30, three plates of sizes 300mm – 750mm shall be used depending upon the reaction loading and maximum grain size of the foundation soil.
- The side of the test plate shall be at least four times the maximum particle size present at the foundation level.
Preparation of Test Pit for Test:
- Excavated up to the proposed Foundation Level
- The test location is prepared after removing all the loose material properly; so that the 750mm diameter test plate rests horizontally in full contact with the foundation soil.
- Width of pit = 5 times of the Plate Width
Plate Load Test Procedure:
- First of all application of seating load equivalent to a pressure of 0.07 kg/cm2 (320 kg for 75 cm diameter plate) is applied and released after a few seconds and then dial gauge is set to zero.
- After that, the load is applied in a cumulative and equal increment of 1.0 kg/cm2 or 1/5th of estimated Ultimate Bearing Capacity whichever is less.
- For each increment, the settlement is recorded at a different time interval of 1.0, 2.25, 4.0, 6.25, 9.0, 16.0, 25.0 min, 1.0hr, 2.0hr, until the rate of settlement is not more than 0.02 mm/min, not less than 1hr for Sandy / Gravelly Soils.
- In clayey soil, the settlement measures are taken either for the time of exceeding 70 – 80% of estimated Ultimate Settlement or for 24 hours for each load increment.
- Application of next higher load and the in the same way the process is repeated.
- The test continues until a total settlement of 25.00 mm under normal condition, or 50.00 mm (under particular condition like dense gravel, gravel-sand mixture) is reached or till failure occurs whichever is earlier
- Calculation of Final Settlement corresponding to each loading intensity (Average of the settlement of all three or four dial gauge readings)
- When settlement does not reach 25.00 mm, the test shall be continued under the following condition:
(i) At least two times of the Estimated Design Safe Bearing Pressure or till the settlement increases rapidly at a certain load indicating shear failure whichever is earlier
(ii) The total settlement is more than 10% of the width or diameter of the plate
Example and Discussion:
Plate Load Test (Routine Type) was conducted in a particular site using a 450mm square plate. Load (vs) settlement plots are made in arithmetic scale to calculate Allowable Bearing Capacity, Young’s modulus. The calculations are presented below.
Project Data PLT-07:
- Maximum recorded plate settlement is 12.27mm under a load intensity of 35.0t/sqm.
- The allowable settlement as per IS:1904 is 50.00mm
- As the settlement observed is less than allowable settlement, the founding stratum is safe for design bearing capacity.
- Recommended safe load =20 t/sqm. Corresponding settlement =4.16mm
From the load settlement curves, the Young’s Modulus of the subsoils can be obtained, which in turn can be used to derive Subgrade modulus and immediate settlement. This immediate settlement will again determine the allowable bearing capacity of the foundation soil.
Subgrade Modulus is obtained from Plale Load Tests of soil as K = P/S, Where, P = Plate Load corresponding to settlement and S is the corrected subgrade modulus for the plate is presented below.
|Test No.||Location||Subgrade Modulus for plate as per IS: 9214, Kg/cm2||Corrected Subgrade Modulus at per IS: 9214, Kg/cm2|
The Subgrade modulus value is inversely proportional to the square root of the foundation area up to a base area of 10.0sqm and then remains constant. The Subgrade Modulus values for PLT-07 have been considered for calculations for other foundation sizes. Based on the above test result, the subgrade modulus for different foundation area is presented below.
Limitation of Plate Load Test of Soil:
- Test results reflect only the character of soil located within the depth twice the width of the test plate and not corresponding to the actual foundation width which is larger than the test plate Width.
- Does not give the total ultimate settlement for cohesive foundation soil.
- Misleading test results in case of the character of foundation soil changes within the shallow depth, which is not uncommon.
- A Satisfactory load test should also include adequate exploratory boreholes with due attention being paid to any weaker stratum.
- From shear consideration, the bearing capacity of clayey soil without any Angle of Internal Friction (phi) is almost the same as for smaller Test Plate.
- Since the bearing capacity of sandy soil increases with the size of the foundation, so the bearing capacity is conservative as derived with the smaller test plate. It may be necessary to test with plates of at least three different sizes and the bearing capacity results extrapolated with actual foundation size.