![Settlement
We can also calculate settlement for given load from plate load
test as follows
• Foundation Settlement Calculation on Clayey Soils
Settlement of foundation (sf) = sp x Bf/Bp
• Foundation Settlement Calculation on Sandy Soils
Settlement of foundation
St = Sp [{B(Bv + 0.3)}/{Bp(B+ 0.3)}]2
• Where
B = the size of footing in m,
Bv = size of test plate in m,
Sp = settlement of test plate in m
St = settlement of footing in m](https://image.slidesharecdn.com/seminar-171105091433/85/Plate-load-test-and-its-interpretations-23-320.jpg)
Plate load test and its interpretations
- 1. Seminar Plate Load Test and its Interpretations
- 2. IS : 1888-1982 Method of Load Test on Soils ( 2nd Revision) This Indian Standard (Second Revision ) was adopted by the Indian Standards Institution on 30 November 1982, after the draft finalized by the Soil Engineering and Rock Mechanics Sectional Committee had been approved by the Civil Engineering Division Council.
- 3. Scope In the case of unusual soil types and for all heavy and important structures, relevant laboratory tests or field tests are essential. This standard covers plate load test method • for determination of ultimate bearing capacity of soil in place which assumes that soil strata is reasonably uniform. • The load test included in the standard is also used to find modulus subgrade reaction useful in the design of raft foundation and in the design of pavements.
- 4. Apparatus
- 5. Apparatus 1. Loading platform truss of sufficient size and properly designed members so as to estimate load reaction for conducting the test shall be used. The typical set up used for gravity loading is given in Fig. 1, for reaction loading in Fig. 2 and for loading truss in Fig. 3. 2. Hydraulic jack of required capacity with properly calibrated load measuring device, such as pressure gauge, electronic load cell, or proving ring shall be used. 3. Bearing Plates - Circular or square bearing plates of mild steel, not less than 25 mm in thickness and varying in size from 300 to 750 mm with chequered or grooved bottom ( see Fig. 4 ), provided with handles for convenient setting and centre marked. As an alternative, cast in-situ or precast concrete blocks may be used with depths not less than two third the width
- 8. Apparatus 4. Settlement Recording Device - Dial gauges with 25 mm travel, capable of measuring settlement to an accuracy of 0.01 mm. 5. Datum Beam or Rod - Beam or rod of sufficient strength capable of maintaining straightness when fitted on two independent supports fitted with arms or magnetic bases for holding dial gauges. 6. Miscellaneous Apparatus - A ball and socket arrangement, loading columns, steel shims, wooden blocks, collar, reaction girder with cradles for independent fitting to the reaction platform as necessary to the particular set up.
- 9. Procedure
- 10. Procedure 1. 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 cohesionless silt and fine sand which cannot be drained by pumping from the sump, the test level shall also be water table level.
- 11. Procedure 2. 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. 3. 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.
- 12. 4. Size and Shape of Plate - 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. NOTE - N is the standard penetration resistance value determined in accordance with IS : 2131-l%l*.
- 13. Test Arrangement • 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, so that the centre of plate coincides with the centre 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 of70 g/cm2 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 resettingin between.
- 14. 6. 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 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
- 15. Settlement and observations 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.
- 16. Interpretations
- 17. Interpretations of results • From the results obtained in plate load test, a logarithmic graph is drawn between loads applied to the corresponding settlement. Load is taken on x-axis and settlement is on y-axis. • Four typical curves are shown in Fig. 5.
- 19. • Curve A is typical for loose to medium non cohesive soils, it can be seen that initially this curve is a straight line, but as load increases it flattens out. There is no clear point of shear failure. • Curve B is typical for cohesive soils. This may not be quite straight in the initial stages and leans towards settlement axis as the settlement increases. • Curve C is typical for partially cohesive soils possessing the characteristics of both A and B. • Curve D is typical for purely dense cohesion-less soil.
- 20. • No difficulty should be experienced in arriving at the ultimate bearing capacity in case of dense cohesion-less soils or cohesive soils ( see Fig. 5, curves D and B ) as the failure is well defined. • But in the case of Curves A and C where yield point is not well defined settlements shall be plotted as abscissa against corresponding load intensities as ordinate, both to logarithmic scales ( see Fig. 6 ), which give two straight lines, the inter-section of which shall be considered as yield value of soil.
- 22. Calculations bearing capacity • Bearing Capacity Calculation for Clayey Soils Ultimate bearing capacity = ultimate load for plate i.e. qu(f) = qu(p) • Bearing Capacity Calculation 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 • Finally, safe bearing capacity = ultimate bearing capacity / factor of safety The factor of safety ranges from 2 to 3.
- 23. Settlement We can also calculate settlement for given load from plate load test as follows • Foundation Settlement Calculation on Clayey Soils Settlement of foundation (sf) = sp x Bf/Bp • Foundation Settlement Calculation on Sandy Soils Settlement of foundation St = Sp [{B(Bv + 0.3)}/{Bp(B+ 0.3)}]2 • Where B = the size of footing in m, Bv = size of test plate in m, Sp = settlement of test plate in m St = settlement of footing in m
- 24. Limitations 1) The test results reflect only the character of the soil located within a depth of less than twice the width of the bearing plate. Since the foundations are generally larger than the test plates, the settlement and shear resistance will depend on the properties of a much thicker stratum. 2) Moreover this method does not give the ultimate settlements particularly in case of cohesive soils. Thus the results of the test are likely to be misleading, if the character of the soil changes at shallow depths, which is not uncommon. A satisfactory load test should, therefore, include adequate soil exploration ( see IS : 1892-1979* ) with ~due attention being paid to any weaker stratum below the level of the footing.
- 25. Limitations 3) Another limitation is concerning the effect of size of foundation. For clayey soils the bearing capacity ( from shear consideration ) for a larger foundation is almost the same as that for the smaller test plate. But in dense sandy soils the bearing capacity increases with the size of the foundation. Thus tests with smaller size plate tend to give conservative values in dense sandy soils. It may, therefore, be necessary to test with plates of at least three sizes and the bearing capacity results extrapolated for the size of the actual foundation (minimum dimensions in the case of rectangular footings ).
Editor's Notes
- #25: Plate load test, though useful in obtaining the necessary information about the soil with particular reference to design of foundation has some limitations :