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Note for Geotechnical Engineering - 2 - GTE-2 by Nusfa Samad

  • Geotechnical Engineering - 2 - GTE-2
  • Note
  • APJ Abdul Kalam Technological University - KTU
  • Civil Engineering
  • B.Tech
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MODULE 3 1. Sounding methods Standard Penetration Test • Procedure – corrections to be applied to observed N values • Procedure for estimation of representative average N value • Numerical examples • Factors influencing the SPT results and precautions to obtain reliable results • Merits/drawbacks of the test • Correlations of N value with various engineering and index properties of soils 2. Static Cone Penetration Test • Procedure • Merits/drawbacks • Correlation of static CPT results with soil properties 3. Dynamic Cone Penetration Test • Procedure • Merits/drawbacks 4. Critical comparison of SPT, static CPT and dynamic CPT 1

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In-situ tests General The in situ tests in the field have the advantage of testing the soils in their natural, undisturbed condition. Laboratory tests, on the other hand, make use of small size samples obtained from boreholes through samplers and therefore the reliability of these depends on the quality of the so called ‘undisturbed' samples. Further, obtaining undisturbed samples from non-cohesive, granular soils is not easy, if not impossible. Therefore, it is common practice to rely more on laboratory tests where cohesive soils are concerned. Further, in such soils, the field tests being short duration tests, fail to yield meaningful consolidation settlement data in any case. Where the subsoil strata are essentially non-cohesive in character, the bias is most definitely towards field tests. The data from field tests is used in empirical, but time-tested correlations to predict settlement of foundations. The field tests commonly used in subsurface investigation are: • Penetrometer test • Pressuremeter test • Vane shear test • Plate load test • Geophysical methods Penetrometer Tests : 1. Standard penetration test (SPT) 2. Static cone penetration test (CPT) 3. Dynamic cone penetration test (DCPT) 2

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1. Standard penetration test The standard penetration test is carried out in a borehole, while the DCPT and SCPT are carried out without a borehole. All the three tests measure the resistance of the soil strata to penetration by a penetrometer. Useful empirical correlations between penetration resistance and soil properties are available for use in foundation design. This is the most extensively used penetrometer test and employs a split-spoon sampler, which consists of a driving shoe, a split-barrel of circular cross-section which is longitudinally split into two parts and a coupling. IS: 2131-1981 gives the standard for carrying out the test. This method is commonly used for cohesionless soil which cannot be easily sampled. Test is extremely useful in determining the relative density and the angle of shearing resistance of cohesionless soil. It is also used for determining unconfined compressive strength of the cohesive soils. Procedure a) The borehole is advanced to the required depth and the bottom cleaned. b) The split-spoon sampler, attached to standard drill rods of required length is lowered into the borehole and rested at the bottom. c) The split-spoon sampler is driven into the soil for a distance of 450mm by blows of a drop hammer (monkey) of 63.5 kg falling vertically and freely from a height of 750 mm at the rate of 30 blows per minute .The number of blows required to penetrate every 150 mm is recorded while driving the sampler. The number of blows required for the first 150 mm is disregarded. The number of blows required for the last 300 mm of penetration is added together and recorded as the N value at that particular depth of the borehole. The number of blows required to effect the first 150mm of penetration, called the seating drive. d) The split-spoon sampler is then withdrawn and is detached from the drill rods. The split-barrel is disconnected from the cutting shoe and the coupling. The 3

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soil sample collected inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests. Sometimes, a thin liner is inserted within the split-barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory. The SPT is carried out at every 0.75 m vertical intervals in a borehole. This can be increased to 1.50 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm. In such a case, the N value can be recorded for the first 300 mm penetration. The boring log shows refusal and the test is halted if 50 blows are required for any 150mm penetration 100 blows are required for 300m penetration 10 successive blows produce no advance. Figure 4.2 shows schematic representation of SPT setup 4

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