SPT Field Test.pdf
- 1. In-situ Testing When it is difficult to obtain “undisturbed” samples Cohesionless soils, Sensitive clays In-situ Test Methods – Standard Penetration Test (SPT) – Cone Penetration Test (CPT) – Vane Shear Test (VST) – Texas Cone Penetration Test (TCP)
- 5. Standard Penetration Test (SPT) The standard penetration test is carried out in a borehole, while DCPT and SCPT are carried 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.
- 6. Standard Penetration Test (SPT) It is an insitu test especially for cohesionless soils. It gives idea about relative Density (ID) Conducted in a bore hole using standard split spoon sampler. SPT value (N) Means, the number of blows required for 30 cm penetration. Cohesion and Angle of internal friction values of soil depends on the N-number.
- 7. Standard Penetration Test (SPT) 63.5 kg Hammer 76 cm free fall Drive sampler over 45 cm Record no. of blows per each 15 cm penetration SPT blow count=blows for 2nd 15 cm penetration + blows for 3rd 15 cm penetration
- 8. Standard Penetration Test (SPT) The test is reported as ‘refusal’ and the test is halted if 50 blows are required for any 150 mm penetration. Importance of N-Value: The N Value indicates the relative density of the cohesionless soil and the unconfined compressive strength of the cohesive soil. If the soil is compact or stiff, the penetration number is high. In general , the greater the N-value , the greater is the angle of shearing resistance. The consistency and the unconfined compressive strength of the cohesive soils can be approximately determined from SPT Number N.
- 9. Standard Split Spoon Sampler Thick wall (0.25in) cylinder Sampling tube is split along the length Hammered into the ground
- 13. Standard Penetration Test (SPT)
- 14. Types of SPT Hammers
- 16. Corrections to SPT blow Counts Factors affecting SPT blow count: Hammer Efficiency (See Table 4.3) Borehole diameter (See Table 4.4) Type of sampler (See Table 4.4) Rod length (See Table 4.4)
- 17. SPT Correction Factors 60 . 0 N C C C E N R S B m 60 hammer efficiency (Em) …. Table 4.3 bore hole diameter (CB)…….Table 4.4. sampler correction (CS) ……Table 4.4 rod length (CR) ………Table 4.4
- 18. Corrections for N-Value Overburden Pressure Correction Dilatancy Correction
- 19. Corrections for N-Value Overburden Pressure Correction In granular soils , the overburden pressure affects the penetration resistance. If the two-soils having same relative density but different confining pressures are tested, the one with higher confining pressure gives a high penetration number. Thus N-Value depends upon confining pressure and which in turn increases with depth. Higher the confining pressure , higher will be the N- Value. N- Value for soils at shallow depth is underestimated and that at greater depth is over estimated.
- 20. Corrections for N-Value Overburden Pressure Correction For Uniformity, the N-Values obtained from field tests under different effective overburden pressure are collected to a standard effective overburden pressure.
- 21. Corrections for N-Value Overburden Pressure Correction
- 22. Corrections for N-Value Dilatancy Correction Dilatancy Correction is to be applied when N’ obtained after the overburden correction exceeds 15 in saturated fine sand and silts.
- 24. N’ > 15 is an indication of a dense sand . In such a soil, the fast rate of application of shear through the blows of a drop hammer is likely to induce negative pore water pressure in a saturated fine sand under undrained condition of loading. Consequently, a transient increase in shear resistance will occur, leading to a SPT value higher than the actual one.
- 27. Use of SPT Data To Determine Relative Density, Dr To determine To determine C