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Standard Penetration Test (SPT)
The document details the Standard Penetration Test (SPT) used for foundation engineering, specifically for determining soil properties such as relative density, shear resistance, and unconfined compressive strength. It describes the test setup, procedure, and the necessary corrections for overburden pressure and water table effects. The SPT results are correlated with the engineering properties of both cohesionless and cohesive soils, providing essential guidelines for analyzing soil behavior.
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Standard Penetration Test (SPT)
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- 2. MachenLink STANDARD PENETRATION TEST(SPT) • This test (IS : 2131 – 1981) is performed in clean hole, 100 to 150 mm in diameter. • A casing or drilling mud is used to support the sides of the hole. 63.5 𝑘𝑔 Drop hammer repeatedly falling 75 𝑐𝑚 Anvil Drill rod Split – barrel (drive) sampler [thick hollow tube: O.D.= 50.8 𝑚𝑚 I.D. = 35 𝑚𝑚 L= 456 to 762 mm • This test is most commonly used for cohesionless soil which can not be easily sampled. • Useful for determining the relative density and the angle of shear resistance. • It can also be used to determine the unconfined compressive strength of cohesive soil. • Test shall be made,… • At every change in stratum • At intervals not more than 1.5 m 30 blows per minute Whichever is less
- 3. MachenLink STANDARD PENETRATION TEST(SPT) 63.5 𝑘𝑔 Drop hammer repeatedly falling 75 𝑐𝑚 Anvil 150 𝑚𝑚 Drill rod Split – barrel (drive) sampler [thick hollow tube: O.D.= 50.8 𝑚𝑚 I.D. = 35 𝑚𝑚 L= 456 to 762 mm First Increment 30 blows per minute • This test (IS : 2131 – 1981) is performed in clean hole, 100 to 150 mm in diameter. • A casing or drilling mud is used to support the sides of the hole. • This test is most commonly used for cohesionless soil which can not be easily sampled. • Useful for determining the relative density and the angle of shear resistance. • It can also be used to determine the unconfined compressive strength of cohesive soil. • Test shall be made,… • At every change in stratum • At intervals not more than 1.5 m Whichever is less
- 4. MachenLink STANDARD PENETRATION TEST(SPT) 150 𝑚𝑚 Split – barrel (drive) sampler [thick hollow tube: O.D.= 50.8 𝑚𝑚 I.D. = 35 𝑚𝑚 L= 456 to 762 mm 150 𝑚𝑚 First Increment Second Increment 150 𝑚𝑚 Third Increment SPT (𝑁𝑜) = the number of blows required for 300mm penetration If the number of blows for 150mm exceeds 50 it is taken as refusal and the test is discontinuous • This test (IS : 2131 – 1981) is performed in clean hole, 100 to 150 mm in diameter. • A casing or drilling mud is used to support the sides of the hole. • This test is most commonly used for cohesionless soil which can not be easily sampled. • Useful for determining the relative density and the angle of shear resistance. • It can also be used to determine the unconfined compressive strength of cohesive soil. • Test shall be made,… • At every change in stratum • At intervals not more than 1.5 m Whichever is less
- 5. MachenLink SPT CORRECTIONS Overburden pressurecorrection Let 𝑁𝑜 is observed value of SPT for 1st correction factor and 𝑁𝑐 corrected value for overburden pressure then, 𝑵 𝒄 = 𝑵 𝒐 × 𝟑𝟓𝟎 𝝈′ + 𝟕𝟎 𝜎′ ≯ 280 𝑘𝑁/𝑚² 𝜎′ = effective overburden pressure at the test level If 𝜎′ > 280 𝑘𝑁/𝑚², then overburden pressure correction is not required. The ratio 𝑁𝑐 𝑁𝑜 should lie between 0.45 & 2. 𝑁𝑐 𝑁𝑜 > 2If, 𝑁𝑐 should be divided by 2 to obtain the design value used in finding the bearing capacity of soil. In granular soil, overburden pressure affects the penetration resistance. Higher the confining pressure, higher will be penetration resistance. Cohesionless soils of same relative density Penetration number increase with depth as confining pressure increase. 𝑁𝑜 - Overestimated 𝑁𝑜 - Underestimated
- 6. MachenLink Water table correction/fine correction/ dilatancy • If water table is present at or above the test level then water table correction is required. If 𝑤 𝑇 is below the test level then it is not required. The corrected value for overburden pressure is further corrected for presence of water table, • The pore water pressure is developed due to presence of water table • In fine soil during dynamic action of hammer the pore water resistance is developed. SPT CORRECTIONS 𝑵 𝒘 = 𝟏𝟓 + 𝟏 𝟐 𝑵 𝒄 − 𝟏𝟓 The correction is required when 𝑁𝑐 is greater than 15. If 𝑁𝑐 is less than 15, then water table correction is not required. Let 𝑁𝑐 be the corrected value for overburden pressure and 𝑁 𝑤 be the corrected value for water table, then
- 7. MachenLink Penetration Resistance andempirical correlation for Cohesionless Soil Penetration resistance N (Blows) Approx. 𝜙 (degrees) Density index % Description Approx. moist density 𝑡/𝑚3 − 25 − 30 0 Very loose 1.12 − 1.6 4 27 − 32 15 Loose 1.44 − 1.84 10 30 − 35 35 Medium 1.76 − 2.08 30 35 − 40 65 Dense 1.76 − 2.24 50 38 − 43 85 Very dense 2.08 − 2.4 − 100 CORRELATION OF SPT (N) WITH ENGINEERING PROPERTIES The value of SPT number depends upon,… Relative density → For cohesionless soil Unconfined compressive strength → For cohesive soil
- 8. MachenLink CORRELATION OF SPT(N) WITH ENGINEERING PROPERTIES Penetration resistance N (blows) Unconfined compressive strength 𝑡/𝑚2 Saturated density 𝑡/𝑚3 Consistency 0 0 Very soft 2 2.5 1.6 − 1.92 Soft 4 5 1.76 − 2.08 Medium 8 10 Stiff 16 20 1.92 − 2.24 Very stiff 32 40 Hard Penetration Resistance and empirical correlation for Cohesive Soils