Friday, August 15, 2014

Standard Penetration Test(SPT)

how have you been?  In this post I shall discuss the importance, and procedure for performing the Standard Penetration Test(SPT) on a specified location of the soil. Along with some correlations of the N value with the Angle of friction and density, corrections for the overburden and dilatancy are also discussed.

  • Aim: To perform SPT(Standard Penetration Test) on an in situ soil sample. 

  • Apparatus: 

  1. Standard Penetration Test Apparatus. The necessary parts of the whole arrangement are a split sampler tube with 50 mm outside diameter and 35 mm inner diameter with a length of 650 mm, a slide hammer with a mass of 63.5 kg with a free fall of 762 mm and the pulley.
    Standard Penetration Test(SPT)

  • Theory: Standard Penetration Test(SPT) is an in situ dynamic penetration test,designed to find out SPT number 'N', that can be used to determine the geotechnical properties of the soil using the various correlations. It is a simple and inexpensive test to find out the relative density and shear strength parameters of the soil.

Test is generally used for testing the granular soils, like sand and silty sands or stiff clays. The reason is that it is almost impossible to get the undisturbed samples of the granular soils. 

There is a sampler tube(Split Spoon Sampler) applied with the dynamic loading to penetrate into the in situ undisturbed soil, and the numbers of blows required to penetrate through 18 inches(450 mm), are correlated through some empirical relations to find out the relative density and shear strength of the soil.

Though the test results are based on empirical values, this test is widely adopted to design the foundations in the sandy soils. 

  • Procedure:

  1. Put the sampler tube in the borehole with proper seating.
  2. After all the necessary arrangements such as connection of hammer to the pulley through the manila rope, and joining the sampler with the number of extension rods, ready to be loaded, apply the dynamic loading of the slide hammer, by making it to freely fall from a height of 762 mm. 
  3. The tube is made to penetrate first 150 mm (6 inches), and then another two 150 mm (6 inches each) and the numbers of blows are noted down.
  4. The 'N' value is reported as the number of blows required in the last two 6 inches penetrations. 
  5. Values for the first 150 mm are neglected.

  • Interpretations and Calculations:
  • Relation between SPT Value and different soil properties in case of Cohesionless soils

SPT Values(N)    Compactness            Angle of Friction      Unit Weight in T/m^3
   0-4                    Very Loose                     < 28                            1.1 - 1.8
   4-10                    Loose                          28-30                           1.4 - 2.0
  10- 30                  Medium                       30-36                            1.7 - 2.2
  30 -50                  Dense                          36- 41                           1.7 - 2.3
   > 50                    Very Dense                    >41                             2.0 - 2.3     

  • Relation between SPT Value and different soil properties in case of Cohesive soils

 SPT Values(N)    Compactness            UCS (Unconfined Compression Strength) in kPa
   0-4                    Very Soft to soft                                25                        
   4-8                     Soft to Medium                                 50                 
   8-16                   Medium to Stiff                                100
  16- 32                 Stiff to Very Stiff                              200
   > 32                    Very Stiff to Hard                            400    

Correction for Overburden Pressure and Dilatancy:

1. Correction for Overburden Pressure: The N value which we get from the SPT depends upon the overburden pressure too, more the overburden pressure, soil will perform better. As we go into the depth, the overburden pressure goes on increasing. 

Bazarra(1967, p99) proposed the following corrections to the actual count N, based on the overburden pressure:

For P<= 75 kPa
                          N' = 4.N/(1+0.04.P)

For P> 75 kPa
                       N' = 4.N/(3.25+0.01.P)
Where, N' = Corrected value for N
              N = Observed value  for N
              P = Overburden pressure (kPa), = Density*D
              D = Depth of testing(m)

2. Correction for Dilatancy in saturated fine sands and silts: In the saturated soils, the presence of the pore water pressure plays a role in the 'N' value. Pore water pressure opposes the penetration of the sample tube into the soil, therefore, the N value will be higher. 

This higher value of N may lead to the unsafe interpretation of the soil properties. Therefore whenever SPT is performed on the saturated silts and fine sands, if the observed 'N' value is higher than 15, a correction has to be applied to the observed value. 

N" = 15 + 1/2(N'-15)

This correction is applied after correcting the N value for the overburden pressure. Therefore the correction will be applied on the value of N'.

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