Standard Penetration Test (SPT) and Bearing capacity Estimation - Part 02

Before bolg we talked about the SPT procedure, from this let's consider about how to calculate the Bearing capacity of soil by SPT test results and several examples.

There are 5 categories of uncertainty in performance of SPT,
1) Category A
   - Includes source that depend on the type of soil
   - Use of the SPT is generally not recommended for cohesive soils
2) Category B
   - Lists source of uncertainty due to the presence of water
3) Category C
   - Includes reducible source related to the equipment and it's maintenance
4) Category D
   - Includes source that are reducible if the test procedure is performed carefully
5) Category E
   - Lists irreducible source in the investigation procedure

Free fall of the hammer,
E1 = mgh
E1 = 63.5 x 9.81 x 0.76
E1 = 473 J (energy available per blow)

E2 = Actual energy in the hammer
E2 = E1 - frictions

Energy ration = Er
Er = E2/E1 x 100%

Energy correction method for SPT  N value and bearing capacity calculation   

There are 5 correction factors,
1) Overburden correction (CN)
   - Overburden correction factor, CN is obtained by the following expression.


P'0 = effective overburden pressure in K Pa

2) Energy correction factor (n1)
   - Variable energy inputs from different SPT hammer system, it was suggested that measured blow counts be corrected to reference value of 70% of the potential energy of the SPT hammer.



Er = Energy ratio

3) Rod length correction (n2)
   - There is a chart to obtain this

4) Sampler correction (n3)
   - Some split spoon samples have liner inside it also affect to the final output. There is a chart to obtain this.

5) Bore hole diameter correction (n4)
   - There is a chart to obtain this

The SPT N value is referred to as N'70, since the energy ratio is converted to that corresponding to 70%.

Methods of drilling - Bore holes 

There are several boring methods use to drill bore hole.

1) Auger boring

Auger boring is the simplest method of making exploratory boreholes. Basically 2 types,
1. Hand augers ( 3 to 5 meters )
2. Truck mounted augers ( deep )

The soil samples obtained from such boring are highly disturbed. For deep boring use temporary casing to prevent the soil from caving in.

2) Wash boring

In this test method a casing about 2 to 3 m long is driven into the ground. The soil inside the casing is then removed by means of a chopping bit attached to a drilling rod. water is forced through the drilling rod and exits at a very high velocity through the holes at the bottom of the chopping bit.

From wash boring we can get only disturbed sample and fine particles are dissolved in water.

3) Rotary drilling

Rotary drilling is a procedure by which rapidly rotating drilling bits attached to the bottom of drilling rods cut and grind the soil and advance the borehole.

Rotary drilling can be used in sand, clay and rocks. Water of drilling mud is forced down the drilling rods to the bits, and the return flow forced the cutting to the surface.

4) Percussion drilling

This is an alternative method of advancing a borehole, particularly through hard soil and rock. chopped soil particles are brought up by the circulation of water.