Cone Penetration Test (CPT) , Pressure Meter Test (PMT) and Vane Shear Test (VST) and Bearing Capacity Calculation

Before 2 blogs we talked about SPT method and how to perform the test, example to how to calculate the bearing capacity using SPT. From this blog let's talk about CPT, PMT and VST methods and how to calculate the Bearing Capacity form that methods.

Cone Penetration Test (CPT)

The test is also called the static penetration test, and no boreholes are necessary to perform the this test. Normally 60 degree cone with base area of 10 cm^2 was pushed into the ground at a steady rate of above 20 mm/sec, and the resistance to penetration ( Point resistance ) was measured.

The cone penetrometers is use at present measure,

(a). The cone resistance (qc) to penetration developed by the cone.

(b). The frictional resistance (fs) or qs which the resistance measured by a sleeve located above the cone with the local soil surrounding it.

In electronic CPT can measure both cone and sleeve resistance electronically. and also there is a device (pizometer) to measure pore pressure also.

Several correlations that are useful in estimating the properties of soils to encountered during an exploration program have been developed for the point resistance (qc) and the Friction Ratio (Fr) obtained from thecone penetration test the friction ration is defined as,

Fr = Frictional or Sleeve resistance / Cone resistance
Fr = qs / qc

There are several correlations between qc and friction angle (phi')

Pressure Meter Test (PMT) 

The pressure meter test is an in-situ test conducted in a borehole. it was originally developed by 'Menard' to measure the strength and deform-ability of soil.

this consists essentially of probe with 3 cells. The top and bottom ones are "guard cells" and middle one is the "measuring cell". guard cells can be expanded by either liquid or gas.

This test is normally use for check rock types, tunneling and very hard materials.

Vane Shear Test (PMT) 

The vane shear test may be used during the drilling operation to determine the in-situ undrained shear strength (Cu) of clay soils - particularly soft clays.

The vane shear apparatus consists of four blades on the end of a rod, the height H of the vane is twice the diameter D.

The vane of the apparatus are pushed into the soil at the bottom of a bore hole without disturbing the soul appreciable torque is applied at the top of the rod to rotate the vanes at a standard rate of 0.1% sec.

This rotation will induce failure in a soil of cylindrical shape surrounding the vanes. the maximum torque T, applied to cause failure is measured,

Cu = T / K

where,
T = torque (Nm)
Cu = cohesion (KN/m^2)
K = Constant ( depending on the dimension and shape of vane)

The test gives good results in soft and medium stiff clays and give excellent results in determining the properties of sensitive clays. There are errors in vane shear test, to get rid of them.