PL-v100 – Portable Point Load Tester

The PL-100V is a cost-effective portable point load tester used for determining the point load strength index for rock strength classification. Its digital readout unit comes with a widespread display battery operated with a keyboard.

The PL-100V point load tester consists of a loading frame, a hydraulic cylinder, a pump and a pressure gauge with a digital readout unit for maximum load reading. An upper conical platen is fixed on the frame and a lower one on the cylinder piston. A graduated scale is fixed on the frame and indicates the specimen diameter.

Test Procedure

Three-point load test configurations are used depending on the available rock specimens:

Diametrical – Axial – Irregular lump

The diametrical and axial tests use core specimens with length/diameter (L/D) ratios greater than 1.0 in the first case, and between 0.3 and 1.0 in the second case. Rock pieces of suitable irregular shapes are used when cores are not available.

The testing steps are the same for all configurations:

1. The specimen is positioned between the conical platens. The platens are then closed to make contact.
2. The distance “De” between the points of contact is read on the scale.
3. The load is increased such that failure occurs within 10 to 60 sec. and the failure load “P” is read and recorded.

* Based on conversion factor determined at calibration

Interpretation

The Point Load Tester allows users to determine an “Uncorrected Point Load Strength Index” (I_s). This index must be corrected to a standard equivalent diameter (D_e) of 50 mm. It then becomes a unique property of the rock tested (I_s(50)) which is most useful in rock strength classification.

Rock anisotropy is quantified by the “Strength Anisotropy Index” (I_a(50)). This index is the ratio of the greatest to least (I_s(50)) index measured respectively perpendicular and parallel to the existing planes of weakness.

The uniaxial tensile (UTS) and compressive (UCS) strengths can be approximated from the  I_s(50) index. The UTS is about 1.25 times I_s(50) and the UCS is normally between 20 and 25 times the I_s(50) index.

I_s is obtained from the following equation:

I_s = P / D_e2 where:

• I_s : Uncorrected Point Load Strength Index, in MPa or psi
• P : Failure load, in MN or lbf
• D_e : Equivalent core diameter, in meters or inches (D_e = D for diametral tests)

The ISRM* “Suggested Method for Determining Point Load Strength” size correction procedure is used. I_s(50) is obtained either graphically, mathematically or by testing 50 mm (maximum diameter) specimens.

*ISRM: International Society of Rock Mechanics.