HOW TO ESTIMATE STRENGTH AND MODULUS OF ROCK FROM SPT VALUES?

Strength and Modulus of Weathered Rock from SPT Values

In the weathered rock it is difficult to drive the split sampler to the full specified depth as in soil. Hence we may estimate the strength and elastic modulus values of weathered rock by driving the spt apparatus for a fixed number of blows and find the penetration. The penetration for the first ten blows is ignored and the penetration for the subsequent 60 blows is measured and taken as the specifi value of penetration to be considered. Depending on the penetration in millimeters for the 60 blows the strength and elastic modulus values of the deposit are read off from the Table-1 as given below. This table has been recommended by Stmatopulos and Kotzas in 1993 after field tests.

Table-1 – Uniaxial compression strength and modulus from SPT test for 60 blows

Penetration for 60 Blows (mm) Uniaxial Compression Strength (N/mm2) Modulus (E) (N/mm2)
10 30 30,000
20 15 16,000
40 5.5 6,000
80 3.0 2,500
150 1.5 1,200
200 1.0 900
300 0.75 650

Note: As the modulus (E) values given above have been obtained by experiments on low strains, calculation of settlement made with these values may yield high value. Hence the following factors of safety are recommended to be adopted for settlement analysis. A high factor of safety of 2 to 3 is given for high E values corresponding 10 to 80 mm penetration and a factor of safety of only 1.2 to 2 for low E values corresponding to 80 to 300 mm penetration shown in Table-1.

Strength and Modulus of Rocks

The values of strength of rocks given in Table-2 can be assumed for rough calculations. In actual practice it is better to take field samples and test for their properties in the laboratory.

Table-2 – Average Strength of Various Rocks

Name of Rock Compressive Strength (N/mm2) Tensile Strength (N/mm2)
Basalt 100 – 350 10 – 30
Gneiss 50 – 250 5 – 20
Granite 100 – 340 7 – 25
Limestone 30 – 250 5 – 25
Marble 50 – 250 7 – 20
Sandstone 20 – 300 4 – 25
Schist 20 – 200 10 – 30
Shale 5 – 100 2 – 10
Slate 100 – 200 2 – 5

Note: The densities of stones vary from 2 to 2.8 tons cm.

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