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

Strength and modulus of weathered rock from SPT values

In weathered rock it is difficult to drive the split sampler to the full specified depth like soil. Therefore we can estimate the strength and elastic modulus values ​​of weathered rock by running the SPT mechanism for a certain number of blows and find out the penetration. For the first ten blows the penetration is ignored and for the subsequent 60 blows the penetration is measured and treated as the typical value of penetration. The strength and elastic modulus values ​​of the deposits based on penetration in millimeters for 60 blows are read from Table-1 given below. This table is recommended stomatopulos And kotzasi After the field test in 1993.

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

Penetration for 60 Warps (mm) Axial compressive strength (N/mm2, Modulus (E) (N/mm)2,
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

pay attention: As the modulus (E) values ​​above have been obtained by experiments at low strains, settlement calculations performed with these values ​​may yield a higher value. Therefore, it is recommended to adopt the following factors of safety for settlement analysis. A high factor of safety of 2 to 3 is given for high E values ​​corresponding to 10 to 80 mm penetration and only 1.2 to 2 for low E values ​​corresponding to 80 to 300 mm penetration shown in Table-1. The safety factor is given.

Rock strength and modulus

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

Table 2 – Average strength of different rocks

name of rock Compressive Strength (N/mm2, Tensile Strength (N/mm2,
bajalat 100 – 350 10 – 30
shell 50 – 250 5 – 20
granite 100 – 340 7 – 25
Limestone 30 – 250 5 – 25
marble 50 – 250 7 – 20
sandstone 20 – 300 4 – 25
shale 20 – 200 10 – 30
shale 5 – 100 2 – 10
slate 100 – 200 2 – 5

pay attention: The density of stones varies from 2 to 2.8 tons cm.


Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.