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.
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Table 1 – Uniaxial compressive strength and modulus from SPT test for 60 blows |
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| 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.
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Table 2 – Average strength of different rocks |
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| 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.
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