Modified Standard Penetration Test for Illinois Weak Fine-Grained Rocks-Article

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The paper presents a Modified Standard Penetration Test (MSPT) for estimating the strength and stiffness of weak fine-grained rocks, including shale, mudstone, and siltstone. Conventional SPT blow counts are often difficult to interpret in these materials because the sampler rarely penetrates the required 45 cm.<br /><br />The MSPT records sampler penetration at regular blow-count intervals, typically every 10 blows, for up to 100 blows. Rather than using a conventional SPT N-value, the method uses the penetration rate derived from the linear, secondary portion of the penetration-versus-blow-count curve. This rate is normalized to account for hammer energy and other test factors. The study also provides procedures for identifying irregular results that may indicate changes in material or obstructions.<br /><br />The authors tested the method at 21 Illinois bridge sites, comparing MSPT results with laboratory unconfined compression tests on shale cores from nearby borings. They developed correlations to estimate unconfined compressive strength (UCS) and undrained Young’s modulus. Two drilled-shaft load tests showed close agreement between measured and predicted ultimate side resistance, supporting the method’s potential for foundation design.<br /><br />MSPT is intended for weak rocks with UCS of approximately 0.48–4.8 MPa. It can reduce reliance on difficult and costly rock coring and laboratory testing when intact samples are hard to obtain. The correlations are based on Illinois weak rocks, so their use beyond comparable materials and conditions requires caution.

Keywords

Modified Standard Penetration Test

MSPT

weak fine-grained rocks

shale

mudstone

siltstone

penetration rate

unconfined compressive strength

undrained Young's modulus

foundation design