During a Standard Penetration Test (SPT), unusually low blow counts in a medium-dense to dense sand foundation may indicate which of the following conditions?

• A. A) The sampler drive shoe is badly damaged
• B. B) Cobbles are encountered
• C. C) The sampler drive shoe is plugged
• D. D) The groundwater in the borehole is much lower than in situ conditions

Answer: (D)

In the context of a Standard Penetration Test (SPT), experiencing unusually low blow counts in a medium-dense to dense sand foundation can indicate that the groundwater in the borehole is much lower than in situ conditions.

When groundwater is significantly lower than the surrounding soil, it affects the effective stress within the soil matrix. This change can lead to the presence of pore pressures and may allow for a lower resistance to penetration during the SPT, resulting in low blow counts. Essentially, the soil may behave more like a loose condition in the presence of lower pore pressures, reducing the overall density offered by the structure of the sand and facilitating easier penetration of the sampler.

This phenomenon is particularly relevant in sandy soils, where the behavior can drastically change with varying groundwater levels. Proper interpretation of SPT results must consider the effects of groundwater levels as they can influence stability and the engineering properties of the soil being tested.