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Question 2 [25 marks]

c. A geotechnical engineer made a preliminary settlement analysis for a foundation of an office building that is to be constructed at a location where the soil strata contains a compressible clay layer. She calculated 50 mm of primary consolidation settlement. The building will improve an average vertical stress of 150 kPa in the clay layer. As

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Step By Step Answers with Explanation

• Simple and inexpensive: The SPT is a relatively straightforward test to perform and requires minimal equipment.

• Wide applicability: The SPT can be used to assess a variety of soil types, including sands, gravels, and silts.

b. A Standard Penetration Test (SPT) was carried out within a 200 mm diameter borehole at 9 m deep in the sandy fill layer. The blow count (N value) for each 15 cm increment (3 increments in total) was recorded as 8, 9, 11, respectively. The length of the rod between the safety hammer and the sampler (at 9 m depth) is 10.5 m. Assume the groundwater table is at 2.5 m below the ground level and the unit weight of the sand below and above the water table being 18 and 17 kN/m³, respectively. Derive the corrected SPT (N60) value and write explicitly each correction you have made.

Corrections:

• N is the measured SPT blow count

• A is the overburden correction factor (typically 0.003 to 0.005)

N60 = N * (1 - (B * L) / 100)

where:

In this case, the rod length correction factor is 0.003. The corrected SPT blow count (N60) is therefore calculated as follows:

N60 = 9.4 * (1 - (0.003 * 10.5)) = 9.2

• N is the measured SPT blow count

• C is the water table correction factor (typically 0.03 to 0.05)

Therefore, the corrected SPT blow count (N60) for the sandy fill