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Soil Investigations

Soil investigations are carried out before designing or constructing buildings, roads, bridges, and other structures. Their findings help consultants understand ground conditions and select suitable foundations.

The investigation plan depends on the structure and its loads. For example, the speaker notes that investigations for a bridge may extend roughly 25–30 meters, depending on the project.

Soil Investigation and the SPT

Soil investigation is the process of exploring and sampling the ground, while soil testing involves examining the collected samples. One method discussed is the Standard Penetration Test (SPT).

The procedure is described as follows:

• A borehole is drilled to the depth specified by the consultant.
• At selected depths—often about every meter or every 1.5 meters, according to the subtitles—a sampling tool is lowered into the borehole.
• A hammer weighing about 63.5 kg is dropped from a height of approximately 75 cm to drive the sampler into the soil.
• The sampler’s penetration is observed in 15 cm increments, and the number of hammer blows required for each increment is recorded.
• The first 15 cm is generally disregarded in the SPT count because it is treated as seating or disturbed material. The counts for the following increments are used to determine the test result, along with other relevant factors.
• Soil is recovered from the sampler at the test locations. The drilling, testing, and sampling process is repeated at the selected depths.

Samples, Soil Profile, and Design

Samples are collected, placed in bags or containers, and sent to a laboratory or consultant for physical and chemical testing. The results are used to build a soil profile showing ground conditions at different depths.

The investigation report helps the consultant assess soil properties, especially bearing capacity, and make decisions about foundation type and depth, piles, pile caps, and other structural design matters. The speaker emphasizes that this information should be available before design decisions are made.

The field process can be time-consuming, particularly when many test locations or samples are required.

Speakers and Sources

• Main speaker/presenter: An unnamed presenter explaining soil investigations and demonstrating the SPT process.
• Other voices: Brief, incidental comments from people at the worksite. They are not clearly identified and do not provide a separate explanation.