Is Subsoil Investigation Specific to Each Structure in Expansive Soil Areas Necessary?

When dealing with expansive soil, it's important to understand the specific characteristics of the soil layers and their distribution. Expansive soil, often referred to as clay, comprises several layers with varying soil classifications and water content. The assessment of these layers is crucial to ensure the stability and durability of structures built on this type of soil.

Understanding Expansive Soil Layers

Expansive soil typically contains multiple layers, varying from coarse sand to fine clay. As you delve deeper into the soil, the composition changes, and each layer needs to be evaluated. Clay, with its high expansiveness, plays a significant role in the soil's overall behavior. Clay particles can absorb and retain large amounts of water, leading to significant swelling and possibly shrinking.

The Importance of Soil Analysis

For accurate classification, the soil is segregated into samples at various depths. Each sample is analyzed to determine the proportion of different soil types, such as clay, silt, fine sand, and coarse sand. This analysis is performed on a per-stratum basis to ensure a comprehensive understanding of the soil's composition. In expansive soil, the percentage of clay can range from 45 to 65%, while the remaining fraction consists of fine silt, alluvial soil, and coarse sand.

Assessing Soil Composition

The composition of soil at various depths and strata thicknesses must be carefully assessed. If a stratum contains more than 50% soft clay with a high liquid limit, and has 25% fine silt and 15 to 20% fine sand, the Standard Penetration Test (SPT) value might range from 3 to 6. The SPT value is a critical indicator of the soil's ultimate bearing capacity and settlement characteristics, influencing the proposed foundation design.

Determining Foundation Design

The design of the foundation, considering the settlement criteria and shear capacity, directly depends on the soil's bearing capacity. The load that can be transferred from the superstructure to the foundation is influenced by the number of stories and the proposed type of structure. It is essential to perform a thorough investigation to determine the appropriate foundation design.

Project-Specific Investigation vs. General Investigation

The need for a specific subsoil investigation for each structure in an expansive soil area depends on the nature, size, and type of the project. If the area is large and contains multiple structures, it is advisable to conduct a site-specific investigation for each structure. This approach ensures that each building's foundation is designed to match the unique soil conditions it will face.

However, if the structures are located in close proximity and the expansiveness of the clay is similar across the area, a general investigation can be conducted, focusing on the worst-case scenario. This can save time and resources compared to conducting separate investigations for each structure.

For areas with known expansive soil properties, the recommended approach is often a general investigation considering the worst case rather than detailed individual investigations, which can be costly and time-consuming in the long run. This strategy ensures that the appropriate soil treatment measures are implemented to prevent structural damage caused by expansive soil.

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Conclusion

In conclusion, while subsoil investigation is necessary for accurate foundation design, it is crucial to consider the project's size and nature. Site-specific investigations for each structure can be costly, but a general investigation considering the worst-case scenario is often more efficient. Consulting engineers can provide the necessary expertise to ensure the structural integrity of buildings on expansive soil.