Which excavation technique would be most appropriate for constructing a substructure requiring a deep excavation in a location with high water tables and sandy soil?

Explanation:

Choosing the Most Appropriate Excavation Technique

When selecting an excavation technique for a substructure requiring deep excavation, especially in areas with high water tables and sandy soil, several factors must be considered. These include soil stability, water control, and the depth of excavation. Given these conditions, the techniques can be analyzed as follows:

1. Trench excavation with timber shoring

   • This method involves using timber to shore up the trench walls, providing support against collapse.

   • While it offers some support, it may not be sufficient for deep excavations, especially in sandy soils with high water tables where soil can easily collapse or become unstable.

2. Hydraulic shoring with sheet piling

   • This method uses hydraulic pressure to drive sheet piles into the ground, creating a watertight barrier that prevents soil collapse and water ingress.

   • It is highly effective for deep excavations in sandy soils and areas with high water tables, providing robust support and control over water intrusion.

3. Open cut excavation with sloped sides

   • Open cut excavation involves sloping the sides of the excavation to prevent collapse. This method is simple and cost-effective.

   • However, it requires a large area and is not practical for deep excavations in urban settings. Moreover, it may not effectively control water in high water table areas.

4. Caisson construction

   • Caissons are watertight structures used for constructing foundations in underwater or high water table conditions. They are highly effective for very deep excavations.

   • However, this technique is often more complex and expensive compared to other methods. It is generally used for specialized projects like bridge piers.

Conclusion

Given the high water table and sandy soil conditions, the most appropriate excavation technique for constructing a substructure requiring deep excavation is Hydraulic shoring with sheet piling. This method provides the necessary support and water control, ensuring stability and safety during the excavation process. Therefore, the correct answer is option 2.