Proper Execution Methods for Excavations and Support Structures

Summary and Recommendations on Excavation Support Techniques

Use of Piles

Using piles in excavation support is a reliable and widely used method in advanced countries. This technique minimizes arching effects and covers most soil surfaces with piles. It’s effective in medium to weak soils and also successful in dense, high-quality soils during deep excavations. Usually, piles are cast in place, but micro-piles are also applied. Often, piles are combined with other methods in excavation projects.

Micro-Pile Technique

Micro-piles are a modern technique enhancing soil strength and excavation stability by injecting cement grout through perforated pipes into surrounding soil. Applications include soil improvement, settlement reduction, water cut-off barriers, liquefaction mitigation, load capacity increase, and foundation tension control. They can be installed vertically, inclined, or horizontally under adjacent building foundations to reduce settlement and lateral deformation.

Nailing Technique

Nailing (or soil nailing) is a new method especially suitable for seismic areas due to its flexibility. Similar to anchoring but less expensive, nails reinforce potential failure surfaces, while anchors act like prestressed cables that compress soil to improve resistance and reduce deformation and settlement in adjacent buildings. Both methods are often combined. Load transfer must occur only in the active soil zone behind the failure wedge to prevent collapse.

Truss Technique

Using the truss method in soils with low cohesion or cementation is not recommended. In medium to high cohesion soils, it should be cautiously applied alongside waling beams, shotcrete, or concrete retaining walls.

Retaining Structures Technique

Truss retaining walls are usually used in urban buildings where precise geotechnical studies are lacking. Changes in soil layers, cracks, interlayers, adjacent building movements, moisture, and groundwater should be closely monitored.

Active State

The active state is a soil failure condition that occurs with soil movement. Soil remains stable until movement begins, making soil retention key to successful excavation.

Retaining Structure Installation

A gap between retaining structure and soil can allow soil movement and failure. This gap must be filled with materials as stiff or stiffer than the soil. The best location for truss retaining walls is near adjacent building columns and immediately next to the excavation. This limits adjacent building movement and local sliding but should not interfere with foundation and column construction. The spacing depends on soil type—closer in loose soils and wider in dense or cemented soils—but should not exceed 4 meters. Most failures occur as soil collapse between trusses, while trusses remain intact without resisting.