Excavation Methods: Benched vs Sheeted Slopes (Bracing and Dewatering)

Flashcards cover the key concepts from the lecture notes on benched vs sheeted excavation, soil types and angle of repose, safety rationale, and the various methods of sheeted walls, bracing, and dewatering (well points and watertight barriers).

What are the two main excavation approaches discussed in the notes?

Benched excavation and sheeted/sloped excavation.

Which soil types are considered frictional (noncohesive) soils?

Gravels, boulders, and sand.

What is another term used for noncohesive soil?

Frictional soil.

What is the angle of repose?

The natural stability angle of soil; cohesive soils can have a high angle of repose.

Why is benching used in excavation?

For safety, to limit erosion to the benches and prevent collapse into the workspace.

What is a major limitation of benching?

It requires a lot of space and may not be feasible on tight sites.

In sheeted slope support, what are soldier beams and lagging?

Soldier beams are vertical steel posts; lagging are wooden strips that brace against the soil.

What is a sheet pile wall, and what materials can be used?

A continuous wall formed by interlocking plates (steel or timber) driven into the ground to retain soil.

What is a slurry wall and how is it constructed?

A trench is filled with bentonite slurry during excavation, reinforced with steel and then concrete is placed; isolates groundwater.

What equipment is used to place concrete in a slurry wall without mixing with the slurry?

A tremie tube and concrete pump to displace the slurry with concrete.

Why is bracing necessary during deep excavations?

To resist increasing lateral earth pressure as depth increases and prevent collapse.

Describe cross knot bracing.

A form of cross-bracing across the excavation; it can obstruct workspace due to posts and beams.

What are rakers and gill blocks?

Timber-constructed temporary wall with struts to push back soil.

What are tiebacks in excavation support?

Anchors drilled through the wall, grouted with tendons, and put in tension to hold back soil; used with whalers.

What is soil nailing and shotcrete?

Soil nails drilled into the face with shotcrete sprayed to stabilize the slope.

What is a secant pile wall?

Drilling overlapping piles to form a continuous barrier wall.

What is a tangent pile wall?

Piles placed side-by-side with little to no interlock to form a wall.

What is the purpose of well points in dewatering?

Lower the groundwater by pumping water from perimeter boreholes to keep the site dry.

What is a watertight barrier and when is it used?

An impermeable barrier (e.g., clay layer) plus a slurry wall to isolate groundwater; used when impermeable strata exist.

How does groundwater isolation help during excavation?

Keeps the excavation area dry so foundation and other work can proceed without flooding.