Excavation 2 Flashcards
(27 cards)
What is the general procedure for mass basement excavation?
- Conduct soil testing and site survey
- Choose excavation method based on site conditions
- Excavate in layers, supporting as needed
- Install drainage and retaining systems
- Begin foundation work after reaching design depth
What are the five common basement excavation methods?
Open-cut method
Top-down method
Bracing method
Anchored method
Island method
What is the open-cut method?
A straightforward excavation technique with two types:
Slope excavation: sloped sides, no retaining walls needed
Cantilevered excavation: uses retaining walls to support terrain and groundwater
What is the main advantage and disadvantage of open-cut slope excavation?
✅ Less expensive due to no retaining walls
❌ Requires more space for sloping
What is the drawback of open-cut cantilevered excavation?
More expensive and time-consuming due to retaining wall construction
What is the top-down excavation method?
Excavation begins after constructing load-bearing walls and a top floor slab, then digging beneath while building above.
Where is the top-down method typically used?
In high-rise urban projects to save time and space
What’s the key benefit of top-down excavation?
Upper structure construction can proceed simultaneously with basement excavation
How does the bracing method work?
Horizontal struts are placed to support retaining walls, redistributing load and stabilizing the excavation
Cost comparison of bracing method?
Cheaper than top-down, but more expensive than basic open-cut
What does the anchored excavation method involve?
Driving steel anchors into strong soil or bedrock through the retaining wall to resist earth pressure
When is the anchored method effective?
Only when anchors are embedded in bedrock or dense clay
What is the island method of excavation?
Excavating from the center outward, forming sloped embankments to the retaining walls, used when site has no nearby obstructions
What are the benefits of the island method?
✅ Safe and stable
✅ Combines slope and bracing methods
❌ Limited to specific project sites
What factors influence the choice of basement excavation method?
Type of construction (new site vs. under existing structure)
Soil type (rocky, sandy, clay)
Topography (flat land = more options)
Building size and scope
What are the two most adopted support systems in excavation?
Soldier pile and lagging
Soil nailing
What is the soldier pile and lagging system?
I-beams (soldier piles) are drilled into the ground
Lagging (wood planks) is placed between piles to hold back soil
What is the spacing limit between soldier piles?
Typically 10 feet between piles
What are lagging boards made of?
2–4 inch thick timber used between soldier piles
What is soil nailing?
A technique where the excavated soil is stabilized using grouted steel bars (nails) and shotcrete to hold the slope
What are the key steps in soil nailing?
- Excavate shallow layer
- Spray shotcrete on face
- Drill and grout nails
- Repeat in layers
When is soil nailing most effective?
Above the groundwater table and in cohesive soils
What data is needed for accurate earthwork calculation?
NGL (Natural Ground Level) data
Design Level data
Benchmark elevation
How is benchmark elevation determined?
Using survey equipment (e.g., total station, GPS) and referencing known elevation points or satellites
