Drilled Shaft Methodologies
Comparing Methodologies
Open (Dry) Method
Method Overview:
The hole is drilled into the ground using an auger or rotary equipment.
The hole remains open and stable without requiring support from slurry or casing.
Concrete is poured directly into the open excavation.
When to Use:
Soils are cohesive (e.g., stiff clays) and self-supporting.
Groundwater table is low or non-existent.
Hole stability is not an issue during the duration of construction.
Advantages:
Fastest and least expensive method.
Simplifies concrete placement since no casing or slurry is in the way.
Less site logistics and environmental concerns.
Disadvantages:
Not suitable for unstable or loose soils.
Risk of cave-ins or soil sloughing.
Cannot be used in high groundwater or granular soils.
Cased Shaft Method
Method Overview:
A temporary or permanent steel casing is inserted into the hole as it's drilled.
The casing provides lateral support to keep the shaft open.
Concrete is placed, and casing is either extracted or left in place (depending on design).
When to Use:
Unstable soils, such as sands, silts, or loose gravels.
Collapsible soils or shallow groundwater conditions.
Urban or tight access sites where hole stability is critical.
When ground vibrations or adjacent structures require shaft stability.
Advantages:
Ensures high hole integrity in difficult soil conditions.
Reduces risk of soil or water intrusion.
Can be used in proximity to existing structures without causing damage.
Disadvantages:
More time-consuming than open hole.
Higher cost due to casing material and installation/removal.
Can restrict shaft diameter during concrete placement.
Slurry (Wet) Method
Method Overview:
Drilling is performed under a column of bentonite slurry or polymer slurry that exerts hydrostatic pressure to stabilize the hole.
Slurry is pumped in during excavation and remains in the hole until concrete is placed.
Concrete is placed using a tremie pipe to avoid mixing with slurry.
When to Use:
Very loose or granular soils, including sands and gravels.
Below groundwater table, especially at significant depths.
Deep shafts where casing would be impractical.
Advantages:
Effective in non-cohesive, wet, or granular soils.
Suitable for deep, large-diameter shafts.
Less structural load transferred to casing (if any).
Disadvantages:
Most expensive method due to slurry handling, testing, disposal.
Requires slurry management infrastructure.
Must ensure no contamination of concrete by slurry during placement.
Relative Cost
Open Hole
Relative Cost: Lowest
Use Case: Stable soils, dry conditions
Speed: Fastest
Complexity: Low
Cased Shaft
Relative Cost: Medium
Use Case: Unstable soils, shallow water
Speed: Moderate
Complexity: Medium
Slurry
Relative Cost: Highest
Use Case: Unstable soils, wet conditions
Speed: Slowest
Complexity: High
While each methodology has its own advantages, cased shafts safer than drilling open holes due to the high risk of collapse. Cased shafts are typically less expensive than the slurry method since there is no required infrastructure to manage slurry handling or disposal.