Comparing Deep Foundation Drilling Methods
Deep foundation drilling methods involve creating a vertical shaft or pile in the ground to transfer loads from a structure to a deeper, more stable soil layer. Key methods include drilled shafts (also called caissons), driven piles, displacement piles, and micropiles. Each has unique advantages, disadvantages, and applications.
Micropiles (a.k.a. Mini-Piles or Pin Piles)
Description:
Small-diameter (typically 3–12 inches) drilled and grouted piles.
Reinforced with steel bars or casing.
Can be installed in difficult ground conditions and restricted spaces.
When to Use:
Underpinning existing structures.
Sites with limited headroom or access (e.g., inside buildings).
Areas with high water tables or variable soils.
Seismic retrofitting or slope stabilization.
Equipment Used:
Drill rigs (limited access, track-mounted, or handheld for tight spaces).
Rotary or rotary-percussive drilling tools.
Grout pumps for pressure grouting.
Casing or hollow-bar systems.
Displacement Piles
Description:
Cast-in-place or precast piles installed by displacing soil rather than removing it.
Includes full displacement (using a mandrel or auger) and partial displacement.
Soil is compacted laterally, minimizing spoil generation.
When to Use:
Sites requiring minimal spoil or contamination management.
Loose or granular soils where soil compaction is beneficial.
Urban areas where vibration from driven piles is not acceptable.
Equipment Used:
Displacement augers or mandrel rigs.
Hydraulic rotary rigs with torque and crowd control.
Grouting systems (for cast-in-place types).
Driven Piles
Description:
Precast or prefabricated piles (concrete, steel, timber) driven into the ground using impact or vibration.
No drilling; soil is displaced.
When to Use:
Cohesive or dense soils with good bearing capacity.
Marine and offshore structures.
Fast-track projects requiring quick load transfer.
Sites where pile monitoring (e.g., PDA) is required.
Equipment Used:
Impact hammers (diesel, hydraulic, or air).
Vibratory hammers (especially for steel or sheet piles).
Pile driving rigs or cranes with leads.
Monitoring equipment for driving resistance and stresses.
Drilled Shafts (a.k.a. Caissons or Bored Piles)
Description:
Large-diameter holes drilled into the ground, filled with concrete and steel reinforcement.
Can be drilled dry, with casing, or with slurry support (bentonite or polymer).
When to Use:
Heavy structural loads (bridges, towers, high-rise buildings).
Variable subsurface conditions.
Urban areas where vibration must be minimized.
Deep foundations where lateral resistance is needed.
Equipment Used:
Rotary drilling rigs (Kelly bar or continuous flight auger (CFA)).
Casing oscillators/rotators (for cased shafts).
Reverse Circulation Drill Rigs (used with oscillator).
Slurry pumps and mixing systems (for slurry-supported shafts).
Tremie pipes (for underwater concrete placement).
Excavation tools: buckets, augers, core barrels, rock tools.