Pile selection for very soft soils that offer little resistance to the lateral flow of fresh concrete: which type is preferred?

Introduction / Context:
Different cast-in-situ pile systems are chosen based on ground conditions and the ability to properly place and compact concrete in the bore. In very soft soils, fresh concrete can flow away or segregate unless positively placed and compacted.

Given Data / Assumptions:

• Soil is soft and provides small resistance to the flow of concrete.
• Objective is to ensure sound concrete with adequate shaft integrity and end formation.
• Pile choices include vibro, pressure, Franki, and pedestal types.

Concept / Approach:
Pressure piles use controlled pressure (often with compressed air or mechanical means) to place and compact concrete into the bore and at the base, countering loss of grout/mortar into soft surrounding soils. Vibro systems are suited to densifying granular soils or driving casings; Franki piles rely on heavy ramming and a plug, requiring confinement; pedestal piles need adequate reaction to form an enlarged base, which soft soils may not offer without pressure control.

Step-by-Step Solution:
Identify the risk: concrete leakage/flow in soft soils.Select a method that uses positive pressure to place/compact concrete.Pressure pile method meets this requirement by counteracting soil inflow and stabilizing the shaft.Therefore, choose the pressure pile.

Verification / Alternative check:
Case histories show pressure-placement methods improve base formation and reduce defects in soft, low-confinement soils compared with gravity-only placement.

Why Other Options Are Wrong:
Vibro pile: Focuses on vibration/densification; not primarily for pressured concrete placement in soft cohesive soils.Franki pile: Needs strong confinement for plug driving; less suited where soil offers little resistance.Pedestal pile: Requires adequate reaction to form an enlarged bulb; difficult in very soft soils without pressure control.Screw pile: Steel helical pile; not a concrete-placement solution to this specific issue.

Common Pitfalls:
Assuming vibration alone prevents washout; ignoring the need for positive pressure in very soft strata.

Final Answer:
Pressure pile.