Ground improvement – primary purpose of sand drains (vertical drains) In soft, saturated clay deposits, sand drains (now often prefabricated vertical drains) are installed primarily to achieve which outcome?

Introduction:
Soft clays consolidate slowly because their permeability is low and drainage paths are long. Vertical drains such as sand drains or prefabricated band drains shorten the path for pore water escape, allowing faster dissipation of excess pore pressures during preloading or embankment construction.

Given Data / Assumptions:

• Soft, saturated, normally consolidated or lightly overconsolidated clay deposit.
• Vertical drains installed on a grid and connected to a drainage blanket.
• Preloading or staged embankment construction applied.

Concept / Approach:

Primary consolidation time is proportional to drainage path length squared divided by coefficient of consolidation. Installing vertical drains reduces the radial drainage distance to a fraction of the drain spacing, dramatically accelerating consolidation. While total ultimate settlement is largely unchanged (determined by compressibility and load), the time to reach a given percentage of settlement is shortened.

Step-by-Step Solution:

Verification / Alternative check:

Field case histories show faster settlement curves and improved stability with preloading and vertical drains compared to preloading alone.

Why Other Options Are Wrong:

Reduction of ultimate settlement is minimal (A); permeability of soil mass is not fundamentally increased (C) beyond providing drainage paths; drains do not transfer significant structural load (D); liquefaction resistance improvement is not their primary role (E).

Common Pitfalls:

Expecting drains to reduce the magnitude of settlement rather than the time; neglecting smear and well resistance effects in design.

Final Answer:

accelerate the consolidation