Ground improvement

When the ground is too soft, loose or compressible to carry a structure economically, it can often be improved instead of bypassed with deep piles. The choice depends on soil type, depth of the problem layer, the load and the time available. Improvement aims to increase strength, reduce settlement, speed up consolidation or prevent liquefaction.

For soft clays, preloading with an embankment surcharge squeezes out water and pre-consolidates the soil, so that later settlement under the real load is small. Clays drain slowly, so prefabricated vertical drains, band drains pushed in at a close grid, shorten the drainage path from the layer thickness to the drain spacing and cut consolidation time from years to months. Vacuum preloading applies suction instead of fill.

Stone columns, holes formed by vibro-probes and backfilled with compacted gravel, reinforce soft soils, act as drains and stiffen the ground under tanks and embankments; IS 15284 (Part 1) gives guidance. In loose sands, vibro-compaction densifies the ground, and dynamic compaction, repeatedly dropping a heavy weight from a height, densifies deeper layers; its depth of influence grows with the square root of the energy per drop. Grouting, deep soil mixing with cement and jet grouting create stronger columns or blocks.

Every scheme needs trials and verification: settlement plates, piezometers, penetration tests before and after, and load tests, so that the improved ground is shown to meet the design.