Soil improvement techniques; explain two

Soil improvement techniques include compaction, preloading with vertical drains, stone columns, grouting, chemical stabilisation, replacement, dewatering, geosynthetic reinforcement and soil nailing. Two of them are explained below.

1. Compaction (dynamic compaction and vibroflotation)

Compaction densifies loose granular soil by reducing its voids using mechanical energy, which increases the strength and reduces settlement and liquefaction potential.

• Surface compaction: rollers (smooth, sheepsfoot, vibratory) compact thin layers of fill to a specified density at optimum moisture content.
• Dynamic compaction: a heavy weight (10 to 40 t) is dropped from 10 to 30 m onto the ground in a grid, which compacts the soil to depths of 5 to 10 m.
• Vibroflotation: a vibrating probe (vibroflot) is sunk by water jet into the sand; the sand around it is densified and the hole is backfilled with sand/gravel.
• Suitable for loose sand and gravel with little fines. Quality is checked by SPT or CPT after compaction.

2. Preloading with vertical drains

For soft saturated clay, a surcharge is placed over the area to force the water out of the clay and speed up the consolidation settlement before the structure is built.

• A sand blanket is laid on the surface, and vertical sand drains or prefabricated band drains are installed at 1.5 to 3 m spacing to shorten the drainage path, so consolidation is completed in months rather than years.
• The fill (surcharge) is left until about 90% consolidation, monitored by settlement plates and piezometers, and then removed.
• It also increases the shear strength of the clay, and is used under embankments, roads, airport runways, tanks and warehouses.