Ground improvement techniques By Deepti

In – situ densification methods in Cohesive soils Preloading Vertical drains Sand Drains Sand wick geodrains Stone and lime columns Thermal methods

Preloading Simply place a surcharge fill on top of the soil that requires consolidation Once sufficient consolidation has taken place, the fill can be removed and construction takes place Surcharge fills are typically 10-25 feet thick and generally produces settlement of 1 to 3 feet. Most effective in clayey soil

Preloading

Advantages of preloading Requires only conventional earthmoving equipment Any grading contractor can perform the work Long track record of success

Disadvantages of preloading Surcharge fill must extend horizontally at least 10 m beyond the perimeter of the planned construction, which may not be possible at confined sites Transport of large quantities of soil required Surcharge must remain in place for months or years, thus delaying construction

Vertical Drains Vertical drains are installed under a surcharge load to accelerate the drainage of impervious soils and thus speed up consolidation These drains provide a shorter path for the water to flow through to get away from the soil Time to drain clay layers can be reduced from years to a couple of months

Vertical Drain Installation

VERTICAL DRAINS Act as free draining water channel. surrounded by a thin filter jacket which prevents the surrounding soil from entering the core. A vertical sand drain accelerates the rate of consolidation. Installation of vertical sand drains is a convenient technique for stabilization of soft and compressible soil . There are two types of vertical drains - sand drains and sand wicks.

Sand drains sand drain is basically a hole drilled in a cohesive soil and filled with sand. Since the sand has larger particle size, its permeability is much higher, thus water will flow through it much more easily. The excess water is collected at the top and directed away from the jobsite. Simply drill the holes and fill them with sand, but if the soil is soft (which is frequently the case,) the holes will collapse.

Sand drains Typically 200-500 mm in dia. Formed by infilling sand in to a hole in the ground. Hole formed by driving ,jetting or augering. Typical spacing 1.5 - 6.0 m. Sand wicks Sand wicks are improved technique of sand drains. A small diameter hole is made by driving mandrel or by boring. Then cylindrical bag with sand is lowered into this.

STONE COLUMNS Done to provide adequate support for relatively light foundation. The method consists of forming vertical holes in ground which are filled with compacted crushed stone, gravel and sand or a mixture. Fig : stone columns