METHODS OF GROUND IMPROVEMENT - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 211

cunette

clay

peat

sand

Figure 13.3 Squeezing below a back-filled cunette

Lightweight fill
Lightweight fill as a replacement material has the advantage that the increase of
load on the soft soils is limited or completely prevented. This method too has old
roots, e.g. road construction in peat areas by excavating the top few metres of bad
soil and replacing it by bales of horticultural peat. As long as the peat bales remain
below the groundwater table, this is an enduring solution. Sawdust, expanded clay
beads, lava products and shredded automobile tyres are other examples of
lightweight fill materials. Expanded polystyrene (EPS), also known as geofoam, is
becoming the preferred material for lightweight fill. It is very light, only 15 to 30
kg/m 3 , and is used in large blocks which can be easily trimmed on site. They are
also used in embankment construction (Fig 13.4), and make fast construction
possible with little or no effect on neighbouring structures and utilities. Where
space is limited, vertical walls are possible by applying a facing. Geofoam has poor
chemical resistance and must be protected against spillage of chemicals.

road

soil cover

geofoam

facing

leveling course

soft ground

Figure 13.4 Lightweight fill (geofoam) application in an embankment slope

C MASS IMPROVEMENT

Dynamic compaction
Densification or solidification can be applied to the entire mass of soft soil.
Menard's method of Deep Dynamic Densification consists of dropping weights
onto the surface of the soil. Weights up to 190 tons and drop heights up to 25 m are
used. The site is covered in a grid of drop points. Large craters can form, which can
either be backfilled or levelled with surrounding fill. The method relies mainly on
expelling air from the soil and is therefore effective only in non-cohesive soils. It is
often used to compact loose fill material, but also for compaction of waste. Very