Soil and ground treatments

Published on 11 February 2008
treatment by spreading lime (photo Razel)

Treatment is an operation that consists in mixing naturally occurring soil with lime and/or a hydraulic binder. By improving its characteristics it allows use of a soil that was unsuitable in its natural state. In addition, this technique helps protect the environment: by using on-site materials, rare natural resources are safeguarded (massive rock formations) and sources of harm and stress are reduced (no heavy goods vehicle traffic on public highways).
Treatment with lime (quick lime) gives very rapid reduction in water content giving soils that are permeable in their natural state good load-bearing characteristics.
If the soil is a clay soil, lime treatment (quick lime, slaked lime or limewash) gives long term enhancement of its mechanical properties. Average lime mixes are around 1 to 4%. Hydraulic binder treatment gives the mix permanent mechanical properties and allows it to be used as a subgrade or surface foundation.
The hydraulic binders usually used for soil treatment are cements and predominantly highway hydraulic binders (a slag-based binder with very little or no clinker). The average mix content of hydraulic binder is of the order of 4 to 8% (if the soil is a clay soil and therefore malleable, it has to be pre-treated with lime - around 1% - to obtain a fine “grist” before treating it with hydraulic binder).

There are two treatment techniques: Either in a mixing plant or on-site. In a mixing plant the natural soil is taken from the borrow pit to the plant, where it is mixed with the binder(s) and then transported back to the place of use. The plant is a permanent installation consisting of a hopper, container silos for the binders, conveyors and/or metering units, a controlled water supply system and a horizontal shaft mixer.
In on-site treatment the natural soil is treated at the point of use. The plant needed for this is made up of:

• A spreader for the binders: a tank wagon trailer of 12 to 16 m3 with metered spreader system linked to vehicle speed
• A sprinkler of 10 to 15 m3, with a ramp equipped with nozzles - this can either be in the form of a towed trailer or a vehicle operating under its own power
• A mixer: either a mouldboard plough towed by a bulldozer, or a self-propelled horizontal rotor pulverizer (pulvimixer) Mixing depth is around 0.40 to 0.50 m.