Subgrade stabilisation

The largest applications of lime treatment are roads, airports, car and lorry parks, where lime may be used to stabilise the subgrade to provide a capping layer , or to produce a stabilised subbase . Figure 26.3 illustrates these terms. 

As an alternative to removing a layer of subgrade and replacing it with suitable material to form the capping layer, the subgrade may be stabilised using lime. Typically, such a capping may consist of two or more layers and may be up to 600 mm thick. 

The subbase is constructed between the top of the subgrade and the bottom of the roadbase. It should have a laboratory CBR of not less than 30 % [26.5], This strength is generally produced by the use of both lime and cement. The lime is applied first to dry and modify the subgrade, which can then be scarified into a fine tilth. The cement is then dispersed, the water content is adjusted to just wet of optimum and the layer is compacted. After compaction, the cement hydrates and sets, and the lime stabilisation reaction proceeds as normal. The required levels of addition of lime and cement should be determined by testing typical levels are 2 % of lime and 4 % of cement,... 

The materials used for the construction of the capping layer or the formation of subgrade may be (a) collective granular soil material (b) unbound mixtures of natural, artificial and recycled aggregates and (c) soil material treated (stabilised) with hydraulically bound binders. 

The minimum values of CBR or to be achieved after chemical stabilisation should be at least equal to the value that an untreated granular soil material possesses, suitable for subgrade material. An addition of 2.0% cement or 2.5% of lime of total mass in a granular soil material normally gives satisfactory strength and performance of the stabilised material. However, higher than 2.5% of lime may be required to be added when the soil material has a lot of fines and has a high plasticity. 

The HBMs in the United States are called chemically stabilised material (CSM). They can also be referred to as cementitious stabilised materials. The latter is a general terminology that also covers soil stabilisation for subgrade improvement. 

Chemically stabilised material used as sub-base, select material or subgrade under flexible pavement rupture (M,d (psi) 250... 

DAMA also computes the cumulative pavement damage due to excessive subgrade deformation leading to surface rutting and fatigue cracking of the asphalt-stabilised layer. More details can be found in the SW-1 user s guide (Asphalt Institute SW-1 2005). 

The thickness removed is typically between 0.5 and 1.0 m. Suitable materials for subgrade improvement are those used for capping layer (see Section 10.3.1 for soil stabilisation, see Section 10.3.2). 

For the selection of the materials, it is recommended to also consider subsequent publications related to the manual (Austroads 2013a,b). The first is related to modified granular materials (unconfined compressive strength <2.0 MPa) and bound materials (hydraulically or bitumen bound) for base/sub-base course. The second is related to selected subgrade and lime-stabilised subgrade material. 

Optional stabilised subgrade, improved subgrade, or embarkment... 

Austroads. 2013b. Proposed Procedures for the Design of Pavements on Selected Subgrade and Lime-Stabilised Subgrade Materials. Publication No. AP-R435-13. Sydney Austroads Ltd. 

An FWD has the ability to provide information for the structural condition of all pavement layers, including the subgrade. In particular, with FWD deflection measurements, the following may be obtained (a) estimation of modulus of elasticity or stiffness modulus of each layer, (b) estimation of remaining life of pavement, (c) detection of weak points in the pavement, (d) estimation of the severity of potential cracks in stabilised layers, (e) determination of the effectiveness of the load transfer at the joints of rigid pavements and (f) detection of cracks on rigid pavements. 

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