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1、Subgrade Stabilisation using Chemical AgentsIntroductionWeak or problematic soils, either sand or clay, have low a low bearing capacity and are therefore unsuitable subgrade materials.The subgrade is the bottom most layer of the road, any deformations to it can affect the entire pavement.Weak subgra

2、des can be improved by borrowing stronger material from elsewhere on site, importing stronger material, granular stabilisation, or chemical stabilisation.Chemical stabilisation allows the in situ material to be used and made stronger.The main chemicals used for stabilisation are lime, cement, fly as

3、h, GGBFS, DPP, and kiln dusts.LimeLime is used in stabilisation in the forms quicklime (CaO) and dry hydrated lime (CaOH2).Quicklime is formed by the chemical breakdown of limestone under high temperatures in a kiln.Hydrated lime forms when quicklime is mixed with water.It is used in different amoun

4、ts for drying, modifying, and stabilising soil.Drying and modifying occur at of 1%-4% the dry weight of the soil, hydration reactions use up the water and releases heat. The hydrated lime produced then reacts with the dry clay particles and flocculates them together, enabling easier compaction.It is

5、 used to dry wet problematic soils and create a stable work platform for construction.LimeStabilisation uses 5% or over of the dry weight of soil, an amount that will raise the pH to 12.4. The amount varies with the soil.The high pH breaks down the clay, providing more material for hydration and cre

6、ating a strong, easily compacted cementitious matrix throughout the subgrade.Lime can be applied as a powder or a slurry. Once it is spread evenly, water is added and is then mixed by a rotary mixer. It is allowed to settle, then mixed again and compacted, then allowed to cure.When applied as a slur

7、ry the added water is not applied.Case Study: I-20 Reconstruction GeorgiaThe GDOT in the USA wanted to widen and reconstruct a section of highway.Geotechnical investigations showed that the in-situ material was wet highly plastic clay.Two main options were considered, lime treatment or Graded Aggreg

8、ate Base.A cost analysis proved that lime was the cheaper option stabilising a 230mm layer.After the lime treatment the strength increases were analysed and summarised.CementCementCementFly AshFly ash, also known as flue-ash, is one of the residues generated in combustion, and comprises the fine par

9、ticles that rise with the flue gasesFly ash is a heterogeneous material. SiO2, Al2O3, Fe2O3 and occasionally CaO are the main chemical components present in fly ashes.Fly AshClass F fly ash, which is mostly used as one of the paving material, produced by burning of harder, older anthracite and bitum

10、inous coal. Some advantages to using fly ash include: greater strength, lower permeability, increased durability, reduced alkali silica reactivity, reduced heat of hydration, and reduced efflorescence.GGBFSGGBFS, which is short for Ground Granulated Blast Furnace Slag, mainly contain CaO (30-50%), S

11、iO2 (28-38%), Al2O3 (8-24%), and MgO (1-18%).GGBFS is used to make durable concrete structures in combination with ordinary portland cement or other pozzolanic materials. GGBFSAdvantages include: environmentally friendly through recycling, reduced cracking risk, increased long term strength, and imp

12、roved durablity.Main uses:Portland blast-furnace cement (PBFC) High-slag blast-furnace cement (HSBFC)DPPDry Powdered Polymer:A dry powdered road stabilising binder consisting of an insoluble polymer thermally bound to a very fine carrier such as fly ash. -The Australian Stabilising Industry Associat

13、ion (AustStab) (1998)DPPs act to preserve the adequate dry strength of water-susceptible gravels by a process of internal waterproofing. This involves creating a hydrophobic soil matrix between the aggregates, which reduces permeability, and limits water ingress. The typical softening and lubricatin

14、g effect of any moisture that enters a granular pavement is also significantly reduced (internal waterproofing). DPPThe field performance of DPP stabilised pavements has shown to increase pavement rut resistance (less moisture sensitive) for granular materials that historically were highly sensitive

15、 to moisture. The subgrade is also further protected because there is minimal to no deformation of the stabilised pavement itself. It is important for a stabilised pavement to function as an impermeable protection of the subgrade to improve its volume stability.CKDCement kiln dust is created in the kiln during the production of cement clinker. CKDMore than half million metric tons of CKD was used for soil stabilization. Because of the free lime content and ability to enhance the ef