Challenge

Clayey soils have a poor rating as subgrade for road construction due to the tendency to swell and shrink. Lime dries out and modifies the structure of the soil. At this stage, the plasticity index of the soil is reduced, and the soil becomes friable and granular, making it easier to work and compact.
Gradual hardening of the mixture, thanks to bonds formed by pozzolanic reactions between clay and lime will increase the bearing capacity. Compacted clayey soils result in flexible, permanent structural layers for subgrade, subbase, base & capping layers.

Solution

Soil treatment with lime is a technique where fine soils are mixed, in-situ, in order to obtain flexible, permanent structural embankment layers for all types of roads, highways and railways construction. Lime addition into fine soils creates different effects.

Immediate effects:

• Soil drying - rapid decrease in soil moisture content due to the hydration of quicklime;
• Soil modification – decrease of soil plasticity, increase in optimum moisture content, decrease in maximum dry density, reduction of the soil’s capacity to swell and shrink, and improved strength and stability after compaction.

Long term:

• Soil stabilization – pozzolanic effect: physical-chemical reaction among clay components and calcium - induced by the pH increase – changing soil particles size.

Assessment of Suitability for Lime Stabilization

Soil classification - Soil stabilization with lime is suitable in very cohesive soils with a “fair to poor” subgrade rating.
Organic Materials < 4%
Total Sulphur Salts < 0.25%
Lime-Soil Interactions - Calcium hydroxide is split into Ca2+ and OH- ions, which are increasing the pH. In these conditions, Ca2+ ions will fix the clay particles around them creating macro-particles which are very stable. This process is called clay flocculation.
Immediate Lime Consumption - The test consists in measuring pH of a soil-lime mixture, starting at 2% by mass addition of lime. Lime addition is increased in steps of 0,5%, until a value superior to 12,4 at 25°C is obtained. The minimum lime content, reaching the threshold of 12,4, is defined as the immediate lime consumption.

Methods: choosing the right solution

Laboratory:
Maximum Dry Density & Optimum Moisture Content – increasing optimum moisture content and reduction of dry density.
Compressive Strength on Proctor Specimens - Natural soil and soil-mixed specimens in accordance with EN 13286-50 at standard Proctor compaction. Determine compressive strength - after a conditioning period of 28 days - that ensures adequate field performance in a cyclic freezing and thawing and an extended soaking environment.
Mechanical Performance - California bearing ratio (CBR) test is described in EN 13286-47 measures CBR after 11 days, this means after 7 days (air-conditioning), followed by 4 days soaking.
Volumetric swelling after 4 days soaking - Volumetric swelling, determined on fully soaked CBR specimens in accordance with EN 13286-47 is recommended to be below 1%.
On site: steps include preparation of soil, spreading of lime, eventually adding water and mixing, compacting to maximum (practical) density, and curing.
Quality control: It is recommended to measure prior the execution the moisture content, during execution the quality of lime spreaded, milling degree, compaction control. After the execution plate load test (static or dynamic).

 

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