Soil Modification by adding small amounts of binders A laboratory study

Detta är en avhandling från Luleå University of Technology

Sammanfattning: Soil stabilization through addition of a hydraulic binder is a method frequently used to modify and improve engineering properties of soft soils. Additives like cement and lime are typically used as stabilizers. More recently, industrial by-products, such as fly ashes, cement kiln dust, blast furnace slags and other slags have been used. The chemical reaction between the soil and the stabilizer alters the physical and engineering properties of the soil and thus desired strength and durability are obtained. The choice of appropriate type and quantity of stabilizer (binder) depends largely on factors such as soil type, moisture content, organic content, sulfate content, curing conditions (time and temperature) and the desired improvement.The objective of this thesis is to increase knowledge and understanding of how small amounts of binders change various engineering properties of stabilized soils in short- and longtime perspective. Extensive laboratory and field programs have been carried out. They cover immediate and long-term effects on the engineering properties by adding various binders. Cement, Multicem, and by-products Petrit T and Mesa were used as binders. Binder was added to the soil at various quantities: 1%, 2%, 4%, 7% and 8% of soil dry weight. The field and laboratory investigation included tests of consistency limits, sieving and hydrometer, unconfined compressive strength, density, solidification, grain size distribution using laser particle size analyzer, leaching tests and pH value. The tests were carried out on the treated soil with different binder contents and after different curing times i.e. 7, 14, 28, 60, 90 days for laboratory tests and 7 and 35 days for field investigation.The unconfined compression tests were used to show the effects of different binders on the enhancement in strength and stiffness over time. Consistency limits were determined to investigate the effects of the binders on the consistency limits, directly after treatment and over time. Laser particle size analyzer tests were conducted to investigate the effects of different binders on the particle size distribution (PSD) before and after treatment. The pH tests were conducted to investigate the effects of different binders on the alkalinity of the soil immediately after treatment and over time. This was used to give an indication of soil-binder reactions. MRM leaching tests were conducted to investigate the acidification potential of soils before and after treatment. Freeze-thaw cycles were conducted to investigate the strength characteristics after freezing and thawing in short- and long-term perspectives. Visual observation and standard dry sieving tests were conducted to optimize the proper mixing times to disintegrate or homogenize the soils by decreasing the size of agglomerated soil particles.The results show, that the variation in soil strength and stiffness of the treated soils are linked to different chemical reactions. Cement is most effective in improving the physical and engineering properties compared to the other binders studied. The plasticity index of soil decreases after treatment and over time. Liquidity index and the ratio of water content to plastic limit are introduced as new indices to illustrate the improvement in workability of treated soil by measuring the reduction in the liquidity index. This is found directly after treatment and it increases with time when the liquidity index is within the plastic range or when the water/plastic vi limit ratio is more than one. Increase of binder content and using longer curing times result in increase of soil density and decrease of water content. Particle size distribution of soil is changed by reducing the clay size fraction and increasing the silt size particles after treatment. This shows that an aggregation of particles take place resulting in coarser material than the initial. The cement-treated soils exhibit a more brittle failure in the unconfined compression tests compared to soils treated with other binder types where a more ductile behavior is observed. Applying freezing-thawing-cycles reduces the strength and stiffness of the treated soil.The appropriate length of time to homogenize and disintegrate the natural soil prior to treatment depends on several factors, such as soil type, water content, and plasticity properties of soil. For high plasticity soil, the disintegration time should be kept as short as possible. The homogenizing and disintegration time is less important for low plasticity soils with low water content than for medium to high plasticity soils.The acidification potential of soils are related to the addition of cementitious binders. The effect is found directly after treatment and over time. The treated soil exhibits higher resistance to decrease in pH value. The strength and stiffness properties found in the field investigation agree in general with those obtained from the laboratory investigation for the same binder type.