Soil improvement Methods.pptx

Soil improvement Methods

1. Soil Improvement Methods Prepared by Asst. Prof. Dr. Mohammed Kadum Fakhrudin Civil Engineering Department College of Engineering

2. 1.Introduction The design of a structure which is economical and safe to construct, is durable and has low maintenance costs . All constructions systems are built either on earth, in earth, and/or with earth. The stability of the foundation of a building, a bridge, an embankment or any other structure built on soil depends on the strength and compressibility characteristics of the subsoil. The terms soil improvement, ground modification and ground improvement are used interchangeably. A number of foundation and slope treatment alternatives are possible, and the choice of the most appropriate method or methods depends on the soil types involved, whether predominately granular or cohesive, and the type, function and performance requirements of the facility . In recent years, the interest in soil improvement techniques for foundations and slopes has greatly increased, primarily due to the world-wide increase in the cost of land together with an increase in environmental awareness. The increased cost of conventional construction, especially deep foundations, undoubtedly has contributed to this interest also.

3. Five major functions of soil improvement applied to foundations and slopes: • increasing bearing capacity; • controlling deformations and accelerating consolidation ; • providing lateral stability for slopes and excavations; • seepage cut-off and other types of environmental control ; and • increasing resistance to liquefaction of loose, saturated granular deposits . The methods of soil improvement are:- 1. Compaction. 2. Mechanical stabilization by Use of Admixtures. 3. Preloading. 4. Vertical drain.

4. 5 . Dewatering. 6 . Electro-Osmosis. 7 . Dynamic compaction. 8 . Stone columns. 9 . Grouting. 10. Reinforcement the soil by Geosynthetics . 11. Using waste materials.

5. Compaction: Soil compaction is the densification or reduction in void ratio of a soil through the expulsion of air by the application of the mechanical energy. Soil compaction is extensively employed in the construction of embankments and in strengthening the subgrades of roads and runways. 2. Mechanical stabilization by Use of Admixtures The physical properties of soils can often economically be improved by the use of admixtures. Some of the more widely used admixtures include lime, Portland cement and asphalt. The process of soil stabilization first involves mixing with the soil a suitable additive which changes its property and then compacting the admixture suitably. This method is applicable only for soils in shallow foundations or the base courses of roads, airfield pavements, etc. Soil-lime Stabilization Lime stabilization improves the strength, stiffness and durability of fine grained materials. In addition , lime is sometimes used to improve the properties of the fine grained fraction of granular soils . Lime has been used as a stabilizer for soils in the base courses of pavement systems, under concrete foundations and on embankment slopes.

6. Soil-Cement Stabilization Soil-cement is the reaction product of an intimate mixture of pulverized soil and measured amounts of Portland cement and water, compacted to high density. As the cement hydrates, the mixture becomes a hard, durable structural material. Hardened soil-cement has the capacity to bridge over local weak points in a subgrade. When properly made, it does not soften when exposed to wetting and drying, or freezing and thawing cycles. Bituminous Soil Stabilization Bituminous materials such as asphalts, tars, and pitches are used in various consistencies to improve the engineering properties of soils. Mixed with cohesive soils, bituminous materials improve the bearing capacity and soil strength at low moisture content. 3. Preloading: Preloading is a technique that can successfully be used to densify soft to very soft cohesive soils. Large-scale construction sites composed of weak silts and clays or organic materials ( particularly marine deposits), sanitary land fills, and other compressible soils may often be stabilized effectively and economically by preload.

7. 4. Vertical drain: Vertical drains are normally used for consolidating very soft clay, silt and other compressible materials as following 1. It consists of a series of vertical sand drains or piles. Normally medium to coarse sand is used. 2. The diameter of the drains are generally not less than 30 cm and the drains are placed in a square grid pattern at distances of 2 to 3 meters apart. Economy requires a careful study of the effect of spacing the sand drains on the rate of consolidation. 3. Depth of the vertical drains should extend up to the thickness of the compressible stratum . 4. A horizontal blanket of free draining sand should be placed on the top of the stratum and the thickness of this may be up to a meter, and 5. Soil surcharge in the form of an embankment is constructed on top of the sand blanket in stages 5. Dewatering: Dewatering is a technique of soil improvement whereby the amount and/or pressure of pore water is reduced .Dewatering usually causes densification.

8. 6. Electro-Osmosis: Electro-osmosis can be used to stabilize unstable sand and silt, coarse-fine silt and clays the application of a direct current (D.C .) potential across an anode and a cathode causes the pore water and part of the boundary film of water that is attached to the soil particles to move to the cathode. This action directs the seepage pressures from the anode to the cathode, increases the effective stress in the soil, reduces the water content of the soil and causes a chemical exchange that strengthens the soil . 7. Dynamic compaction: Dynamic compaction is carried out by repeatedly impacting the ground surface by dropping a pounder from a given height from a heavy duty crane at a rate of one blow every 1–3 minutes. Usually the blows are concentrated at specific locations, the distances between the centers of impact frequently ranging between 4 and 20 m. 8. Stone columns: Stone columns have particular application in soft inorganic, cohesive soils and are generally inserted on a volume displacement basis . The size of the stones used for this purpose range from about 6 to 40 mm.

9. 9. Grouting: Grouting usually refers to the injection of suspensions, solutions and emulsions into pores in soils to improve their geotechnical characteristics . Particulate grouts consist of cement-water, clay-water or cement-clay water mixes. Grout used to reduce the permeability of the ground must be able to develop sufficient strength to withstand the hydraulic gradient imposed .

10. Reinforcement the soil by Geosynthetics : Reinforced earth is a composite material consisting of soil containing reinforcing elements which generally comprise strips of galvanized steel or plastic geogrids . In situ reinforcement is the inclusion of resistant elements in the moving soil mass in order to improve its shearing resistance. Geosynthetics Types In general, there are nine types of geosynthetics : Geotextiles, Geogrids , Geonets , Geomembranes , Geopipes , Geocells , Geofoam, Geocomposites and Geosynthetic clay liners.

11. CBR value for the crest point increased more than three times when compared with unreinforced soil.

12. Gypseous Soil for all samples the use of geogrid reinforcement increase the value of California Bearing Ratio (CBR), CBR value for the optimum point increased almost two times when compared with unreinforced soil . The position of first layer of geogrid under the surface have a large influence on CBR, the position 0.15D gives a higher value of CBR for all samples, therefore it can be say that the reliable depth is 0.15D where D is the diameter of CBR mold.

13. 11. Using waste materials: Using CKD as Lining 1. Granular Materials the CBR value of the granular material increased with increasing the depth of CKD lining layer until the optimum position which were 0.2H from the top of the subbase layer. Moreover, CBR value for the optimum point increased almost three times when compared with untreated material.

14. 2. Sand There are obvious increases in shear strength in stabilized soil with CKD in all four cases of study. The higher results obtained in case of CKD lining at depth of 0.5B. The angle of internal friction φ increased by using CKD lining. The higher value of φ happened in soil samples with CKD lining at depth of 0.5B. In this situation, φ equal to 42.77°, and the increase in it to the φ in soil without CKD is about 2.14 times.

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