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brief about ground improvement its need and various methods to improve soil properties
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Ground Improvement Techniques By Prof. Neeraj Singh
Course Outcomes • Behavior of soil based on its particle size and mineral content • Ability to understand the Earth work equipment • Ability to understand the necessity of ground improvement and potential of a ground for improvement • Understand the soil reinforcement mechanisms • Understand the grouting and injection method
References • JoyantaMaity- Ground Improvement Techniques- PHI Learning Pvt Ltd. • Dr. B. C. Punmia- Soil Mechanics & Foundation- Laxmi Publication Pvt. Ltd. Delhi. • Dr. K. R. Arora-Soil Mechanics & Foundation Engineering-Standard Publisher Distributor Delhi. • Hausmann H.R.- Principles of ground modification- Mcgraw Hill book co. • Shashi K.Gulati & Manoj Dutt- Geotechnical Engg- Tata Mcgraw Hill Education Pvt ltd. New Delhi. • Koemer R.M.- Construction & geotechnical methods in foundation Engineering- Tata Mcgraw Hill New Delhi. • P. Purushottam Raj-Ground improvement techniques- Laxmi Publication Pvt. Ltd. Delhi.
WHY GROUND IMPROVEMENT • Development of infrastructures in cities compelled the engineers to improve the properties of soil to bear the load transferred by the structures. • As the land available may not be suitable for the structural load. • The purpose of these techniques to increase bearing capacity of soil and reduce the settlement to a considerable extent.
Ground Improvement • The process in which in-situ soils are improved for the support of the foundations in known as ground improvement. • Ground improvement technologies are geotechnical construction methods used to alter and improve poor ground conditions in order that embankment and structure construction can meet project performance requirements, where soil replacement is not feasible for environmental or technical reasons, or it is too costly.
Purpose of Ground Improvement • Increase the bearing capacity of weak soil • Decrease the amount of settlement or time in which it occurs • Improve the stability of a slope or a vertical cut or an underground opening • Accelerate the rate of drainage • Reduce seepage • Eliminate the possibility of liquefaction
Problem with In-Situ Soil • Collapsible soils • Liquefiable soils • Foundations on dumps and sanitary landfills • Expansive and shrinkage soils • Marshy and soft soils • Sand, gravelly and Karst deposits • Organic soils and peaty soils
Collapsible soils: They are unsaturated soils that undergo a large volume change upon saturation. The sudden and usually large volume change could cause considerable structural damage. The volume change may or may not occur due to an additional load.
Expansive soils: When they absorb water they increase in volume. The more water they absorb the more their volume increases. This change in volume can exert enough force on a building or other structure to cause damage. Example: clay.
Peat : It is an organic complex soil, well known for its high compressibility and low stability. Peat soils are characterized by large porosity, low density and large water contents.
Soft Clay: As the bearing capacity is very low, shear strength is low, consolidation settlements are going to be very high and permeability is very low. These are all very peculiar. So, this needs to be improved.
Karst: It is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves.
Landfills are locations where disposable materials are sent, which are then buried underground. When the landfill has reached its capacity, the waste is covered with clay and another plastic shield.
Liquefiable soils : Poorly drained fine-grained soils such as sandy, silty , and gravelly soils are the most susceptible to liquefaction. Liquefaction is the transformation of a saturated non-cohesive soil from a solid to a more liquid state by earthquake shaking or other rapid loading.
Effects of Ground Improvement • The process reduces the permeability of soil mass. • The process reduce compressibility and consolidation. • The process improves the bearing capacity of soil mass. • The process is used to make an area trafficable within short period of time for emergency & military purpose.
Factors Affecting Selection Of Method • Soil type : This is one of the most important parameters that will control what approach or materials will be applicable to only certain types of soil types and grain sizes. • Area , depth and location of treatment required- Many ground improvement methods have depth limitations that render them unsuitable for applications for deeper soil horizons. • Desired/required soil properties- Different methods are use to achieve different engineering properties, and certain methods will provide various levels of uniformity to improved sites. • Availability of materials- Depending on the location of the project and materials required for each feasible ground improvements approach. • Availability of skills, local experience, and local preferences- While the engineer may possess the knowledge and understanding of a preferred method. • Environmental concerns- With a better understanding and a greater awareness of effects on the natural environment, more attention have been placed on methods that assure less environmental impacts. • Economics- The final decision on choice of improvement method will often come down to the ultimate cost of a proposed method, or cost will be the deciding factor in choosing between two or more otherwise suitable methods.
Basic Structural Units of ClayMinerals • The clay minerals are a group of complex alumino-silicates, i.e., oxides of aluminium and silicon with smaller amounts of metal ions substituted within the crystal. The atomic structures of clay minerals are built up of two basic units • Silica tetrahedral units • Aluminium (or magnesium) octahedral unit
Tetrahedral Units • Silica Unit Consists of a silicon ion surrounded by four oxygen ions arranged in the form of a tetrahedron. The basic units combine in such a manner as to form a sheet.
Octahedral Unit • The octahedral unit has an aluminium ion or a magnesium ion endorsed by six hydroxyl radicals or oxygen's arranged in the form of an octahedron. In some cases, other cations (e.g. Fe) are present in place of Al and Mg.
Types of Clay Minerals • The variation in the stacking of the two basic sheet structures and nature of bonding has given rise to over dozen clay minerals which have been identified. • From an engineering point of view, three clay minerals of interest are • Kaolinite • Montmorillonite • Illite.
Kaolinite • Basic structural unit consists of alumina sheet combined with silica sheet. • Thickness 7 Aº. • There is no interlayer swelling. • Electrically neutral. • Ex: China clay
Montmorillonite • Basic structural unit consist of an alumina sheet sandwiched between two silica sheets. • Thickness: 10 Aº • Negatively charged surfaces of silica sheet attract water in the space between two structural units expansion of material. • High shrinkage and swelling. • Water can be removed by heating b/w 200 to 300 ºC.
Illite • The basic structural unit is same as montmorilloniteexcept for the fact that there is some substitution of aluminium for silicon in the silica sheet and the resultant charge deficiency is balanced by potassium ions. • Swells less than montmorillonite but more than Kaolinite. • The basal spacing is fixed at 10 Å in the presence of polar liquids
Soil Structures • Soil structure refers to the arrangement or state of aggregation of particles in a soil mass. • The engineering behaviour of soils is influenced by soil structure to varying degrees. Types of Soil Structures • Single grained structure. • Honeycomb structure. • Flocculated structure. • Dispersed structure. • Course-grained skeleton structure. • Cohesive Matrix Structure.
Single Grained Structure • Found in the case of coarse-grained soil deposits. When such soils settle out of suspension in water, the particles settle independently of each other. • Major force causing their deposition is gravitational and the surface forces are too small to produce any effect. • After attaining the final position, each grain is in contact with the surrounding grains and formed structure is called as single grained structure.
Honeycomb Structure • Associated with silt deposits. • When silt particles settle out of suspension, in addition to gravitational forces the surface forces also play a significant role. • When particles approach the lower region of suspension they will be attracted by particles deposited as well as the neighbouring particles leading to the formation of arches. • The combination of a number of arches leads to the honeycombstructure. • There will be large reduction in volume due to breakdown of structure
Flocculated Structure • These are the types of structures found in clay particles which contains larger surface area. • There are two type of configurations edge to edge and edge to face contact between the particles. • These are charged particles which have positive charge on the edges and negative charge on the face of the particle. • When there is net attractive force between the particles, then positive charged particles attracted towards negatively charged faces which results the formation of flocculated structure.
Flocculated Structure • This type of soils has high shear strength. • Because of edge to face orientation void ratio is high in this type soil and water content also optimum but they are light in weight. The compressibility is very low for this type of soils.
Dispersed Soil Structure • Dispersed structure also occurs in clay particles when the clay is remoulded. • Remoulding reduces the shear strength of soil due to repulsion between them, the edge to face orientation turns into face to face orientation. Finally, dispersed structure of clay will form. • This type of soil is highly compressible and less permeable.
Coarse Grained Skeleton Structure • Found in the case of composite soils in which the coarse grained fraction is greater in proportion compared to fine grained fraction. • The coarse-grained particle forms a skeleton like structure and voids between them are filled by fine grained or clayey particles. • If it is undisturbed, it will give good results against heavy loads. If disturbed, the strength extensively reduced.
Cohesive Matrix Structure • Found in composite soils in which the fine-grained fraction is more in proportion compared to coarse grained fraction. • In this case the coarse grained particles will be embedded in fine grained fraction and will be prevented from having particle to particle contact. • This type of structure is relatively more compressible compared to the more stable coarse grained skeleton structure.
Compaction • Compaction is a process that brings about an increase in soil density or unit weight, accompanied by a decrease in air volume. There is usually no change in water content. • The degree of compaction is measured in terms of its dry unit weight. • The densification of soil is done by the application of mechanical methods
Factors Affecting Compaction of Soil • Water Content. • Compactive Effort. • Type Of Soil. • Method Of Compaction.
Water Content • At low water content, the soil is stiff and offers more resistance to compaction. As the water content is increased, the soil particles get lubricated. The soil mass becomes more workable and the particles have closer packing. The dry density of the soil increases with an increase in the water content till the O.M.C is reached.
