Effect on Shear Strength Due to Various Water Content in Clay Soil Proposal in civil engineering regarding to clay soil.
This research is focusing on clay soil. The shear strength that is or will be developed by each soil type is different. Shear strength of soil is able to influence the stability of buildings structure. Soil with high shear strength will be able to support buildings structure without failing. Otherwise, the structure will not be stable and side effects will occur either in short term or long term depends on the shear strength. That is why research regarding to shear strength should be carried out to ensure the capability of soils in supporting loads.
The shear strength of soil depends on the water content. Most geotechnical failures involve a shear-type failure of soil. This is due to the nature of the soil, which is composed of individual soil particle that slide when the soil is loaded. The shear strength parameters of soil are known as cohesion, c and angle of friction, Ø. The higher values of cohesion, c and the angle of friction, Ø is higher the shear strength of the soil.
Clay soils can be classified into four conditions according to the amount of water content. They are solid, semisolid, plastic and lastly liquid [1]. The continuous increment of water content will change solid clays soil to semisolid, followed by plastic and liquid. This process not only changes the condition of clay soils, it also weakens the cohesion value of the soil.
The variations in water content of clays are important for a wide variety of reasons. They affect the engineering properties of the soil and thus the building design. For the contractor, the effect can be more prosaic but very important. For example, the jobsite that is dry and easy to move construction equipment about can become a quagmire if enough rain raises the water content of the soil towards and beyond the liquid limit. This may necessitate matting in order to move equipment around, or even affect the type of equipment that is brought on site. The contractor must be prepared for this type of change if conditions are not the same as when the job was bid or began [9].
Very soft to medium clays are very troublesome when they must be supported or excavated and often are unsatisfactory materials for temporary foundations in shoring systems [9]. In brittle soils, yielding may lead to the formation of shear slip surface, over which sliding movement takes place, e.g. landslips, rotational slope and excavation failures. Measures of shear strength are required in the analyses and design of geotechnical structure, such as foundations, retaining walls, earth slopes and road bases [2].
Any change in the ambient conditions will bring about a change in moisture content. If water is taken in a swelling, pressure will be exerted and the volume will tend to increase. Shrinkage will take place if the adsorbed layer is compressed, forcing water out or if suction reduces the moisture content. The swelling potential of clays is very high. In soil masses in general, shrinkage manifests itself as a series of polygonal cracks emanating downward from the surface [2]. Base on the statements, shows that the amount of water content may influence the stage of soils strength.
The main objectives of this study are:
The chosen location for this research is RECESS which is located in KUiTTHO’s campus in about 20 km from the Batu Pahat town center towards Ayer Hitam. The scope of this study is to determine the shear strength of clay soil according to the water content. It includes of disturbed and undisturbed samples. Samples will be taken at depth of 1.5 meters from the surface. There are 31 samples will be needed. The total of 21 samples will be used for direct shear test, 3 samples for free swell test and the other 7 samples will be used for vane shear test.
The percentages of water content that will be involved in both tests are starting from 15% until 65% with gap of 10%. Clay soil with optimum water content will produced the highest shear strength.
Importance and Contribution of Study
In producing buildings, there are many factors that should be considered. One of them is the capability or strength of soils in supporting loads. Any failure that happens to supporting soils will affect the building structures. This aspect is also important in process of digging and removing soil in order to prepare a site for construction and services. The problems here are closely related those of support. In the case of both natural and built slopes (embankments), it is necessary to determine their intrinsic ability for self-support. Problems of shear failure, in which possible collapse mechanisms are investigated where rupture surfaces develop due to the shear strength of the soil being exceeded.
Based on the research that will be carried out, graph of shear strength versus percentage of water content will be plotted. From this graph, the suitable percentage of water content to perform a high shear strength soil can be determined. This information helps engineers in estimating the load that can be transferred to the clay soil. However, the achieving results are limited for clay soil in RECESS KUiTTHO because the properties, behaviour and structures of clays are different base on area.
Study Area
This research is focusing on clay soil. It includes of soil investigation for the purpose of ground improvement. Clay soil will be investigated from the aspect of shear strength. There are many factor influences the level of shear strength. One of them is the percentage of moisture content. That is why the moisture content in clay soil will be investigated to find out their capability in changing the shear strength of clay soil.Literature Review
The term soil conveys varying shades of meaning when it is used in different contexts. To a geologist it describes those layers of loose unconsolidated material extending from the surface to solid rock, which have been formed by the weathering and disintegration of the rocks themselves. An engineer, on the other hand, thinks of soil in terms of the work he may have to do on it, in it or with it. In an engineering context soil means material that can be worked without drilling or blasting. Pedologists, agriculturalists, horticulturalists and others will also prefer their own definitions [2].
For engineering purposes soil is best considered as a naturally (mostly) occurring particulate material of variable composition having properties of compressibility, permeability and strength. All soils originate, directly or indirectly, from solid rocks and these are classified [2]. According to McCarthy, in nature, soils are made up of particles of varying size and shape. To distinguish between soil where size cannot be visually discerned (the particles are too small), an additional property, plasticity is used as criterion. Study has proved that a soil’s important behavioural properties are not always controlled by particle size and plasticity. Soil structure and mineralogical composition and their effect with water, may also have significant influence on the properties and behaviour deemed important for design and construction [8].Clay Soil
The most significant properties of clay are its cohesion and plasticity. If when pressed together in the hands at a suitable moisture content the particles stick together in a relatively firm mass, the soil shows cohesion. If it can be deformed without rupture (i.e. without losing its cohesion), it shows plasticity. Clay dries more slowly than silt and slicks to the fingers; it cannot be brushed off dry. It has a smooth feel, and shows a greasy appearance when cut with a blade. Softer consistencies behave rather like butter, and harder consistencies like cheese. Dry lumps can be broken, sometimes with difficulty, between the fingers, but cannot be powdered. A lump placed in water remains intact.
Clay does not exhibit dilatancy. Lumps shrink appreciably on drying, and show cracks which are the more pronounced the higher the plasticity of the clay. At a moisture content within the plastic range, clay can easily be rolled into threads 3mm diameter which for a time can support their own weight. Threads of high-plasticity clay are quite tough; those of low-plasticity clay arc softer and more crumbly [12].Properties of Clays
Because of the small particle diameter and plate-like shape of clays, the surface area to mass ratio is much greater than in other soils. This ratio is known as the specific surface. For example, montmorillonite has a specific surface of about 800 m2/g, which means 3.5 g of this clay has a surface area equal to that of a football field.
The large specific surface of clays provides more contact area between particles, and thus more opportunity for various interparticle forces to develop. It also provides more places for water molecules to attach, thus giving clays a much greater affinity for absorbing water. Some clay can easily absorb several times their dry weight in water. The interactions between this water and the clay minerals are quite complex, but the net effect is that the engineering properties vary as the moisture content varies. For example, the shear strength of a given clay at a moisture content of 50% will be less than at a moisture content of 10%.Formation of Clay Soils
On a slightly larger but still microscopic scale, clay minerals are assembled in various ways to form clay soils. These microscopic configurations are called the soil fabric, and depend largely on the history of formation and deposition. For example, a residual clay, which has weathered in-place and is still at its original location, will have a fabric much different from a marine clay, which has been transported and deposited by sedimentation. These differences are part of the reason such soils behave differently [10].
Although we sometimes encounter soil strata that consist of nearly pure clay, most clays are mixed with silts and/or sands. Nevertheless, even a small percentage of clay significantly impacts the behaviour of a soil. When the clay content exceeds about 50 percent, the sand and silt particles are essentially floating in the clay, and have very little effect on the engineering properties of the soil [10]Plasticity of Clay
Because of the controlling importance of the effect of surface activity on the behaviour of fine grained soils, description of these materials by reference to their particle sizes is practically meaningless. The practical distinction between silt and clay is made, not on the basis of an arbitrary size distinction, but on the basis of material behaviour in the presence of water. The consistency of fine soil varies according to the amount of water present. Completely dry, the soil may be hard (solid), while at high water contents it may be almost a slurry (liquid). Intermediate states of consistency are semisolid and plastic states. A plastic material is one that deforms readily without cracking or rupture. The boundaries of these states of consistency are defined in terms of soil water content [9].
Different soils may be distinguished by their plasticity characteristics because these characteristics vary with surface activity of the constituent particles. The more active soils (claylike) are more plastic than the inactive soils (silts). This phenomenon may be explained by examining the nature of the water near the surface of a clay particle. Since water’s molecular structure is dipolar, the water near the clay particle is effectively immobilized by the surface charge. It is adsorbed and may be considered essentially solid. As distance from the particle surface increases, the orientation of water is reduced in degree until, at the boundary of the particle’s influence (limit of diffuse double layer), the viscosity is that of free water. With an abundance of water, soil particles would be separated by free water and the mixture would be fluid. As the amount of water decreases, the particles are separated by increasingly stiffer water. The mixture becomes like a solid. The soil water system, therefore, has a range of water contents over which it may be plastic. Changing the chemical composition of the constituent phases (soil type or fluid) causes a change in the plasticity range or the plasticity index. This permits to distinguish among soils on the basis of their plasticity [9].Expansion of Clays
The generic name for clay minerals that swell (expend) as change in soil water occur is smectite. Clays expand in volume if the soil water content is below a stability value when becomes available. The volume change is related to the thickness and mobility of the water film adsorbed onto or surrounding the clays particle, being increased relatively easily during natural wetting conditions. Probably most of swelling that occurs is due to water moving in. A requirement for significant swelling is that the soil have many fine pores in the 0.001 mm to 0.002mm range. Pores of this size permit rapid acceptance.
The expensive force created by a clay undergoing an increase in water content and volume can be considerable, being capable of lifting heavy structures and imposing lateral pressure that can move retaining walls and basement walls. Swelling pressures in excess of 500 kPa have been measured. High swelling pressures can occur even in already high-moisture clays if additional water is adsorbed. Damage to structure can result when a clay swells.Shear Strength Theory
The strength of soil is a variable and elusive property. Strength characteristics of some materials are represented by concepts such as yield point or tensile strength. For compacted fills, engineers deal with soils as they are in nature. Because they are not manufactured products, variations in properties are the rule, not the exception. They are natural materials, and in most cases engineers use what is available, adjusting designs and working methods to accommodate conditions [9].
Strength is the measure of the maximum stress state that can be induced in a material without it failing. Although strength can be stated in terms of compressive stress or tensile stress, fundamentally it’s the ability to sustain shear strength that provides strength. The shear strength of a soil, τ is the internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it. The shear strength of a soil is indicative of the stability and strength of the soil under various conditions of loading, compaction, and moisture content [2].
The shear strength of a soil is the maximum resistance which it can offer to shear stress. When the maximum has been research the soil is regarded as having failed, its strength having been fully mobilized. However, the shear strength value determined experimentally is not a unique constant which is characteristic of the material but varies with the method of testing [13].
The stress on any plane surface can be resolved into the normal stress, σn, which acts perpendicular to the surface and the shearing stress, τ, which act along the surface, the magnitude of the resistance being given by Coulomb’s equation:
τ = c + σn tan Ø
Where,
τ = shear strength of soil
c = cohesion
σn = normal stress
Ø = angle of friction
Shear Strength of Clays
The shear strength of clays depends not only on the soil type and composition, but also on factors related to the mineralogy, grain, size and shape, adsorbed water and water chemistry of the clay minerals present. Shear strength also depends to a great extent upon the initial moisture content of the clay and the rate at which the soil structure can expel or take in water during a test.
In a shear test on a particular type of clay the three factors which are of the greatest significant here are:
The undrained shear strength of saturated plastic clays is usually determined from compression tests. The shearbox test being less satisfactory for these soils. However their shear strength can be measured directly by the vane apparatus [11]. Clay soils are cohesive in that they possess some strength at zero normal pressure. Consequently, their strength envelope looks like that shown in Figure 2.3. The slope of the strength envelope depends greatly on drainage conditions as the clay is being loaded. For most short-term loading conditions, the usual situation during construction, the slope of the strength envelope is near zero if the clay is saturated. The strength of the clay is, therefore, expressed by the cohesion term, c [9]. For a given clay, unconfined strength or cohesive strength will depend strongly on the water content. Table 7.2 indicates typical ranges of strength corresponding to standard nomenclature for soil consistency. Soils of stiff to hard consistency seldom present problems when encountered in construction [9].
Moisture Content
Naturally occurring soils nearly always contain water as part of their structure. The moisture content of a soil is assumed to be the amount of water within the pore space between the soil grains which is removable by oven drying at 105-110°C, expressed as a percentage of the mass of dry soil. By “dry” is meant the result of oven drying at that temperature to constant mass, usually for a period of about 12-24 hours. In non-cohesive granular soils this procedure removes all water present.
There are several ways in which water is held in cohesive soils, which contain clay minerals existing as plate-like particles of less than 2 μm across. The shape and very small size of these particles, and their chemical composition, enable them to combine with or hold on to water by several complex means [12]. For the purpose of routine soil testing, moisture content relates only to the water which is removable by oven drying at 105-110°C. Moisture content is usually expressed as a percentage, always on the basis of the oven-dry mass of soil.
Methodology
Research methodology is used to described the overall work plans of the research for the whole semester. It is normally written in flow chart that can be easily to read and understand.
Research flow chart is a guideline that will be followed throughout the research to achieve the best result as shown below.
Moisture Content Test (BS 1377: Part 2: 1990: Clause 3)
The standard method that will be used for this test is the oven-drying method and this is the procedure recommended for a soils laboratory.
This moisture content test includes of disturbed and undisturbed sample. Undisturbed sample is needed to determine the actual moisture content of sample taken from the research area. To change the percentage of water for the purpose of this study, disturbed sample will be used. The percentage of water will be increased from 15% to 65% with gap of 10%. To determine the mass of water needed to produce 15% until 65% of water content, the following equation can be used:
w (%) = mw x100
mp
where;
w = moisture content
mw = loss of moisture
mp = dry mass
Free Swell Test
About 50g of undisturbed sample will be needed to carry out this test. Because “free swell” is defined as the change in volume of the dry soil expressed as a percentage of its original volume, it can be calculated from the equation:
free swell = V – 10 x 100%
10
where;
V = volume of settled solidsDirect Shear Test (BS 1377: Part 7: 1990: Clause 4)
The direct shear test apparatus for performing single shear is essentially a rectangular or circular box having separated lower and upper halves. For this research, a rectangular shearbox will be used.
In the standard type of apparatus, the box is 60 mm x 60 mm. Both disturbed and undisturbed sample will be used in this test. Three different loads will be applied as the vertical load. They are 1.25 kg, 2.5 kg and 3.75 kg with 7 different samples for each load. The straight-line equation for the limiting shear stress that suggested by Coulumb in 1776 is given below:
τ = c + σn tan Ø
Where;
c = apparent cohesion (assumed to be constant)
σn = normal stress or slip surface
Ø = angle of friction (or angle of shearing resistance)Vane shear test
The vane shear test consists of inserting a vane into the clay soil and then rotating it by applying a torque [14]. The torque is measured and for a known-size vane, is easily related to the shearing strength of the clay soil. The equation for the applied torque is given by:
T = cu πhd x d + 2π d2 x d
2 4 3
where;
T = applied torque
cu = undisturbed undrained strength
h = height of blade
d = width of blade
Hence cu = T
(1/2) πd2 [h + (1/3)d]
This research basically study on the shear strength of clay soil with certain water content. It includes of swelling analysis, shear strength analysis and water content analysis. The purpose of shear strength analysis is to ascertain the suitable water contain needed for clay soil in RECESS KUiTTHO to perform optimum shear strength.Shear Strength of Clay Soil in Recess kuittho
At the end of the study, the highest shear strength that clay soil in RECESS KUiTTHO is able to produce can be determined according to the percentage of water content that had been applied in the range of 15% until 65%. Meanwhile, the percentage of optimum moisture content can also be identify in producing the highest shear strength clay soil for the purpose of load supporting such as building structure and so on. As conclusion, the objectives of this research will improve the original shear strength of clay soil in RECESS KUiTTHO.
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