Experimental Investigation on Soil Reinforced with Geosynthetics - Literature review Example

EXPERIMENTAL INVESTIGATION ON SOIL REINFORCED WITH GEOSYNTHETICS By The of the school (University) The City and State Date Experimental Investigation on Soil Reinforced With Geosynthetics Both soils and rocks have highly complex compositions and behaviors thanks to the numerous natural components that have developed over millions of years. The geomaterials present in the layers of the earth are diverse and vast owing to the unimaginable possibilities of combinations of these naturally occurring compounds. Mayne et al. (2009) believe that the mechanical and chemical properties of soils and rocks depend on the geographical location, geological origin, Ionian histories, climatology, weathering, temperature, environment and moisture. They also address the importance of geosynthetics in altering these chemical and mechanical properties. During construction, certain soil characterization and behavior is required for building and is quite difficult to find in naturally occurring soils. Reinforcement is therefore required to obtain these desired soil properties. Since the advent of polymeric materials, the original composition of the soils has undergone continuous reinforcement to alter the strength and resistance. These materials which are incorporated into the earthen structures and other geomaterials make them proper for different uses. Importance of Reinforcement Soil reinforcement has been widely accepted as a backfill material because of a number of factors including; Economic benefits Ability to adapt to a wide range of conditions Aesthetic values Reliability Simple construction techniques High strength Ability to prevent pore water pressure from developing The Mechanism of Soil Reinforcement Reinforced soil alters the mechanical and chemical properties of the original oil sample because of the transfer of stress from the soil to the interface of reinforcement. The transfer of stress is basically what makes it possess high resistance and strength. For such a mechanism to occur, it is important to ensure sufficient interaction between the soil and the reinforcement. Clay soil for instance has an exceptionally low interfacial strength which results in an early interface failure before maximum mobilization of the reinforcement’s full strength. There is therefore a high likelihood that the strength of the reinforcement will not be fully utilized (Abdi, et al., 2009 p.2). Short discreet fibers are quite easy to be incorporated into the soil mixture through random mixing with the soil part. Both geogrids and geosynthetics can be used to facilitate reinforcement. Cement, lime and other additives have been known to change the characteristics of soil and improve their properties. Kaniraj, et al. (2001) and et al., (2009) conducted a research on the effect of cement additives on the strength of soil and came up with the inference that it can be a great reinforcement material. They discovered that when a composite is reinforced with fiber, it shows small losses of peak strengths and more ductility compared to the unreinforced composite. When cement is used together with the composite material, the brittle behavior of the material is lost which makes it applicable in geotechnical engineering projects. Geosynthetics as Reinforcement Materials Geotechnical engineering construction has undergone tremendous changes and discoveries in altering the specification and characterization of cohesive soil. Research on this topic has been improving over the years as more inventions are realized. One of the huge milestones is the design of reinforced soil structures which improve the mechanical behavior of the soil. Asmirza, (2005) carried out an investigation on soil improvement and found out that geosynthetics can be great reinforcement materials. Since geosynthetics have high tensile strengths and low installation costs, their benefits are highly attractive Contractors in the construction business have accepted and used geo synthetics as a soil reinforcement material for their building requirements. One of the advantages of reinforcement objects as construction materials is that they have high resistance due to the amount of tensile force provided by the elements used for reinforcement. This means that overall stability of the soil structure is increased because of reduced horizontal deformations. Settlement is also guaranteed at the shortest time if the right combination of the soil mixture and reinforcement material is adhered to. Use of planar inclusions for instance geogrids, metallic strips, and geotextiles within soil structures has been a traditional method of reinforcement for quite a while now. It was widely used in the past to strengthen and add resistance to soils. The same technology is applied now but with more advanced and effective technology. The reinforcement materials have gone through great advancements because their applications have gained in popularity. Researchers continue to investigate better ways and materials of reinforcement and keep inventing new materials and new ways of reinforcing soil (Consoli and Dalla, 2009) Kitchen cloth was used as the geosynthetic material used to facilitate the reinforcement by some experts. The reason for the choice to use geosynthetics is their ability to function as both a reinforcement material as well as a lateral drain. Zohnberg and Mitchell (1994 p. 105) made an observation on soil reinforcement materials and concluded that making a decision concerning the type of reinforcement material is crucial. They argue that concerns on the build-up of pore pressure, compatibility, higher construction creep potential and lower frictional strengths need to be addressed during reinforcement of cohesive soils. These issues may be unrealistic in the real world for instance where compacted clay is used to construct highway embankments. Permeable geosynthetics take care of all these concerns and eliminate the need for costly backfill materials. They also reduce transportation and structural costs and improve the performance of compacted clay. Ghavazi and Ghaffari (2013) investigated the interaction between the soil and the geosynthetic material and have shown that it is influenced by the following factors; The mechanism of interaction between geosynthetics (direct shear or pull-out) and geomaterials The mechanical and physical properties of the geomaterials. These include water content, grain size distribution, density, grain shape and size and clayey plasticity. The geosynthetic mechanical properties (tensile strength and peak), geometry and shape. Practical Applications Most researchers have thoroughly investigated the effect of reinforcement on soil at failure strain. It is however necessary to have knowledge on the effect of reinforcement of both reinforced and unreinforced soil at various strain levels. This helps in making the right choice on the kind of soil and reinforcement properties. It is important to specify the strength for any given type of soil in terms of density and the reinforced layers at any confining pressure. The value of strain (settlement) ought to be limited to the value of allowable strain. An experiment seeking to show that the level of strain imposed on the clay/sand mixture specimen is crucial in predicting the behavior of reinforcement of cohesive soil was conducted. The results obtained from the experiment can help in determining the initial strength estimates of the wet reinforced soil used in this study and other soils of similar characteristics and grading (Abu-Farsakh, and Coronel, 2006). Definition of Terms Failure stress: This is the specimen stress which corresponds to the maximum deviator stress (also known as the principal stress difference attained) attained, or by the stress of the deviator at an axial strain of 15% depending on what is obtained first in the experiment. Triaxial Compression Test: This refers to the test where a soil or rock cylindrical specimen enclosed in an impermeable membrane is induced with a confining pressure then axially loaded to failure in compression. Deviator Stress: Also known as principal stress difference, deviator stress is the difference between the minor and the major principal stresses in a triaxial experiment. This can be equated to the axial load put on the specimen divided by the cross-sectional area of the specimen. The deviator stress and the chamber pressure constitute the major principal stresses while the chamber pressure makes up the principal stress in the specimen. Total stresses: This is the sum of the cohesion, the undrained shear strength and shear resistance. Effective stresses: Sum of angle of shear strength and the cohesion Liquid Limit: This is the moisture content (empirically established) where a soil changes state from liquid to plastic. It is useful when classifying cohesive soils with fine grains; especially where plastic limit is already known. Plastic limit: This is the moisture content (empirically established) where a soil becomes too dry to be plastic. Both the plastic and liquid limits are necessary when classifying cohesive soils Soil Properties This experiment was carried out to determine the effect of reinforcement of geosynthetics (kitchen cloth) on a mixture of sand and clay. Both triaxial and direct shear tests were conducted to ascertain the differences of shear strengths and stresses between reinforced and unreinforced clay/sand mixture. Table 1 below shows the properties of the soil and clay used to conduct this experiment. Soil Property Value Clay Percentage Composition 75% Plastic Limit 25% Liquid Limit 47% Sand Percentage Composition 25% Table 1: Sand, clay and plastic compositions Reason for the Soil Mixture The soil used to carry out this experiment was a mixture of sand and clay at percentage compositions of 25 and 75 respectively. The reason for the choice to use the soil mixture was particularly due to the numerous advantages of both soils in the construction industry as well as other industries. The individual properties of the sand and clay make them highly attractive therefore the need to conduct studies and experiments using them. Some of the properties include the following; Clay: Clay is one of the soil components with extremely fine particles. It consists of hydrous aluminium silicates with an incredible amount of chemical variation. Because of its high cohesive force, clay holds any type of mixture and binds well with it securing the mixture on the wall; a property that is very attractive during construction of structures. In addition, clay has plastic properties when wet therefore makes the plaster mixture workable. Sand: Sand gives earthen plasters structure, bulk and strength. Because it consists of millions of tiny mineral granules of its parent material; rock, it promotes the structural strength of any mixture. The predominant composition of sand is silicon dioxide (quartz) which makes it a non-reactive substance. Lastly, the most attractive character of sand is that it is found in almost all naturally occurring soils. The attractive benefits of a clay and mixture mentioned above led me to the decision of basing my experiment on reinforcement of geosynthetics on a mixture of sand and clay. A mixture of sand and clay means a combination of the properties which makes a highly useful tool for building structures. I believe that the results obtained from the investigation will be beneficial far and wide because soil and clay are soils used with each passing day (Ghazavi and Ghaffari, 2013). The Experiments Direct shear tests and triaxial tests were conducted to determine the effect of the geosynthetic reinforcements on the clay and sand soil mixture. Both tests were conducted on the reinforced and unreinforced soil. For the direct shear test, a small 100mm x 100mm shear box was used to investigate change in strength, shear stresses and resistance of the reinforced sample compared to the unreinforced sample. The triaxial tests which took place in confined undrained conditions sought to determine the friction angle, total stress, cohesion concept and principal stresses of the reinforcement on the sand and clay soil specimen. The test was done repeatedly for confined pressures of 150kpa, 300kpa and 500kpa for both wet and dry soil samples (Zornberg, and Mitchell, 1994). Consolidation tests Consolidation of the specimen during the triaxial tests was performed in accordance with ASTM D2435. The specimens were trimmed from undisturbed samples to represent the vertical consolidation properties of soil. Other samples were also consolidated to make for the horizontal soil consolidation properties. This was followed by loading the specimens at regular intervals using various load-increment ratios. From here, strain- stress curves were plotted. Direct Shear Test Soils have different strengths depending on two factors namely; the reinforcement and density. One of the oldest and efficient methods used to measure strengths of soils is by using a direct shear apparatus to conduct direct shear tests. This test was used in this study to determine the strength of the sand and clay mixture. Chegenizadeh and Nikraz (2012) argue that soil stabilization and strengthening entails enhancing the subgrade stability as well as mitigation of slope hazards. Randomly oriented fibers can be mixed on the soil to enhance its stability. In this investigation, the reinforcement consisted of geosynthetic plastic also known as “the kitchen cloth”. This geosynthetic makes a wonderful reinforcement material due to its fiber. There have been quite a number of researchers investigating the strength of the soil-geosynthetic interface including Bergado et al. (1993), Alfaro et al. (1995), Farsakh et al. (2003) and (Fox and Kim) have investigated all methods of performing the strength tests on soil reinforced with geosynthetics such as torsional ring shear tests and tilt table tests, but all argue that direct shear test is the most effective. They have investigated the effect of continuous failure on shear strength of a geomembrane-geosynthetic-clay interface by using a wide range of specimen clamping/gripping systems to assess the failure. In addition, they have drawn the conclusion that the progressive failure leads to an increase in the displacement at peak, a reduction in the peak shear strength, a distortion of the shear stress-displacement relationship and an increase in large displacement shear strength. The Testing The direct shear test can be carried out either in re-moulded or undisturbed soil samples. A soil sample is compacted at optimum moisture content inside a compaction mould to facilitate the re-moulding purpose. The direct shear test specimen is obtained by following the correct cutter provided. The sand sample can alternatively be placed in a dry state in the assembled shear box. The next step is the application of a normal load to the specimen then shearing of the specimen between the two halves of the shear box in a predetermined horizontal plane. From there, measurements of sheer displacement, shear load and normal displacements and recorded which eventually help in determining the shear strength parameters (Chegenizadeh and Nikraz (2012) p.6) A direct shear test machine has three components. These are the direct shear test machine, the specimen preparation equipment and the balance. The testing program The direct shear test machine consists of a shear box made of metal where the soil specimen was placed either in a circular or a square plan. The small shear box (100mm x 100mm) was used in this experiment. The shear box was split into two halves then normal force applied on the top of the box. Shear force was then induced by moving one half of the shear box relative to the other. This was done to cause failure in the soil specimen. The shear force was applied in equal increments until the soil specimen failed as seen along the shear’s plane of split. After applying continuous incremental load, the horizontal dial gauge was used to measure the shear displacement at the top half of the shear box. The difference in the specimen’s height which translates to the specimen’s volume change during the test was obtained from the dial gauge’s readings that record the vertical movement of the top loading plate. For a given direct shear test, the normal stress can be obtained using the following calculation: δ = Normal stress = Normal force /Cross sectional area of the specimen The resisting shear stress for shear displacement can be obtained using the equation below: Ƭ= Shear stress = Resisting shear force/ Cross sectional area of the specimen Disadvantages of Direct Shear Testing Machine Direct shear testing machine may be the most commonly used perhaps because it is fast, simple and quite easy to use when determining the strength and stress of reinforced and unreinforced soil specimen but it has a few shortcomings as well as seen below: Inability to control drainage Stress concentrations at the sample boundaries Failure planes has to be strictly horizontal Uncontrolled rotation of both the principal stresses and planes. Triaxial apparatus takes care of these shortcomings. Not only does it minimize the stress build up at the boundaries of the sample and limits rotation of the principal stresses but it also restricts drainage completely. In addition, the failure plane can occur anywhere hence is more versatile. Triaxial Tests Triaxial tests are methods used to determine the compressive strength of specimens of soils by use of a triaxial chamber where the soil sample is subjected to a fluid pressure that confines it. This happens in condition that is undisturbed and for this experiment; confined and undrained. The strain-controlled application used in the triaxial test is an axial compression-test load. In simple terms, the triaxial apparatus measures the amount of stress applied to any composite specimen, which translates strength properties and strain-stress soil relations. Triaxial tests can occur in three conditions namely: Unconfined undrained condition (UU) Confined undrained condition (CU) Confined drained condition (CC) This study concentrated on the confined undrained conditions where different confining pressures were subsequently applied to the specimen. History of Triaxial Tests Studies of clay reinforced with other materials using the triaxial apparatus were first investigated by Ingold (1979). These studies were accompanied by more experiments on undrained compression on consolidated cylindrical samples by use of several disks of reinforcement (mainly aluminium foil or porous plastic). The above tests registered more than 50% reductions in undrained axisymmetric compressive strength of the reinforcing clay specimen relative to unreinforced samples. Pore water pressures which were induced in the reinforced specimen in excess of the unreinforced specimen were the reason for the premature failure of the specimen. When another experiment using the same sample but with a horizontal porous reinforcement was investigated, it showed that there was generation of pore pressure during shear. In this particular case, the premature failure was averted because the porous reinforcements partially dissipated the pore water pressures which were induced in the clay. As the porous spacing in the soil reinforcements was decreased, the strength of compression of the reinforced sample increased substantially compared to that of the unreinforced sample (Zohnberg and Mitchell (1994). Fabian and Fourie (1987) also performed undrained triaxial tests on silty clay samples with different reinforcement materials including nonwovens, woven geotextiles and geogrids. The results showed that the clay with high-transmissivity reinforcements registered up to 40% increase of the undrained strength while low transmissivity reinforcements decreased the undrained strength of the clay by almost the same magnitude. Other studies performed on the use of non-woven geotextiles during the reinforcement of a near saturated silty clay specimen using a plane strain device showed that reinforcement improved the strength and stiffness of drained tests compared to undrained tests (Ling and Tatsuoka, 1993). Importance of Triaxial test In the quest to understand the interaction between soil and the different reinforcement systems, several experimental studies have been conducted using the triaxial tests. Conclusions and characteristics drawn from such experiments have been helpful in the process of reinforcing cohesive soil with geosynthetics. The objective of triaxial tests is to determine the strength of the unreinforced soil sample as well as the apparent strength of the reinforced sample. The strength of the reinforced sample is done for different reinforcements placed on the cylindrical sample. The resulting change of strength due to the reinforcements is calculated using a strength ratio which is the stress of deviator at failure obtained from the reinforced sample divided by the stress of deviator at failure obtained from the unreinforced sample. Shear failure can occur at the interface because of increased stresses near the reinforcement. The shear stresses are usually high around the reinforcement and decrease further away from it. This knowledge comes in handy during construction; especially when poor quality backfill is used. High-strength granular soil layers can be placed around the reinforcement so as to reduce the shear stresses at the interface. The Testing Program In this experiment, the triaxial test was conducted to determine the friction angle, cohesion intercept, total and effective stress of both reinforced and unreinforced soil samples. The test underwent unconfined undrained conditions with pressures of 150kpa, 300kpa and 600kpa. This was done for both dry and wet conditions with a rate of 0.4mm/in. The two important parameters during the design of soil geosynthetic structured are the interface adhesion ca and friction angle δ. Ghavazi and Ghaffari (2013) have explained the basic issues that need to be addressed when designing reinforced structures. These include development of sufficient anchorage (especially in slope reinforcement and retaining walls), prediction loads of the geosynthetics and the potential for sliding along the reinforcement. The triaxial tests determine these parameters that assist during reinforcement of soils. Consolidated Undrained Test The triaxial test of this study was conducted in consolidated undrained conditions. This means that the clay and sand mixture was first consolidated by a chamber confining pressure and full drainage from the allowed. Drainage was done until there was a complete dissipation of excess pore water pressure after which the deviator stress was increased to cause specimen’s failure. The drainage line from the specimen was kept shut during the time of loading. Because drainage was not allowed at this stage of the experiment, the deviator stress caused the pore water pressure to increase in the specimen. Progressive and simultaneous measurements are recorded during the test. Reason for using wet soil During construction, structures built on saturated soils take a relatively short period of time to attain settlement compared to those built on dry soils. Mayne et al 2009 explains that when saturated soils are used to build structures, the load is initially carried by incompressible water situated at the soils’ voids. The additional load on the soil causes the water to be squeezed out of the voids ultimately leading to volume reduction in the void. Course-grained soils with high permeability take a shorter time to complete this process which means that total settlement of a structure can happen at the time of construction. This is not the case however in soils with low permeability as the process may take months or even years to complete. Mohr -Coulomb Failure Criterion Mohr (1900) invented a theory for failure in materials that argues that a material fails due to a combination of normal and shearing stress and not necessarily from either shear stress or maximum normal stress alone. Therefore, the functional relationship between shear stress and normal stress on a failure plane can be presented in the form below: Tf = f (δ) The failure defined by the equation above represents a curved line. Most soil mechanics problems, the shear stress is a linear function of the normal stress. This function can be written in the equation below; Tf = c + δ tan (Φ) where c : cohesion Φ : angle of internal friction δ : normal stress on the failure plane Tf: shear strength The equation above is called the Mohr-Coulomb failure equation. For saturated soils, the equation is slightly different because the total normal stress at any point is the sum of the pore water pressure (u) and the effective stress (δ’) or; δ = δ’+ u The Mohr-Coulomb equation failure criterion expresses the effective stress carried by soils in the following equation: Tf = c’ + δ’ tan (Φ’) Where c’: cohesion and Φ : friction angle, based on effective stress. All the equations above express shear strength based on effective and total stress. Sand and inorganic silt have no cohesion so c’ is 0. Normally consolidated clays also can be approximated with 0 cohesion while over consolidated clays have values of c’ that are more than 0. The friction angle can also be referred to as the drained angle of friction. The over consolidated ratio OCR can be calculated as follows: OCR= δ’c /δ3 Where is δ’c = δc is the maximum chamber pressure at which the specimen is consolidated and then allowed to rebound under a chamber pressure of δ3. References Abdi, M., Sadrnejar, R. and Arjomand, M., 2009. Clay Reinforcement Using Geogrid Embedded In Thin Layers of Sand. International Journal of Civil Engineering. Vol. 7(4), pp.2-4. Abu-Farsakh, M. and Coronel, J., 2006. Characterization of Cohesive Soil-geosynthetic Interaction from Large Direct Shear Test, 85th Transportation Research Board Annual Meeting, Washington, D.C. Alfaro, M., Miura, N. and Bergado, D.,1995. Soil-geogrid reinforcement interaction by pullout and direct shear tests, Geotechnical Testing Journal, 18(2), pp.157-167. Asmirza, M., 2005. Experimental and Model Behavior of Geosynthetic Reinforced Residual Soil Components, Jurnal sistem, Teknik Industri, 6(4), p. 23 Bergado, D., Chai, J., Abiera, H., Alfaro, M. and Balasubramaniam, A., 1993. Interaction between cohesive-frictional soil and various reinforcements, Geotextile and Geomembranes 1(2), pp.327-349 Chegenizadeh A. and Nikraz H., 2012. Numerical Shear Test on Reinforced Clayey Sand, Bund. E 17(7) pp 12-13 Consoli, N. Vendruscolo, M. Fonini, A. and Dalla, F., 2009. Fiber reinforcement effects on sand considering a wide cementation range. Geotextiles and Geomembranes, 2(7), pp. 196–203. Fabian, K. and Fourie, A., 1986. Performance of geotextile reinforced clay samples in undrained triaxial tests. Geotextiles and Geomembranes 4(11), pp.53-63 Fox, P. and Kim, R. 2008. Effect of progressive failure on measured shear strength of geomembrane/GCL interface. Geotech. Geoenviron. Eng., ASCE, 134(4), pp. 459-469. Ghazavi M. & Ghaffari J. 2013. Experimental Investigation of Time-dependent Effect on Shear Strength Parameters of Sand. Geotextile Interface, 37(C1), pp 97-109 Ingold, T., 1979. A laboratory investigation of grid reinforcements in clay, Geotechnical Testing Journal, ASTM, Vol, 16, No. 3, pp.112-119. Kaniraj, S., and Havanagi, V., 2001. Behavior of cement-stabilization fiber reinforced fly ash-soil mixtures. Journal of Geo-technical and Geo-environmental Engineering. 127(7),pp. 574-584. Ling, H., and Tatsuoka, F., 1993. Laboratory evaluation of nonwoven geotextiles for reinforcing on-site soil. Proc. of Geosynthetics, 93( 2), pp. 533-546. Mayne, P., Springman, S., Huang, A.B., and Zornberg, J. 2009. State-of-the-Art Paper (SOA-1): GeoMaterial Behavior and Testing. Proc. 17th Intl. Conf. Soil Mechanics & Geotechnical Engineering, vol 4 (ICSMGE, Alexandria, Egypt), Millpress/IOS Press Rotterdam: 2777-2872. Zornberg, J. and Mitchell, J. 1994. Reinforced soil structures with poorly drained backfills. Geosynthetics International. 1(2),pp.103-148. Read More