Soil Mechanics and Historic Techniques - Essay Example

Soil Mechanics Introduction Earth is extremely rich in soil and minerals and the terrain we find is of different types and kinds. Soil is made up of three components: solid particles, air, and water. The particles are classified by size as clay, silt, sand, gravel, cobbles, or boulders. The amount of air and water within a sample of soil affects its behavior. The sizes and types of particles that constitute a particular soil affect its properties and thus its load-carrying ability and compressibility. However the soil we find is hardly perfect for any kind of construction before any steps are taken to harden the soil. This is usually because soil is by natural forces in tension and shear, although it is comparatively stronger in compression. Soils have relatively low tensile strength so that they are not able to transfer all of forces arising in a structure when it is loaded (Jan 2007). Tension failure can be marked in the form of tension cracks in certain situations, however, slopes and embankments are predominantly the areas where shear fails. Under a load, soil distorts when placed under a load (Thinaga 2004). Historic Techniques In historic times, builders realized early that the soil at one place is different from another. They also realized that building structures built at one place couldn't be copied as they were to another place. This created many challenges for the builders as the instability of the ground left many buildings enveloped by natural disasters such as floods and earth quakes. They tried to build solid foundations for the buildings, however, due to unstable soil, the foundations also swayed. This led to the invention of strengthening the soil first and then building their structures afterwards. The soil strengthening techniques have been utilized since times immemorial. The concept of reinforcement of soil by introducing a foreign material in the ground has been around for ages. The earliest record of strengthening the soil was through using available material. Bamboo and straw were used to strengthen soil mass in many parts of Asia. One of the best examples for this concept has been utilized by the workers building the Great Wall of China. Natural materials such as reeds and twigs were reinforced at every 150mm lift to strengthen the wall. Other materials were also used by other workers building the walls in different provinces (Thinaga 2004). Techniques Strengthening soil through layering reinforcements and its methodologies have been known for more than 3000 years. Materials as such reeds, twigs, grass and bamboos were reinforced into the ground serving their purpose well at that time. But as technology evolved, people found out that these natural materials possessed only limited strength and durability. Although the changing times have increased our knowledge, but the methods used today have been largely unaltered. The major difference today is that engineering technology has traveled as great distance, especially, in the type of materials used (Thinaga 2004). Here are some of the techniques that are being used today to strengthen the soil shear strength. Soil Compaction The soil generally is unstable and has many weak and hard areas. Soil compaction is the movement of soil particles (sand, silt and clay) closer together due to external forces. These external forces can come in a shape of voluntary force or non-voluntary. People trudging over the soil, cars passing over it or things such as littering can all pressurize the soil into compaction. The soil, as a result, becomes denser and soil pores become smaller. Increased soil density results in higher strength and lower hydraulic conductivity. Most of the times, compaction is done on purpose. Firstly it increases maximum load-bearing capacity and heavier and higher buildings can then be built. Secondly it prevents soil settlement and frost damage. It gives the adjacent area a sense of stability too. When the soil is compacted this tightly, it reduces the chances of water seepage, swelling and contraction which can be harmful for the structures on top of the hard soil (Petersen, Ayers, Westfall, 2007). Compaction can be achieved by using a variety of techniques involving Vibration, Impact, Kneading and applying direct pressure. Rammers, Vibratory Plates, Rollers and heavy trucks are mainly used to attain the required level of compaction (Contractors Depot, 2007). Geo-Textiles Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. It is a product used as a soil reinforcement agent and as a filter medium. It is made of synthetic fibers manufactured in a woven or loose nonwoven manner to form a blanket-like product (Environmental Protection Agency Glossary, 2006). These geosynthetics provide tensile strength to the soil, thus enhancing its shear strength characteristics. This enables walls, slopes and embankments to be constructed cost-effectively and quickly. Geotextiles are commonly used as integral components in reinforced soil structures such as retaining walls, over steepened slopes and embankments constructed on weak soils. They help in embankments on weak subgrades, in voiding bridging and in making veneer (Armtec, 2007). Geo-grids Geogrids are open-mesh polymer-based geosynthtics designed to enhance the performance of granular soils. Superior grades of high tenacity, high molecular weight and low carboxyl end group polyester yarns are formed into a grid structure. Then they are given a high quality polymeric coating to produce a strong, flexible, tough, uniform aperture, dimensionally stable and durable geogrid. By combining the tensile strength of the synthetic grid with the resistance and knitted properties between the soil and the grid a composite soil-grid structure can be constructed. The chemical resistance of the grid ensures long-lasting structures with high performance (Techfab India, 2005). Uni-Axial and Bi-axial geogrids are used mainly to reinforce the soil, to block and segment retaining walls, to reinforce steep slopes and also for stockpiling and aggregate storage (Armtec, 2007). Geo-web Geoweb is a cellular synthetic mat which is designed to be used in combination with granular or weaker soils in order to create an engineered-grade composite fill material. Keeping the weaker soil within the individual cells of the Geoweb, and within the expanded mat itself allows these soils to exhibit the characteristics of stronger granular soils. The use of Geoweb can greatly reduce the need for expensive granular materials. They are usually used for constructing road base and railroads in weak soils. It protects against the problems of steep slope by stabilizing the soil. It is also helpful in retaining gravity type wall construction (Armtec, 2007). Vegetations It has been observed that vegetation and tillage increases the shear strength of the soil as well. The deep channeled roots of plants and vegetation inside the soil, brings the soil particles closer together taking the effect of compaction. The hard roots of the plants act as reinforced materials making the soil more stable. However this process takes place as a result of natural growth of vegetation rather than human induced vegetation. This is generally so because it takes time for the plants to grow their roots inside the earth's surface and generally people have no idea how strong the surface would be after some time. Another reason why this technique is not generally used is because first these vegetations would have to be taken out before anything is built on it. This involves cost and also people will have no knowledge what will be the strength of the soil once the tillage has been removed. Cementation and Liming The soil comes in all kinds and shapes. One of the major reasons of soils being weak is that it contains water somewhere deep down. And once pressure is applied, the water displaces and the ground buckles underneath. Therefore the ground under any building has to bear the weight of the structure. For this foundations are built before for the building. Therefore soil for a good foundation that can carry the weight of a house must be well drained so that it is dry and not waterlogged. Waterlogged soil can become liquefied in an earthquake - turn to a semi-liquid - so that structures sink into the ground. This problem can be solved by the addition of quicklime to the soil instantaneous drying the surface. Once treated with lime even the most plastic soil will breakdown into fine particles. This makes it suitable for the addition of cement, fly ash and ground granulated blast furnace slag that will give significant strength and make the material suitable for sub-base applications. The treated soil can then also be compacted to a high standard to promote further strength gain and long-term durability. The soil after this process becomes suitable for stockpiling and reuse. It loses it plasticity to an extent and reduces susceptibility to swelling and shrinkage (Britpave, 2005). Foundations If the techniques described above still don't work in increasing the shear strength of the soil, the job lands onto the foundations of the buildings. These foundations need to strong to support the buildings. If the soil is weak, the foundations must be made stronger. If the building has more than one floor, the foundation must also be made even stronger. Two types of foundations can be used when the soil is unstable or it is weak. Trenchfill foundations fill the excavation, almost to the top, with concrete. The Trenchfill foundation is used when soil is loose or in areas where there is a high water table. In areas with heavy clay and in the presence of trees, Trenchfill foundations may be taken deeper, to a level where the moisture content of the subsoil is unchanged (Buildstore, 2006). Raft foundations are used where the ground is by nature stable but where conditions deep below the surface, such as mining, might lead to ground movement. The reinforced raft is cast on top of consolidated hardcore and is fashioned at the edge to provide a step upon which both leafs of the wall are constructed (Buildstore, 2006). These foundations provide the builders with alternate measures of strengthening the soil. Problems Increasing the soil strength is not always a good measure. Compaction is not always beneficial for the soil. In cultivated soils, if the soil is excessively compacted it may result in poor internal drainage, higher potential for increased runoff and decreased yields. Growing roots do not penetrate high-strength soils, and the result is poor root development in the subsoil. The stressed plant cannot take full advantage of subsoil moisture and nutrients (Petersen, Ayers, Westfall, 2007). These increases in soil strength can make it very difficult for the roots of seedlings to penetrate soil. The effects of even limited increases in soil strength can be dramatic even in favorable environments. In one study, equipment operation increased soil bulk density only 12% on logging decks and skid roads, but reduced seedling survival 90% (Lockaby and Vidrine, 1988). In the desert or in dry lands, increases in soil strength are even more critical as they can prevent roots from keeping up with the drying front as the moist soil dries out after a rain event. Conclusion The weaker soil needs to be made stronger so that it becomes stable for any building to be erected there. Techniques such as compaction and cementation are efficient and cost effective ways to increase the shear strength of the soil. Newer and evolving techniques of geosynthetics such as geotextiles, geoweb and geo grids are more than helpful in making the soil stronger. Shear strength being the maximum stress that can be applied tangentially on a plane within a soil mass before sliding occurs on that plane is affected with the increase in soil strength. In cases where water has to be dried, the shear strength of the soil increases and in other cases it decreases too (Terzaghi, 1942). Where the soil is made denser by using the techniques described above, soil may contract slightly before granular knitting prevents further contraction. In order to continue shearing once granular knitting has occurred, the soil must expand in volume. Through continued shearing the resistance provided by the soil to the applied shear stress reduces. Other affects that can result include cementation and bonding of particles (Roscoe, Schofield, Wroth, 1958). The constant volume shear strength is a built-in feature to the soil, and independent of the initial density or packing arrangement of the soil grains. In this state the grains being sheared fall over one another, with no significant granular interlock or sliding plane development affecting the resistance to shearing. At this point, no inherited fabric or bonding of the soil grains affects the soil strength (Roscoe, Schofield, Wroth, 1958). Increasing the strength of the soil does in-fact increases the shear factor which is helpful as the soil is expected to be much more stable and construction work on it is safer. Works Cited 1. Armtec (2007), "Products - Soil Reinforcement and Retention Products", Available at , Accessed on July 27, 2007 2. Britpave (2005), "Zero Landfill Option" [Internet], Available at , Accessed on July 27, 2007 3. Buildstore (2006), "Foundations" [Internet], Available at , Accessed on July 27, 2007 4. Contractors Depot (2007), "Soil Compaction Handbook", Available at , Accessed on July 27, 2007 5. Environmental Protection Agency Glossary (2006), "Geotextiles" [Internet], Available at , Accessed on July 27, 2007 6. Jan Varga (2007) "SOIL REINFORCEMENT" [Internet], Available at , Accessed on July 27, 2007 7. M. Petersen, P. Ayers and D. Westfall (2007), "Managing Soil Compaction" [Internet], Available at , Accessed on July 27, 2007 8. Robin D Willison (2007), "Handbook on Good Building Design and Construction" [Internet], Available at , Accessed on July 27, 2007 9. Roscoe, K. H., Schofield, A. N. and Wroth, C. P. (1958), "On the Yielding of Soils", Geotechnique, Vol. 8, pp 22-53 10. Techfab India (2005), "High Performance Geogrids for Soil Reinforcement" [Internet], Available at , Accessed on July 27, 2007 11. Terzaghi, K. (1942), "Theoretical Soil Mechanics" [Book], Wiley, New York 12. Thinaga Rans(2004), "Soil Mechanics" [Internet], Available at , Accessed on July 27, 2007 Read More