Role of Index of Plasticity in Structural Design - Research Paper Example

Foundation design factors: Introduction: The determination of soil type and the soil properties are some of the most fundamental prerequisites of foundation designing. A foundation is designed so as to minimize the settlement and distribute the load of the structure over the underlying soil. Load of the structure is one factor that needs to be considered while designing the foundations, while the underlying soil’s properties are equally important. If not considered in the designing phase of the substructure, underlying soils can undergo volumetric changes large enough to cause danger to the integrity of the whole structure. The Atterberg limits: The two limits, namely the plastic limit and the liquid limit are together called as Atterberg limits. The Atterberg limit tests are considered as necessary for designing a structure’s foundation. The identification of the right state of soil from a range of liquid, plastic, semi-solid and solid states is vital for determining the strength of fine grained soils. The state of a soil is influenced by the water content in the soil, which can be realized with the help of Atterberg limit tests. Atterberg limits are the water contents that cause the soil to change from one state to another. In addition to that, the Atterberg limit tests serve to identify the hydraulic conductivity coefficient, “k” of a soil, that is actually a measure of the average velocity of water through the pores of the soil under the unit hydraulic gradient. The plasticity index of a soil: Before getting into a detail of how plasticity index of a soil can influence the design of foundation to be constructed over it, it is customary to give a brief description of the term. Plasticity index is the difference between the percentage of moisture content in the liquid limit of a soil and that in its plastic limit, since soil at both Atterberg limits always contains a moisture content. Plasticity index of a soil is essentially a measure of the range of values over which it would maintain its plastic characteristics. Clay starts to exhibit its plasticity when water is added to its dry solid sample. When that happens, the percentage of moisture the sample contains is referred to as its plastic limit. The plastic state is essentially the state at which the sample starts to soften without developing cracks. Soil transforms into a liquid state from a plastic state as more water is added to the plastic soil sample. The point at which it begins to act like a liquid is identified as its liquid limit. Highly plastic clays, or Fat clays are those that have a plasticity index more than 50 and they are highly expansive in nature. Thus, as the plasticity index of a cohesive soil rises, it begins to be increasingly expansive in nature. Why is plasticity important? Plasticity is associated with fine grained soils and not with course grained soils. Coarse grained soils can be classified on the basis of the size of their constituent particles, whereas for fine grained soils, particles are too small to be sized. That is why, fine grained soils are classified on the basis of their plasticity. Also, the content of water in soil alters its properties and is thus, a matter of major concern when the foundations are being designed. While the size of particles becomes smaller, it effectively serves to increase the surface area of contact between the soil particles and water. The increased contact area enhances the water absorbing capability of fine grained soils. Hence, fine grained soils, typically Clay has a lot of capability of absorbing water. As a result of that, the clay minerals and the entrapped water particles interact in such a way, that the engineering properties of the soil become a function of the moisture content it contains. Shear strength of Clay is influenced by the moisture content of Clay and also on the rate of water absorption or expulsion. This is evident from the fact that the shear strength of Clay which contains 50 % water will be far less than that achieved at a moisture content of 10 %. (scienceray.com, 2010). Besides, plasticity index is also used together with liquid limit to realize the category in which a particular soil sample belongs. This is achieved with the help of charts commonly known as the Plasticity charts or the A-line charts. It is shown in the figure below:  (www.mite.com.au, 2007). The chart shown above categorizes soils as organic or inorganic, coarse and fine grained soils on the basis of their plasticity indices and liquid limits. As the plasticity index of a soil rises, its volume change characteristics become enhanced. “Fat or plastic clays plot above the line”. (www.mite.com.au, 2007). Dangerous plasticity index range: According to the Section 1802.3.2 of the International Building Code and Section R403.1.8 of the International Residential Code states that if a soil has a plasticity index of 15 or greater, when determined according to ASTM D 4318, it is one of the four provisions for classifying a soil as Expansive in nature. (www.fairfaxcounty.gov, 2009). Thus, when the plasticity index of a soil falls in the range prescribed above, it is essentially dangerous for the structure in a number of ways mentioned below. Plastic soil’s interaction with the structure: Common issues: Because of the extremely small size of the Clay particles and increased surface area, the presence of Clay in a soil causes it to expand. The higher the amount of Clay in a soil, the more expansive the deposit becomes. One of the most common causes of issues, a structure experiences in its foundations is the nature of such shrink – swell soils, also known as “Expansive soils”. Plastic soils can develop huge variations in their volume as their moisture content raises or lowers. The variations can be as large as 30 % of the original volume. If this characteristic feature of Expansive soils has not been considered much in the design of foundations, it can later result in drastic consequences. By changing volume, Expansive soils can result in both lifting up and settlement of the whole structure that rests on the underlying Expansive soil. Besides, Expansive soils can also cause lateral movements due to the pressure exerted on the walls of foundations and the retaining walls as the soil swells. The rise in moisture content of these soils also causes a loss of strength which can result in disastrous settlements and imbalances brought along with the inadequate load distribution channels. Similarly, when these soils loose water as a result of evaporation or by gravity, the soils shrink causing settlement of the structure constructed over them. Conventional slab-on-grade design employed in the construction requires footings to be designed for loads greater than those which are borne by the grade slab. In such a design of footings, footings are able to withstand the lateral pressure exerted by the expanding soil lying beneath the grade slab. So the expanding soil basically serves to cause deflections vertically. This phenomenon is commonly referred to as “differential heave”. It causes the hump in the roof. Cracks originate and start to develop sideways from the humped roof. This phenomenon can also be observed in conventional column and beam foundation system, which is based on differently loaded isolated and strip footings. Middle portions are humped as compared to sides in a similar fashion. However, soil would not cause any load imbalances unless it experiences a change in the moisture content over the life of the structure. This characteristic feature of Expansive soils can eliminate chances of structural settlement only if the underground moisture levels can be controlled. The following text lays down certain methods that can be adopted to eliminate the foreseeable ways in which the high plasticity index-soil can be dangerous for the integrity of a structure. Commons solutions: The best solution to prevent the swelling of soils is by protecting them from experiencing changes in moisture content. Unfortunately, that may not always be possible given the natural fluctuations in the underground water table, seepage and leakage, or because of the very high cost of the design and implementation of such a system. Therefore, alternate strategies need to be designed and adopted for ordinary buildings. Some of them are listed below. For new structures: Pre-wetting of soil: Prior to the construction of foundations and grade slab over the soil, it should be tested for plasticity index and liquid limit. If the plasticity index of the soil is found to be above 15, it should necessarily be watered enough to allow all expansion to occur. The design of foundation and the elevation of grade slab should follow the dimensions suggested by the expanded soil. Replacing expansive top with non-expansive soils: If the project is too fast to allow time for the wet soil to expand, an alternative to the pre-wetting of soil can be the replacement of the top of expansive soil with non-expansive soil. The thickness of removable layer can be up to several feet. For existing structures: General measures: Strategies for existing structures have to be more carefully designed so that the structure is not exposed to additional risks. In such cases, measures to reduce the effect of expanded foundation soil can be addition in the thickness of the grade-slab, deepening of the foundation walls so as to extend the footing-walls to more stable layers of soil. Replacing grade slab with structural slab: The humped grade slab of an existing structure can be replaced by a “Structural slab”. A structural slab is normally thicker than an ordinary grade slab. Its thickness is about 5 to 6 inches and is usually reinforced with half-inch reinforcing bar (#4 bars) at about 18 to 24 inches center to center both ways. (www.foundation-repair-guide.com, 2007). In such a case, it may not always be possible to replace the grade slab completely including the portions immediately above the walls of footings with a structural slab. If that happens, those portions should be left as such and should be connected to the new structural slab with the help of rebar dowels. Conclusion: Plasticity index of a soil should be a major consideration while designing the substructure and grade slab. Before constructing the footings, it is imperative that the soils are tested for the plasticity index. If it lies in the range specified for expansive soils, special care must be taken and the activities should be planned in a way that would ensure that the structure would not later be subjected to loads because of the expansion of the underlying soil. References: Scienceray.com. “Effect on Shear Strength Due to Various Water Content in Clay Soil”. 2010. Web. 21 June, 2010. www.mite.com.au. “"A" line charts and what they mean”. 2007. Web. 21 June, 2010. www.fairfaxcounty.gov. “Clarification of the definition of expansive soil”. Land Development News. 2009. Web. 21 June, 2010. www.foundation-repair-guide.com,. “Getting Control of Expansive Soil”. 2007. Web. 21 June, 2010. Read More