Unconfined Compression Test and Foundation Design - Essay Example

Civil Engineering Laboratory Unconfined Compression Test and Foundation Design Submitted Robert Samuel December 9, Thefocus of this experiment is to introduce and familiarize the students with the important testing techniques and materials used to set up the column footing of a building under a given load conditions. The experiment also gives an account for various types of soils and their associated properties in construction point of view. It has been shown that by using elementary techniques from geotechnical engineering along with various instruments the potency of soil samples in a given region can be predict. Also by calculating various soil parameters it can be evaluated that how much pressure per square meter a type of soil can be tolerate. Introduction: Understanding of soil is an essential area of study in civil engineering. It is critically important to carry out the analysis of soil prior to initiate any construction. The analysis of soil yields soil related various important factors that include the interaction of soil with the structure over a time through various weather conditions, types of structure soil can support etc. The analysis of soil composition reveals that there are three types of soil namely cohesion less, cohesive, and organic soils. In cohesion less soil the soil particles remain apart from each other. The example of such soil is sand, gravel and silt. Cohesive soils consist of very tiny particles that have characteristic to stick together in presence of water due to attractive forces present among them. Clay is an example of such soil2. Organic soils have spongy, compressible and crumbly nature and because of because of its unstable nature are strongly undesirable for any scale of construction. Depending of the soil various layers depth, all three types are further divided into various sub-categories and construction is carried out while considering these sub-layers i.e. shallow foundations is located in the first layer below the structure, followed by the individual footing (sub plate) between boundary layers, and uprooting sub paneling that sometimes used extreme environments (tundra)1. As enlarge footing size will increase the area of contact (area = B2), hence it increase the allowable bearing capacity qa of soil. By evaluating all these variables, the column load can be calculated which will be equal to the load on footing. As soil degradation also affect the footing hence the effect is indirectly transfer to columns and hence to the whole construction. Procedure: In this experiment a specimen of soil with known % of water by weight has been prepared. The mass of sample is determined using an analytical balance while using a metric sale various dimensions are recorded. The dried sample has been then placed inside an unconfined compression test setup as shown in figure 1 below1. Figure 1: Unconfined Compression Test Setup The sample can be fit well in test setup by palling off the top layer of sample. The value of pressure exerted on average surface is monitored through a pressure tracking software booted on a system. Now to increase the value of pressure/area of sample, the test contraption wheel is continuously cranked and recording the corresponding value through system. Various values of pressure for predetermined time intervals that were also recorded. By repeating the same procedure the corresponding values has been recorded. The software compiled the data and calculate the compression that has been experience the by soil samples1. In fig. 2 an individual footing of a column has been shown. In fig .1 (a), both the top and elevation view of column footing has been shown while (b) represents the footing load. Figure 2: Individual Footing Experimental Results: The observed values of various parameters of experiment have been recorded and give in table 1. The results highlight stresses values involved in experiment and also account how the dissipation of the force acting on soil affects the system as a whole. Based on the various values given in table 1 further calculation has been carried out and results are given in table.2. along with units and conversations factors used to calculate the data from the stress levels involved1. Table 1: Measured Quantities The % of water present in soil sample is derived as; (150 grams of soil) * (0.15) = 22.5 grams of water. Secondly the amount of clay present in sample is (150.0 grams – 22.5 grams) = 127.5 grams. The important information received regarding the duration of clay in experiment is recorded in table 2 below2. The relation of stress and corresponding strain has been visualized through a graph as given in graph. 1. The graph has been used to calculate unconfined compressive strength with the 0.15 (15%) strain value in the soil mixture. The sample displacement change corresponds to the reading of compression scale. The values of strain have been evaluated dividing change in displacement by initial height (from table 1) 2. From Gp values load was calculated in pounds by multiplying proving ring constant (from table 1) tp Gp (from table 2)2. Area is recorded in inches squared by dividing initial area by (1-strain) while stress is measured in psi that equated to pounds per square inch on table 2 below. Table 2: Clay Stress Analysis Represented below in graph 1 is the stress and strain relationship that was established by plotting values from stress and strain sections within table 2 above. Graph 1: Stress and Strain Projection Discussion: The unconfined compression test and foundation design test accounts the various aspects of soil samples with a varying water composition. The tests also describe the particular stress levels involved in specific locations of the foundations in a structure. The basic objective of the experiment is to determine the type of soil used in sample i.e. sample is composed of the stiff clay or soft clay based on data projections involving stress and strain applied. The experiment results with 15% water content in soil sample indicated that soil involved in sample is of moderate nature. The clay is neither too soft clay nor too stiff; it gives a moderate behaviors between the two extremes1. However, if 15% more water is added to sample then the sample curve was close to the soft clay as its major principal stress lies closer to the trend of a soft clay curve2. Further investigation shows that as axial strain increased, the both soft and stiff clay reached an unconfined compressive strength maximum on each curve and then dissipated rapidly back to lesser strength. Also shear strength capacity and axial strain applied do not have a direct relation, but parameters managed to follow a parabolic shape. This fact again suggest that each soil sample was heavily dependent on the amount of water involved affecting the clay properties. Major errors can also have affected the trend line and data. If large air packets are present in sample then as density will decreases resulting a decrease in strength capacity. Another major error factor is an inaccurate utilization of equations to derive specific values for quantities as this leads to large variation of data in the charts. Further error can be contribute in experiment due to inaccurate equipment reading of scales on equipment and inconsistent rotation forces being applied to the stress apparatus. Conclusion: From experiment it can infer that a stable solid structure foundation involves various types of acting on it. Further there are various types of soils and each type have specific characteristic with construction point of view. A thorough examination of how stress and strain are intimately related was highlighted as a core concept in civil engineering. References: 1 Information is collected and referenced from Professor Robert Samuel’s laboratory handout. 2 Armstrong, Sam. “Stress and Strain of North American East Cost Construction Soil.” Proc Inst. ASCE, Vol 226, no 20, pp. 36, Dec. 2009. Appendix: A) q = footing contact pressure in psf = lbs per ft2 B) Q = total vertical load from column in lbs C) B2 = area of square footing in ft2 D) qa = allowable bearing capacity of underlying soil in psf = lbs/ft2 E) ε1 = strain (dimensionless) F) Δh = axial deformation = change in length of soil samples in inches G) h0 = initial length of soil samples in inches H) A0 = cross sectional area of sample = d02/4 I) d0 = initial diameter of the soil sample in inches J) σ1 = stress in psi = lbs/in2 K) P = applied force in lbs Read More