Soil Testing Methods for Civil Engineering Projects - Coursework Example

Soil Testing Methods for Civil Engineering Projects Topographic survey and soil investigation are two very important requirements for the design and construction of the civil engineering projects. Where the topographical survey deals with the surface conditions of the site, on the other hand the soil exploration accounts for the underground conditions of the soil and to check whether the soil on which the structure is going to be constructed can it sustain that or not. In certain time the soil can be of crucial importance and can be a factor in the failure of structure. In order to understand the behavior of soil upon different loadings we have to first investigate that which type of soil is present on site and then test it under different conditions. These steps are of prime importance to the design of foundations. Objectives of soil exploration and testing are to assess sequence and thickness of strata. To determine the position of the Ground water Table and characteristics and their types of subsoil. Then bring these samples to the lab and test them for minor details. Which include the determination of their shear strength, compressibility and chemical characteristics. At the end if there are some problems with the soil and its does not fulfill the requirements for the sustenance of the structure, consequently the geotechnical engineer proposes solutions to the problem depending on the results of the soil testing he has done. There are majorly two types of soil testing in civil engineering; first one is In-situ testing and the other Lab testing. As the name clearly depicts the location of the tests, Insitu is done on site and then the samples acquired from there come to the lab for further analysis. Although there are many soil tests that are performed at both the locations but the most prominent and used among them are the following. Insitu soil tests: 1. Standard Penetration Test (SPT) 2. Static Cone Penetration Test (CPT) 3. Insitu Density Test 4. California Bearing Ratio (CBR) 5. Pressuremeter Test 6. Etc Laboratory Soil Tests: 1. Grain size Analysis ( Sieve analysis, Hydrometer Analysis) 2. Atterberg Limits 3. Permeability Tests ( Constant Head/variable head Permeameter) 4. Triaxial Compression Test 5. Direct Shear Test/ Lab Vane Shear Test 6. Etc The above mentioned tests are the most widely used in the soil testing and geotechnical investigations for the civil engineering projects. In my report I will not be able to discuss all of them and have to select one because each test has a vast area of application, inaddition to this the diverse arrangements for the performance of these tests make it further more difficult. For that purpose I have selected one of the above tests which is the Insitu Test, Standard Penetration Test (SPT). There are majorly three reasons for this selection, the first is that SPT test is among the most widely used tests for soil exploration and sampling of the soil for further tests which are to be performed in the lab, so consequently in every geotechnical study you have to acquire a soil sample and then bring it to the lab, hence you have to perform this test in any case. Second reason of choosing this is that in Standard Penetration Test (SPT) we will cover all the aspects of testing procedures which include design, construction, safety, limitation and effectiveness. Whereas the third reason for selecting this test is that from the results of this test we can have correlations with other results as well hence making the result spectrum of the test wider, consequently we can have more information of soil and its mechanics from lesser testing. Design & Construction of SPT test For the design and procedures of the Standard Penetration Test (SPT) I have conformed to the major agencies that deal with the testing of soils and standards they have set for the industries. All the description done in this report will be in direct conformation of the majorly British Standards and partly ASTM standards. For this purpose the BS 1377 and ASTM D 1586 are been used. The working principle of the test is 63.5 Kg load is applied with impact loading with a free fall of height 760 mm using a range with pulley and rope assembly or by the automatic mechanical means as shown in the Appendix A. Then the no of blows for each 300 mm penetration are counted and reported and this is regarded as penetration resistance (N). The apparatus used during the testing process include Boring equipments, Split barrel sampler assembly, Drive rods and assembly. The boring equipment is used to develop a clean hole before the sample is acquired and the process of jetting through an open tube sampler and then testing when the desired depth is reached shall not be permitted. Then it is the split barrel sampler that is an integral part of the test. The detailed dimensions are shown in the Appendix B. The material used is steel for the drive shoe and the barrel. In addition these two parts must have same diameter and a ball and its seat is used to maintain the watertight conditions in the sampler. Then to extend the sampler into deeper levels of soils drive rods are employed. These should be of hardened steel and shall not be heavier than 10 kg/m and the straightness of the rod should be ensured. Finally and most important is the drive assembly, this include a steel hammer of 63.5 kg and a pickup and release mechanism for a height of 760mm. It should be ensured that the hammer falls free on the drive head (anvil) and no resistance is applied during the fall which could increase or decrease the speed. Performance of the test First step is to clear the ground and borehole for the commencement of drilling and then test. Lower the sampler assembly to the bottom of the borehole on the drive rods with the drive assembly on top. Record the initial penetration under this total dead-weight. Where this penetration exceeds 450 mm omit the seating drive and test drive and record the “N” value as zero. The seating drive will be the first among the 150 mm penetration or 25 blows. At the end is test drive in which penetration resistance (N) is determined and this is the number of blows required for a further penetration of 300 mm. Another condition is that if the sampler does not achieve a penetration of 300mm in 50 blows then terminate the test. During the last actions of the procedures we have to acquire the soil sample from the split barrel sampler and place a representative portion of the soil core from the bottom 300 mm drive in an airtight container to prevent loss of the soil moisture. It is also advised that the data recorded on site should include as much specifications as possible but few things are of prime importance which includes depth of penetration, length of recovery, standard penetration resistance (N) and borehole diameter. Factor affecting N-Values There are many factors, direct and indirect that affects the N-values of the Standard Penetration Test but the significant among these are few. Poor cleaning of the borehole, difference in drilling methods, insufficient height of fall of the hammer and not using correct weights affects the N values directly. It is to be ensured that the weight hits the drive cap at right angle and there is no eccentricity, hence the impact load is sufficient. The performance attitude of the labour and technical staff is a major contributing factor in the accuracy of results acquired. Then the last essential factor that affect is failure to maintain sufficient hydrostatic head in boring, and it should be guaranteed that the water table in the borehole and sand have the same piezometric pressure and hence there is no decrease in N-value. Correlation between Standard Penetration Test and Soil Properties There are four major soil properties that can be determined if we have the knowledge of the N-Value from the SPT test. These are 1. State of packing 2. Relative Density 3. Effective Stress Friction Angle 4. Undrained Shear Strength The graphs and the tables are drafted so that the results and the correlations are made in a swift manner and a better understanding is developed. General Correlations Limitations of SPT test Numerous studies have shown considerable variation in the procedures and equipment used in test. Blow counts (N) are influenced by operational procedures, by the presence of gravel, or cementation. N- values do not reflect fractures or slickensides in clays, which may be very important strength characteristics. The test gives highly accurate estimates of the sand but the chances of faulty reading increase in case of soft clays or where water content of soil is high. Environment, Health and Safety According to HSE code of practice Regualtion 22(2) the contactor is bound to give his labour the induction, information and training about the issues on health and safety. This regulation is also assisted by Regulation 13(4)(a) and Regulation 19. The type of hammer operation also poses the type of safety problems during the testing. This should be noted that if you have manual hammer system then there will be more safety problems and if you have automatic system then there is less danger to the workers. . Then comes an important aspect which is to be considered before the process of construction starts which is the to develop a controlled access into the site so that there is no trespassing and no one who is unaware of the activities may get harmed by entering the area. To cater to this very problem HSE code of practice dictates the contractor to obey the Regulation # 4, 5, 6 and this target can be achieved by placing temporary fencing and barriers with sign boards and advisory plates hence demarcating the whole area where construction is taking place. An effective emergency action plan should be developed which is in accordance to HSE code of practice and especially regulation # 39. In addition it is advised that there will be an HSE inspector on site at all times, if it is not possible to have one, for instance in small projects such as our then it is suggested that there should be at least one person on site who is educated in paramedics and experience of on site injuries. Conclusion There are many advantages of SPT test which include its rapid and swift result production and it provides you with a representative soil sample. It reading can be translated into relative strength and compressibility of the soil. Above all the test has been with us for now a very long time and hence all the geotechnical investigations of the past are interpreted in the same N-values of SPT test. The SPT is an in situ test that gives as an idea of soil density, soil fabric, stress and strain history effects, and horizontal effective stress, which are very important to know in order to gauge influence on the liquefaction resistance of the soil of the specific soil. Appendix A Appendix B Appendix C References 1. Health and Safety Executive (2007). Construction (Design and Management) Regulations 2007 Approved Code of Practice. UK: Crown copyright. 1-107. 2. STANDARD, BRITISH. "Methods of test for Soils for civil engineering." BS 1377-9:1990. BRITISH STANDARD, 1990. 3. ASTM Designation: D 1586 –. Standard Test Method forPenetration Test and Split-Barrel Sampling of Soils. American Society for Testing Materials , 2005. 4. Health and Safety Executive. (). Construction design and management (CDM). Available: http://www.hse.gov.uk/construction/cdm.htm. Last accessed 03-12-2010. Read More