Laboratory Testing and Equipment - Research Paper Example

Table of Contents Introduction 3 Bearing capa determination in granular soils. 3 Standard penetration test.4 Evolution of the Standard Penetration test 4 Equipment used in standard penetration test 4 Procedure for Standard penetration test. 5 Determination of bearing capacity in Cohesive soils (Clays and silts). 6 Triaxial tests 6 Evolution of the triaxial test 6 Sample preparation 9 Consolidated undrained test 9 Unconsolidated
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Undrained 10 Granular soils (sands and gravels) settlement 10 Evolution of Plate loading Test Equipment 11 Plate Loading Test Procedure 11 Test set-up 11 Test Procedure 11 Gravity loading method 12 Loading truss method 12 Application of Plate loading test in Engineering 13 Clay swelling and shrinkage assessment 13 Atterberg limit 14 Determination of plastic limit of the soil. 14 Conclusion 16 References 17 Laboratory testing and equipment Introduction A foundation of a structure is defined as the part of the building that is designed specifically to facilitate the transmission of the structure’s load directly to the underlying soils. The design of a foundation, just like any other geotechnical structure, goes through different phases. These include investigation of the site (site investigation), determination of the soil parameters, and finally the design. Following of the above mentioned phase is paramount since it determines the lifecycle of a foundation. Site investigation involves the process of gathering and assessing the required information about the location of the structure to be built. On the other hand, determination of the soil parameters in foundation design involves the determination of soil parameters that are useful in finding the soil’s bearing capacity, settlement of the structure, and assessing of shrinkage and swelling, in case of clay soils, and determination of mitigation solutions to the shrinkage and swellings. And finally, the design phase involves designing the proposed structures basing on the information gained in the site investigation phase and determination of soil parameter phases. The tests carried out in the determination of soil parameter phase are referred to as geotechnical tests. The geotechnical tests are useful since it helps in the determination if the proposed structure can be supported by the soil, the impacts of fillings and excavations, stability of the slopes among others This paper is aimed at evaluating the different type of geotechnical tests that are carried out before and after the design and construction of a foundation. The paper focuses on reviewing existing geotechnical tests, their evolution and the application of the tests to foundations Bearing capacity determination in granular soils. Granular soils refer to gravel and sands. To be able to determine the bearing capacity of the soil where the foundation is to be designed, it is paramount to determine the soil’s shear strength. There are various geotechnical tests that are used in the determination of the shear strength of a given soil. The commonly used in – situ method in the determination of the shear strength of the soil is standard penetration test (Craig). Standard penetration test. Standard penetration test is an in-situ geotechnical tests used in the determination of soil shear strength. Also, the test is used in checking the stony cohesive soils and weak rocks for consistency. The standard penetration test can be used on all types of soils. Most commonly, the test is used in granular materials but also, the tests can be used in other materials requiring simple in – place bearing strength tests. The test is also applicable in situations where recovering of materials is difficult by other means. Evolution of the Standard Penetration test The use of Standard penetration test is dated back to 1902 when the owner of Gow Construction Company, Colonel Charles, made exploratory borings by use of drive samples that were 1 – inch (Rogers). In the late 1920’s and early 1930’s, Harry Mohr adopted the procedure and the standardized it. In the standardization of the procedure by Harry Mohr and his company, a slightly larger split – spoon was developed. Also, the number of blows per penetration foot was recorded while undertaking the procedure. To make the blows, a hammer measuring 140 lb. was used and it dropped freely from a height of 30 inches (Rogers). The first value recorded was discarded as an outlier due to contaminations and fall – in in the sample hole. The second values of the resulting from the procedure were combined and used to obtain the blow SPT blow count value (Rogers). The test become popular, but the sample used in each case was different. In the 1950’s, The American Society of Testing and Materials Standards (ASTM) adopted the procedure and standardized it. In the standardization of the procedure, free falling hammer was required, though there was no standardization of the shape and drop method. The ASTM standardization of the procedure has been documented in ASTM D1586. The procedure was also adopted in Britain and standardized according to BS 1377:1990 Part 9, Section 3.3. Since the test is very popular in almost all parts of the world, worldwide standardization of the procedure has been adopted and documented in the International Reference Test Procedure (IRTP). Below is a description of the standardized procedure when carrying out the standard penetration test. Equipment used in standard penetration test The equipment required in SPT tests are as listed below i. Sampling rods – a sample rod is made commonly made of steel and is usually utilized when connecting the drive weigh assembly and the sampler. ii. Drilling Equipment – A drilling equipment that is capable of producing a clean hole is required in the SPT test. The Equipment used must be able to produce a hole that is 5 mm bigger than the sampling rods and the sampler and a diameter that is approximately 175 mm less. iii. Split – barrel sampler – This equipment is made up of three main parts. This are split – barrel, the shoe and the head. While using the equipment, it must be ensured that a core catcher has been installed to aid in the prevention of sample loss. iv. Drive – weight assembly – This is made up of a hammer with a mass of 63.5 kg, a driving head and guide that allows a free flow of up to 0.76 m. Also, the guide should allow 100 mm capability (at least). Procedure for Standard penetration test. A hole to the required sampling diameter and depth is drilled and all then disturbed material removed from around the hole. The equipment is then assembled by attaching the rod and the split sampler. The split – sampler is then lowered into the drilled hole till it rests on the bottom of the hole that consists of undisturbed soil sample. The drive weight assembly is then attached and the 63.5 kg hammer lifted to approximately 0.75 m and allowed to fall on the anvil. This delivers a single a single blow. Marking of the drill rod in successive increments at 0.15 m is done. The drive weight is then marked in indication of 0.75 m hammer lift. In carrying out the penetration test, the hammer is raised for 0.75 d dropped using means of a rope and a cathead. In this process, the hammer is used such that it drops freely. The raising and dropping is repeated until a penetration of 0.45 m has been achieved or the number of blows is equal to 100. The results are recorded and a graph plotted. Determination of bearing capacity in Cohesive soils (Clays and silts). The calculation of the long term drained bearing capacity is paramount. This involves finding out the drained shear strength. The drained shear strength of a soil sample is determined on the laboratory using triaxial tests. Triaxial tests There are three types of triaxial tests. These are; i. Consolidated – drained triaxial test ii. Unconsolidated – drained triaxial test iii. Consolidated – drained triaxial test. Each of the above stated test are carried out in determination of drained shear strength at different conditions, which is later used in the determination of the bearing capacity of cohesive soils. Evolution of the triaxial test The triaxial test has seen a huge evolution since its inception. The first triaxial device was introduced to the world in the year 1924 by Buisman. Casagrande at Harvard University and Rendulic in Vienna adopted the device and performed various improvements. The developed device by Casagrande and Rendulic is what resembles the current modern equipment. Currently standards have been published to guide the triaxial tests. The British Standards for the trial tests are published BS 1377-7:1990, and The ASTM standards have been published in ASTM D2850 and ASTM D4767. The diagram below shows a typical modern day triaxial apparatus. Typical modern day triaxial apparatus (Geotechdata.info) The triaxial equipment is made up of the following major equipment. The triaxial cell, the universal testing machine and the pressure control panel. The triaxial cell is transparent to allow viewing from outside. The triaxial cell is composed of three ports which are controlled by independent valves. One of the port is used in filling the cell with water to facilitate application of confining pressure. The remaining two ports are used to drain water into and out of the triaxial cell. These two ports connect the sample at the bottom and at the top. The ports are used during the performance of the tests and also during the in sample preparation. The figure below shows the triaxial cell. The triaxial Cell The universal testing machine made up of loading platen and the loading frame. The triaxial cell is placed on the platen that has been designed such that it is able to move up and down. The pressure control panel is another major component on the triaxial equipment. The Pressure control panel is interlinks the pressure system. The Pressure Control panel has three vertical sections of which there is connection of each of the sections to the ports. The figure below shows the control panel Pressure control panel of triaxial equipment. The procedure below describes the triaxial tests using the ASTM standards. Sample preparation A sample of the soil to be tested is taken and prepared before placing it into the triaxial cell. In cohesive soils, the soil sample is trimmed while for granular soils, the preparation of the sample is done directly on the by the use of split – part moulds. During the preparation of the sample, the preparation should be done such that there is minimum soil disturbance on the specimen. Consolidated undrained test This test is also referred to as quick triaxial test. The procedure for consolidated undrained test is as outlined below. i. Consolidation of the sample under the required stress. ii. The drainage involves on the triaxial equipment are closed. This implies that there is no water allowed to flow out of the specimen. Unconsolidated
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Undrained The prepared sample is placed in the triaxial cell while the drainage valves are closed. Confining pressure is applied, there is no occurrence of consolidation. In this test, pore water pressure is not measured. The soil sample is loaded to failure. The resultant stress is the total stress of the sample. The resultant results are used to draw stress string curves as illustrated in the figure below. Granular soils (sands and gravels) settlement To determine the settlement of granular soils, the stiffness of the soil needs to be determined. The measurements of this settlement must be done in-situ. The best method for the determination of the stiffness factor of any given soil is the plate loading test. Evolution of Plate loading Test Equipment Many equipment have been constructed to aid in determination of soil bearing capacity in the field of geotechnical engineering. These equipment range from the old simple mechanical and analogue to digital electronic equipment like the Electronic Plate Load Device AX 01 that are in accordance with ASTM BEING used today. The operation principle of the equipment remains much the same despite the modifications. Plate Loading Test Procedure Plate loading test is a field test carried out to determine the ultimate bearing capacity of a foundation soil and prediction of the likely settlement when the soil is subjected under a given amount of load. The test basically involves loading a steel plate that is placed at the level of the foundation and recording down the settlements as they correspond to each increment in load. The test load on the plate is gradually increased to a point where the plate begins to sink into the foundation at a rapid rate. The soil ultimate bearing capacity is given by the total plate load divided by the actual area of the steel plate. To arrive at a safe bearing capacity, the ultimate bearing capacity of the foundation is divided a recommended factor of safety, which varies from 2 -3. Test set-up A test pit is prepared by digging at the site up to a depth proposed for the laying of the foundation. In standard test applications, the width of the test pit should be five times, or slightly more, than the width of the plate. A small depression, square in shape and of the same size as the plate and a bottom level corresponding to that of the foundation is made. The depth of the depression should be in such a way that the ratio of width to depth of the loaded coverage is approximately equal to the ratio of the actual width to depth of the proposed foundation. The bearing steel plate to be used should be at least 25 mm thick with size varying from 300-750mm. The shape of the plate can either be circular or square. Square plates are used in all types of foundation footings and a circular plate is used in circular footing. The edges and sides of the plate are machined. Test Procedure A test load is applied to the plate through a column that is centrally placed. This load is transmitted using two methods: i. Gravity loading ii. Loading truss method iii. Gravity loading method Gravity loading method In this method, a loading platform is made over a column placed on the plate and the test load is applied by use of sand bags, concrete blocks, pig iron, lead bars etc. placed on the platform. A hydraulic jack may be placed between the column top and the loading platform for applying the test load to the plate. This technique of loading is also termed as reaction loading- as the reaction of the jack is borne by the load on the platform. Figure 1: Plate load test method- Reaction by gravity loading method Loading truss method In this method, a steel truss of reasonable size is made to bear the reaction resulting from the jack, as opposed to constructing a platform. The truss is anchored firmly to the ground using steel anchors and guy ropes to provide lateral stability. A load is then applied to the test plate, sinking slowly, and the plate settlement recorded with accuracy of 0.02 mm using two dial gauges placed diametrically at the opposite ends of the test plate to take into account the differential settlement. During sinking of the plate, the dial gauge moves down as the settlement is recorded. The loading gauge of the hydraulic jack indicates the load magnitude. The load is regularly applied in increments of 2KN or one fifth of the anticipated ultimate bearing capacity based on whichever is less. Settlement is observed for every increment intervals of 1, 4, 10, 20, and 40 and 60 minutes, then at hourly intervals, until the settlement rate becomes 002 mm per hour. The maximum load recommended for the test is about fifteen times the anticipated ultimate bearing capacity. Figure 2: Plate load test method- Reaction by truss loading method. Application of Plate loading test in Engineering Plate loading tests is applied in site geotechnical investigations to enable the determination of the soil bearing capacity and settlement of in-situ foundations. This ensures that the soil on which a structure is laid meets the required bearing capacity to prevent structural failure by settling. Clay swelling and shrinkage assessment Addition if water to clays results in the clay to swell while removal of water from the clay soil results in the shrinkage of the soil. The shrinkage and swelling of the soil results in the change in volume. The change in volume has a negative impact on the foundation in that they can result in movement of the foundation and consequently, damage the structures being supported by the foundation. The occurrence of swellings and shrinkage is when the soil is plastic. Thus, to be able to assess the shrinkage and swelling of a clay soil, it is important to determine the liquid and the plastic limit of the clay soils. Plastic limit and liquid limit are collectively known as Atterberg limit. Atterberg limit Atterberg limits were developed by Albert Atterberg in 1911. The development of the experimental procedure followed an extensive study that he had carried out on soil plasticity (Specifically clay soils). Since then, the tests have undergone numerous amendments and recently, there are standardized methods used in the determination of the Atterberg limits. Determination of plastic limit of the soil. In the determination of the plastic limit of a soil, a thread of soil is rolled on a flat surface or between hands. The thread of the soil is considered to be at its plastic limit when it crumbles at a diameter of 3 mm. Set of equipment used in the process of determining Atterberg limits. Figure showing equipment used in the determination of Atterberg limits (University of Minnesota). Figure showing palms rolling the soil sample (University of Minnesota). Figure showing the rolled soil thread (University of Minnesota). Step by Step procedure i. A handful of soil is selected from the soil sample ii. The soil sample is then rolled between the palms iii. The rolling of the soil sample is repeated until the sample has a diameter of approximately 3 mm. iv. If the soil cannot be rolled to a diameter of 3 mm, then the soil has no plastic limit v. Plastic limit exist when the soil can be rolled to a diameter of 3 mm before breaking, the soil has a plastic limit. This is followed by taking a sample of the rolled and taking its moisture. Conclusion Foundations are very vital in constructions of any structure. This is due to the fact that they are responsible for transmitting the structure’s load to the ground. Thus, it can be concluded that the strength and stability of a structure is dependent on the quality of the foundation laid. If the foundation is weak, then the building will be weak. The quality of the foundation lies in the three phases of foundation development. These are the site investigation, determination of the soil parameters and the design process. Geotechnical testing plays an important role in determining various parameters of the soil where the foundation is to be laid. The geotechnical tests help in determining such things as soil bearing capacity, settlement of the soil, and finally assessing shrinkage and swelling. References Craig, R. F. Craigs Soil Mechanics. New York: CRC Press, 2004. Print. Geotechdata.info. Triaxial Test. 10 10 2010. Online. 30 April 2014. Rogers, David. Advanced Engineering Geology & Geotechnics. Columbia: Missouri University, 2004. University of Minnesota. Procedure for Determining Plastic Limit. 22 October 2010. Online. 30 April 2014. Read More