Rocks are usually classified based on their mode of formation, texture and chemical and mineral composition. Considering these parameters, the three main classes or types of rock formation are: metamorphic, sedimentary and igneous (Science Learning Hub, 2011).
These rocks form as a result of collection, deposition and accumulation of remains of living and non-living things, including soil particles, plant remains, animal remains, etc. (generally referred to as sediments) (Del Pozo, et al., 2015), on land or sea/ocean floor. These sediments are usually eroded by various processes including erosion and weathering (Andrei, 2017). As more sediments get deposited, their self-weight compresses them together thus forming a solid rock known as sedimentary rock. The weight increases the intensity of pressure at the bottom of the sediments where he rock gets formed through a process known as lithification. These rocks form in layers and it can take several years for them to form a layered appearance. Sedimentary rocks include: sandstone, gypsum, limestone, conglomerate and shale. The main classification of sedimentary rocks are: clastic, organic and non-clastic. Clastic sedimentary rocks are formed from different sizes of pre-existing rock particles. They include shale, conglomerate, siltstone, sandstone and breccia. Organic sedimentary rocks are formed from organisms such as fossils. They include limestone, dolimites and coal (Geology.com, 2018). Non-clastic sedimentary rocks are also known as chemical sedimentary rocks. These rocks get formed when a saturated mineral solution evaporates. They include rock salt and gypsum.
Sedimentary rocks are relatively soft and can crumble or break easily. It is estimated that sedimentary rocks cover 75% of the earth’s surface whereas the remaining 25% is covered by metamorphic and igneous rocks (Sirisha, (n.d.)).
Formation of igneous rocks is associated with volcanic processes or the occurrence of plate tectonics. These rocks form as a result of cooling and solidification/crystallization of melted rocks (Liu, et al., 2013). The melted rock can be found below the surface of earth (where it is called magma or silicate melts) or be released on the earth’s surface when a volcanic eruption occurs (where it is called lava) (du Bray, 2017). When these rocks form below the surface of the earth, they are called plutonic or intrusive igneous rocks, and when they form on the earth’s surface they are known as volcanic or extrusive igneous rocks (Wu, et al., 2010). Igneous rocks include: granite, basalt, pumice, scoria, peridotite, gabbro, dacite and obsidian (Villanueva, 2009).
Igneous rocks are generally compact, hard and crystalline in nature. However, some of them are cemented or welded and others remain weak when they do not undergo complete cooling process like others.
These are rocks formed when sedimentary or igneous rocks are subjected to different temperature and pressure conditions thus changing their forms (Liu, et al., 2016); (Ratliff, 2017). These conditions are usually different from the original condition under which the sedimentary or igneous rocks were formed and that is why they are able to be transformed into new forms (Singh, et al., 2017). As these rocks are subjected to extreme temperatures or greater pressures, their chemical composition, mineral arrangement and structure changes (Pantuhan, 2017); (Yang, et al., 2013). Metamorphic rocks include: slate, gneiss, phyllite, migmatite, quartzite, schist and marble (Ozbek, et al., 2018). Metamorphic rocks usually have shiny crystals and ribbon like layers (Udagedara, et al., 2017).
Rocks are formed from different minerals. By definition, minerals are naturally occurring non-living materials that have specific chemical composition and which are usually crystalline, solid and stable at room temperature. There are more than 5,000 mineral species. However, most of the rocks are formed from a mixture of a few minerals known as rock-forming minerals. These rock-forming minerals include the following:
Quartz – this mineral is commonly known as silica. It is among the commonest minerals present in the crust of earth. It mainly comprises of silicon dioxide. The shape of quartz is usually prismatic and hexagonal. When quartz is pure, it is colorless but impurities can give it several colors including orange, pink and violet. Quartz is chemically and physically resistant to weathering. This means that the mineral remains stable and unaltered even when exposed to different weathering conditions. The mineral does not have a cleavage and it is very hard, making it very resistant to mechanical weathering (Yao, et al., 2017). It is also resistant to chemical weathering because it is made up of interlocking silica tetrahedra. Quartz’s resistance to weathering makes it one of the most stable rock forming minerals.
Mica – this is a group of silicate minerals containing iron, magnesium, potassium, water, silicon and aluminium. These minerals form book-like, flat crystals. They are usually found in intrusive igneous rocks, but can also be found in metamorphic and sedimentary rocks. There are 28 mica species but only 6 are known to be rock-forming minerals. These are: muscovite, paragonite, phlogopite, biotite, lepidolite and glauconite. These species are susceptible to different levels of weathering. During chemical weathering, the minerals’ color changes. In general, mica is relatively resistant to weathering.
Olivine – this is a glassy, green, silicate mineral containing magnesium and iron. It is very common in ultramafic and mafic rocks. Olivine provides very minimal resistance to weathering hence it is highly susceptible to weathering. When exposed to a weathering environment, olivine loses its appeal very quickly thus becoming dull, yellowish brown and earthy (ten Berge, et al., 2012). The mineral is also susceptible to hydrothermal metamorphism, which converts it from igneous to metamorphic rock (Sand Atlas, 2013). Leaching changes olivine principally resulting to elimination of magnesium and addition of iron and water. Most of chemical reactions of this mineral are complex and they include carbonation, oxidation and hydration.
Pyroxene – this is a group of silicate mineral containing aluminium, calcium, magnesium, silicon, iron and oxygen. Pyroxenes re usually in form of columnar or short prismatic crystals. This mineral is usually found in metamorphic and igneous rocks. Pyroxene is relatively susceptible to weathering because it solidifies easily when exposed to high temperatures.
Amphibole – this is a group of silicate minerals containing aluminium, calcium, magnesium, iron, water, oxygen and silicon. Amphiboles are usually in form of needle-like or prismatic crystals and are very common in metamorphic and igneous rocks. This mineral is not very susceptible to weathering nor is it very resistant to weathering. When exposed to certain conditions, amphibole can weather.
Feldspar – this is a group of silicate mineral that is the commonest mineral on earth. On Mohs scale, the hardness of feldspar is 6 (Alden, 2017). This means that the mineral is relatively resistance to weathering.
Calcite – this is a clear or white carbonate mineral containing calcium carbonate. It is very common in sedimentary rocks. Calcite is susceptible to weathering, especially chemical weathering. When exposed to chemical weathering conditions, calcite will break down easily and its sediments carried away by groundwater solution and surface water.
Garnet – this is a group of silicate mineral that is found in different colors – red, blue, colorless, orange yellow, black, purple, green, pink, brown, etc. On Mohs scale, the hardness of calcite ranges between 6.5 and 7.5 making it resistance to weathering. Some species of garnet include almandine, spessartine, grossular, andradite, pyrope and uvarovite.
Rocks and un-cemented sediments have been and are still widely used for construction purposes all over the world. These materials are used as construction materials because of the suitability of their mechanical, physical and chemical properties for the intended use (Saul & Lumley, 2013). Rocks can be used for construction of both temporary and permanent structures. Un-cemented sediments (also referred to as unlithified sediments) are materials that include loose aggregates and sand, collected from unconsolidated sediments of seas, lakes and rivers.
The commonest use of rocks and un-cemented sediments is construction of temporary structures. These structures are mainly used for providing support, access or protection to the permanent structure being constructed. They can either be incorporated into the completed work or dismantled and removed once the permanent structure is completed or self-supporting (Ratay, 2014). Strength requirements of these structures are less than those of permanent structures. Rocks and un-cemented sediments can also be used to construct temporary structures for the purposes of decoration, exhibitions and modelling so as to assess the aesthetics, efficiency, function and appearance of the structure. One of the fundamental characteristics of un-cemented sediments is that they do not contain cement, which is a major binding material in construction.
The figure below is an illustration of a structure built from rock and un-cemented sediments. The structure, which is a wall, is made of large and smaller rocks. The rocks have been arranged by hand and without a binding material (cement). The structure appears to be less strong than a similar one built from rock and cemented sediments. Looking at the wall, it has larger rocks at the base so as to lower the center of gravity and make it stronger. Once the rocks have been placed appropriately, the small gaps remaining between the rocks can be filled with un-cemented sediments. This kind of a wall is known as dry rock wall.
Below are other images that illustrate how rock and un-cemented sediments can be used for construction. The structures seem to have been constructed for different purposes including decoration, exhibitions, modelling and assessing the function.
Generally, rock and un-cemented sediments have unprecedented usage in construction. Even though most of structures are built using rock and cemented sediments, the others also have numerous uses. It is up to engineers and geologists to ensure that the rock and un-cemented sediments used are suitable for the intended purpose.
In geotechnical engineering or investigation, there are two main categories of soil: in-situ and sampled soils.
In-situ soil is basically undisturbed soil. This is soil that is used or tested while in its original, natural or existing condition. The soil is tested or used without having to disturb its natural conditions such as moisture content, texture, stress condition, structure and density. This soil is usually considered to be the actual representative of the underground conditions due to minimal disturbance (if any) (Feng, et al., 2013). Therefore the original properties of in-situ soil are considered to be retained. Some of the common properties tested from in-situ soils are: strength, fracture patterns, compressibility and permeability. The illustrations below are some of the in-situ soil tests and samples.
Testing of in-situ soil involves placing the testing machine into the soil at its original or natural position so as to measure or determine appropriate properties (Wenjun, et al., 2014). Collecting in-situ soil for testing should be carried out carefully so as to minimize disturbance as the soil has to retain its structural integrity. Some of the sampling tools used for collected in-situ samples include spoon samplers, pitcher barrel sampler, piston sampler and drill rig. Common in-situ soil tests include: vane shear test, standard penetration test, borehole shear test, dilatometer test, cone penetration test, California bearing test, and pressuremeter test. (Briaud, 2013); (Federal Highway Administration, 2017).
Sampled soil refers to disturbed soil. This is soil that has been extracted from its natural positioned and transported to another place for use or testing (Li, et al., 2012). The natural state of the soil is usually interfered significantly during extraction and transportation (Zhao, et al., 2010). As a result of this, sampled soils are not considered to be actual representatives of underground soil conditions (Moberg, et al., 2013). Some of the common properties tested from in-situ soils are: moisture content, texture, type of soil, contaminant assessment, etc. (Barnhart, (n.d.)). Once the sampled soil is extracted from its natural place, it is usually put in tubes, containers, boxes or other packing materials and taken to the laboratory for testing. The illustrations below are some of the sampled soil tests and materials
Soil classification entails grouping of soils with the same physical, biological and chemical properties (Toksoz, et al., 2016). The soils that are in the same group or class tend to behave in a similar way because their mechanical properties are similar. Soil classification is very important in engineering as it helps to determine the suitability of different soils for particular construction projects. These are different systems that are used to classify soils. Some of these are:
This system classifies soils based on their geological origin (Gougazeh & ali-Shabatat, 2013); (Marchalko, et al., 2013). A soil’s origin can be determined by its components or activities that are responsible for its current status. The two main groups of soil based on its components are: organic soil and inorganic soil. Organic soils contain carbon and are made up of once living organisms while inorganic soils do not contain carbon and do not comprise of any living or once living organisms. Based on activities that are responsible for their current status, soils can be classified as: transported soils, residual soils, glacial soils, sedimentary or alluvial soils, lacustrine soils, Aeolian soils and marine soils.
Soils can also be classified based on their structure, which is determined by the average grain size, conditions responsible for the formation and deposition of the soil and the arrangement of individual soil particles. The structure of soil affects how water circulates in the soil (i.e. permeability). Based on structure, soils can be classified as: single-grained, flocculent and honey-comb structures. The structure of soil can also be described based on the individual aggregates’ average size as: very fine, fine, medium, coarse and very coarse. Additionally, soil structure can be described based on individual aggregates’ shape as: granular structure, blocky structure, prismatic structure or platy structure (Food and Agriculture Organization, (n.d.)).
This system classifies soils into 3 main classes: organic soils, coarse-grained soils, fine-grained soils and organic soils (Robertson, 2016). The classification is based on the size of grain and texture of the soil (Gadouri, et al., 2016). Fine-grained soils are the ones where over 50% of their particles pass through #200 ASTM sieve while coarse-grained soils are the ones where less than 50% of their particles pass through #200 ASTM sieve. Fine-grained soils are further classified as inorganic clays, very fine sands and inorganic silts, and organic matter and organic clays and silts. Coarse grained soils are broadly classified as gravels and sands.
This system classifies soils based on the size of their grains or particles. This is most common soil classification system. The system classifies soil as clay, silt, sand, gravel, etc. depending on their particle or grain sizes. There are numerous grain-size classification systems that have been developed. Some of these include: International Classification, Indian Standard Classification, S. Bureau of Soils and Public Roads Administration Systems, and Massachusetts Institute of Technology System.
Different types of soils have unique behaviors when put into different uses. Soil-type classification is the most common and effective classification system used by most geologists. This system classifies soils depending on their composition, particle size and susceptibility to various environmental conditions. Some of the different types of soils include: clay, bentonite, boulders, fills, gravel, cobbles, dune sands, loam, muck, humus, silt, tundra, top soils, varved clays, sand, peat, etc. (Venkatramaiah, 2015).
Soils have different properties, which influence their behaviors. For instance, clay will behave differently from sand when subjected to the same amount of load or environmental conditions. Therefore it is always recommended for engineers and geologists to determine the basic soil properties before determining the suitability of a site for a particular construction project. These properties significantly influences the stability of engineering structures (Roy & Bhalla, 2017). Some of the basic soil properties include: permeability, particle size, specific gravity, shear strength, consolidation, compaction, density index and consistency limits.
This is the ability of soil to allow water or air movement and distribution. Soil mass can be divided into 2 main zones: below the water table or above the water table (Raj, 2012). Permeability affects foundations’ stability, subgrades’ drainage, open cut excavations, and water flow rates into wells. For this reason, it is important to determine permeability of the soil so as to know how it allows movement and distribution of water. Permeability can be determined from permeability tests. The two main permeability tests are: falling head permeability test and constant head permeability test.
Constant head permeability test is carried out by passing water through a cylindrical column of soil specimen under constant pressure difference. It can be performed in a permeameter or permeability cell whose size can vary depending on the soil grain size. The testing device has an outlet reservoir and a variable constant head reservoir that are used for retaining a constant head throughout the test. It also has a loading piston used for applying constant axial stress to the soil specimen. The soil specimen has to be saturated first before the actual test is started. In this test, the quantity of water moving through the column of water is recorded at preset time intervals. Permeability of the soil sample is then calculated using the following formula:
Where Q = amount of water passing through the column of soil, L = length of soil specimen column, A = cross section of soil sample, ?h = constant pressure difference and ?t = time interval.
Falling head permeability test is carried out by passing water through a short soil sample that is connected to a water pipe. This water pipe provides water head and allows the amount of water flowing through the specimen to be measured. The test can be conducted in an oedometer cell or a falling head permeability cell. The soil specimen has to be saturated first and the water pipe filled with de-aired water up to a particular level before the flow measurements can start. The test is then completed by allowing water to pass through the soil specimen until water in the water pipe reaches a particular lower limit. The time taken for the water in the water pipe to flow from an upper level to a lower level is recorded. Permeability of the soil sample is then calculated using the following formula:
Where a = cross section of water pipe, L = height of soil specimen column, A= cross section of soil sample, ?t = time interval for the water to pass through the soil sample column, h_U = upper water level in the water pipe, and h_L = lower water level in the water pipe.
This results from interlocking and friction of soil particles, and bonding or cementation when soil particles collide with each other. This property is defined by the friction angle and cohesion of the soil. It is also determined by particle density, effective stress, strain direction, strain rate and drainage conditions. Shear strength determines the soil’s ability to support a structure’s loading, its overburden, or slope in equilibrium. This soil property is very essential when designing foundations, air fields, highways, dams, etc.
Shear strength of soil can be determined through the following tests: triaxial compression test, vane shear test, direct shear test and unconfined compression test. Direct shear test is carried out by placing a soil specimen in a direct shear box. The specimen is then compacted by applied a load or force on it until failure. The data collected can be used to develop failure envelope, stress-strain curve and Mohr’s circle of the soil specimen.
Vane shear test is used for determining soft clays’ undrained strength. The test is conducted using a vane that has a vertical steel rod and 4 steel blades fixed at the bottom. It can be done in the laboratory or in the field. The shear strength is then calculated from readings of height of vane, diameter of vane and torque applied (The Constructor, 2017).
Triaxial compression or shear test is conducted by applying stress on a cylindrical soil specimen. This test is conducted in stages. It starts with consolidation stage where pressure is subjected on the sample at the top, bottom and on the sides. The second stage is known as shearing stage. This is where a ram is used to apply extra deviator and axial stresses on top of the sample. Triaxial test can be conducted in cohesive and cohesionless soils.
Unconfined compression test is carried out by applying a compressive force on the soil sample until failure. Confining pressure in this test is zero and that is why it is only carried out on clayey soils.
Consistency is the ability of soils to resist rupture and deformation. In most cases, consistency of soil is indicated by unconfined compression strength. This property is largely influenced by water content in the soil. The amount of water content in the soil causes the soil to transform from one state to another. When water content increases gradually, the soil can transform from solid state to semi-solid state, from semi-solid state to plastic state, and finally to liquid state. Each of these states has a corresponding water contents, which are commonly known as Atterberg limits (liquid limit, shrinkage limit and plastic limit). Liquid limit (LL) refers to the water content of soil expressed as a % of the weight of soil that has been oven dried at the liquid-plastic boundary. Plastic limit (PL) refers to the water content of soil expressed as a % of the weight of soil that has been oven dried at the plastic and semi-solid states boundary. Shrinkage limit refers to the maximum water content expressed as a % of the weight of soil that has been oven-dried at a point where further decrease in water content will not reduce the soil mass’ volume (Suits, et al., 2009).
LL is the water content at the point where soil changes from plastic state to liquid state. This property is determined using Casagrande cup. It involves determining water content of soil corresponding to the number of drops (usually between 15 and 35) required to close a 13mm-diameter groove that is cut into the soil specimen. The moisture content of the soil is adjusted until the groove is closed within this range of drops. Once the grove is closed within this range of drops, the moisture content of the soil is determined by oven drying the soil specimen for 18 to 24 hours at a temperature of 105°C.
PL is the water content at the point where soil changes from semisolid state to plastic state. This is basically the water content at which a soil specimen can be rolled into a 3.2 mm diameter thread without breaking between the hands. When this is achieved, the water content of the soil is determined by oven drying the crumbled thread of soil for 18 to 24 hours at a temperature of 105°C.
Shrinkage limit is calculated using the following formula:
Where W = moisture content of wet soil sample, V = volume of wet soil sample, Vo =volume of mercury displaced by the dry soil sample and Wo = weight of oven dried soil sample.
Soils undergo compression when they are subjected to compressive stress caused by construction activities. The main causes of compression are water seepage, rearrangement of soil particles, elastic distortions and particles crushing. When a structure consolidates or settles beyond acceptable limits, its stability gets compromised. Compression data is used to predict the amount and rate of settlement that a structure will undergo. This data is used to design the foundation of the structure such that settlement does not reduce its stability beyond acceptable limits.
Compaction is very important as it increases the soil’s shear strength d reduces its permeability and compressibility. A properly compacted soil is more stable than the one whose particles are loosely arranged. When soil gets compacted, it becomes more dense thus reducing water seepage loss.
Density index is the measure of compactness degree and stratum stability of the soil. It is expressed as a percentage and defined as a ratio of difference between void ratio in loosest state and cohesionless soil and void ratio of difference between densest and loosest states of soil. The compactness of soil is largely influenced by its relative density. Loose soils have lower relative density while very dense soils have high relative density. Shear resistance angle of the soil also increases with increase in relative density.
Soils have different particle sizes that are widely used in soil classification. The particle sizes can be used for predicting water movement through the soil (Li, 2013). The particle size of soil is usually determined by sieve analysis.
Each type of soil has a different specific gravity. By definition, specific gravity refers to the ratio of soil solids mass to the mass of an equivalent amount of water. This property influences the soil’s qualitative behavior and is also used in classifying soil minerals. Specific gravity is also used for calculating several other parameters of soil such as void ratio, degree of saturation, porosity, etc. Many studies have found that an increase is specific gravity of soil increases its shear strength and California bearing ratio.
Grain size affects several properties of soil. The figure below shows grain size distribution
The relationship between dry density and water content of soil is that an increase in water content results to a gradual decrease in dry density (U.S. Department of Transportation, 2017), as shown in the figure below. This relationship is used to determine the optimum water content for maximum compaction.
Generally, an increase in particle size results to a corresponding increase in shearing resistance (Vangla & Latha, 2015); (Wang, et al., 2013). A graph showing this relationship is as shown below
The coefficient of permeability of soil increases with increase in void ratio (Cai, et al., 2014). Void ratio, e is calculated as follows:
Ground investigation is very essential in determining the suitability of a site for the proposed construction project (Kowalska, 2010). The investigation is usually undertaken by collecting geotechnical and geological data before the start of geotechnical design works (Kumor, et al., 2017). There are several geotechnical design parameters that must be considered. Some of them include: soil bearing capacity, shear strength, soil permeability, soil compressibility, void ratio, soil porosity, angle of friction, young’s modulus, cohesion, dry unit weight, etc. (Ameratunga, et al., 2015) The geotechnical design parameters investigated in this question are soil bearing capacity, shear strength and soil permeability.
Soil bearing capacity refers to the capability of a soil to support the amount of load that is applied to the ground. It is important to note that the total load of a structure gets deposited to the ground via the foundation. For the structure to remain stable, the soil on which it is resting must be able to provide adequate resistance to the force or load resulting from the structure. Therefore it is important to determine soil bearing capacity so as to establish whether the soil has adequate capacity to support the load or if there is need to improve the soil’s bearing capacity using various techniques. Some of the techniques that can be used to improve bearing capacity of soil include: draining the soil, confining the soil, compacting the soil, increasing foundation depth, use of grouting materials, replacing poor soil with good soil, and using chemicals to stabilize the soil (Suryakanta, 2015). The amount of pressure that the soil can bear easily against the load it is supporting is known as allowable bearing pressure.
Soil bearing capacity is usually measured using plate load test. This test is used for determining the soil’s ultimate bearing capacity. This is the maximum pressure that the soil can support beyond which it will fail. The apparatus needed to perform this test include: square mild steel test plate, hydraulic pump, hydraulic jack, pressure gauge, loading columns, load cell or proving ring, dial gauge stands, 4 dal gauges, supporting channels of the dial gauge, dial gauges’ magnetic bases, spirit level, plumb bob, loading platform device, pulley block and tripod.
The test starts by excavating a test pit whose size is five times that of the test plate and depth equal to the foundation’s depth. The loading platform is then erected vertically over the test pit. The test plate is placed centrally at the test pit’s base, ensuring that the ground surface underneath the steel test plate is perfectly level. The hydraulic jack is positioned over the test plate. A loading column is placed between the hydraulic jack and the test plate so that the hydraulic jack can reach the loading platform. The dial gauges are positioned on a stable base at the four corners of the test plate so as to record the plate’s settlement. The positioning of the dial gauges is such that their plunger is at the rebound’s beginning. To apply load on the plate, the hydraulic pump is used to pump hydraulic pressure into the hydraulic jack. Before actual loading is started, a seating load of approximately 0.7T/m2 is applied. The dial gauges’ initial readings are noted. Application of the load via the hydraulic jack is done in expedient increments, usually a tenth of anticipated ultimate bearing capacity of the soil. Readings of the applied load can be obtained either from the proving ring fitted between the reaction platform and hydraulic jack or from the pressure gauge fitted to the hydraulic pump. As the load is applied, the plate’s settlement readings are obtained from the dial gauges at specific times, usually 1, 10, 20, 40 and 60 minutes. The readings are recorded in a table. After recording all the needed settlement readings of the plate under a particular load, a new load increment is applied then readings from the dial gauges are recorded. The loads are increased subsequently and their corresponding settlements recorded until reaching the maximum load.
The data collected is used to draw a logarithmic graph of settlement (y-axis) against load applied (x-axis). The typical settlement-load curves for various types of soils are as shown in the figure below. From the graph, the ultimate bearing capacity of the soil is taken as the load at the point where the plate’s settlement starts sinking or reducing rapidly, that is, the point where the settlement-load curve breaks. This break point is where the curve drops to a vertical line. It is also important to note that different soils produces different kinds of settlement-load curves. This shows that different soil types have different ultimate bearing capacity that makes them behave differently when supporting the same amount of load.
The value of ultimate bearing capacity of soil determined above can be used to calculate safe bearing capacity of the same soil. This is done by dividing ultimate bearing capacity with factor of safety (which is usually 2, 2.5 or 3).
Shear strength, in geotechnical context, refers to the maximum amount of shear stress that a particular soil can resist, sustain or withstand. For a structure to be safe, the shear strength must be greater than applied stress by the factor of safety (Remai, 2013). Shear strength of soil is influenced by several factors including: soil density, particle shape, soil gradation, types and amount of fine particles, pore pressure and loading conditions. Shear strength of soil can be determined either in the field or laboratory. The most common methods used for determining shear strength are: vane shear test, direct shear test, standard (SPT) penetration test and cone penetration test (CPT). The test discussed in this question is direct shear test.
The apparatus needed to carry out direct shear test include: direct shear box, dial gauges, loading frame (attached with motor), loading pad, loading yoke, compaction devices (dynamic or static), proving ring, weighing balance, tamper, spatula, straight edge, grid plates, base plate, porous stones and box container. The test starts by measuring the shear box’s internal dimensions followed by fixing the upper and lower parts of the box. The grid plate is placed in the shear box then the porous stones are placed over the plate. The whole shear box (plus its components and constituents) is weighed. The soil specimen is placed in the shear box then tamped at the needed density. The loading yoke and dial gauges are fixed then weights are placed on the loading yoke for a normal stress (25 kN/m2) to be applied. All dial gauges and proving ring are adjusted to zero. Horizontal shear load is applied at a constant strain rate (0.2mm/min). The readings of dial gauges and proving ring are recorded at regular intervals. The test is continued until when the soil specimen fails. The shear load at which the specimen fails is used to calculate the shear strength of the soil as follows:
Soil permeability (also known as hydraulic conductivity) is the soil’s ability to conduct water. This geotechnical parameter depends on several factors including type of soil, depth of soil, location, moisture content of the soil, flow direction, etc. There are two main methods used for determining soil permeability: falling head permeability test and constant head permeability test. These tests were discussed in task 2.3 above.
Ground investigations are tests undertaken by engineering geologists or geotechnical engineers so as to collect information about physical properties of soil and other ground conditions, and use it to design foundations and earthworks for proposed structures. These tests are very important as they help engineers and geologists to establish groundwater level, underground obstructions, mechanical properties of soil and presence or nature of fissures or faults, and make appropriate decisions before the start of design or construction works.
There are numerous methods of ground investigation. Some of these include the following:
This method involves excavating the ground using a backhoe excavator or by hand. It is used for investigating ground conditions within shallow depths (not more than 4.5m) with an aim of understanding soil profile of the ground. Trial pitting can be done from machine excavated trenches or hand dug pits. This technique is usually used to examine the ground visually or collect soil samples for further in-situ or laboratory tests. Trial pitting is carried out in places where the ground can stand unsupported temporarily. Trial pitting makes it easy to collect soil samples at various depths or to observe natural soil strata.
This is a ground investigation technique involving use of handheld or percussive pneumatic samplers. The technique is most suitable when investigating contaminated sites and those sites that have restricted access. It is also recommended for use when disturbance of the site has to be minimum. In this method, cylindrical steel tubes of different length are inserted or driven into the ground by use of a percussive or hydraulic hammer fixed on tracked or wheeled rigs. The sampling is done via percussive action. When the device reaches the desired depth, a hydraulic jack is used to extract drill rods and sample tubes. After withdrawing sample tubes from the ground, the soil sample gets logged and removed from the window (RSA Geotechnics Ltd, (n.d.)). In some cases (where the ground is unstable), casing systems can be used to prevent boreholes from collapsing.
This method is commonly known as shell & auger boring. In this method, steel casing are driven into the ground for stabilizing the borehole’s sides. In most cases, the diameter of casing is bigger at the top of the borehole and reduces with increase in depth. A diesel engine-driven winch and a tripod rig and are installed on site and clay & shell cutter tools are used to extract soil. A clay cutter is used for forming borehole sin cohesive soils while shell cutter is used for forming boreholes in non-cohesive soils. This technique can be used to obtained disturbed or undisturbed soil samples. To collect undisturbed samples, a hollow tube is driven into the ground through which the sample is obtained. The disturbed samples can be obtained from shell and clay cutter.
This technique is usually used to obtain soil samples from solid geological formations like in rocky areas (bedrocks or very dense gravel soils). The technique can be used to obtain samples up to 100m deep into the ground. In this method, a mechanical rotary cutting head is mounted on a shaft then driven into the ground. As the cutting head moves into the ground, it disintegrates the rock layers. The rotary drilling bit is cooled using a jetting liquid or drilling lubricant, which transports the soil sample from the ground to the surface. There are two main types of rotary boring: rotary cored boring and rotary open boring. In rotary open boring, the drilling lubricant gets mixed with the material or soil sample collected. The collected material is then taken to the laboratory for testing or it can be tested in the field.
Once the in-situ samples are collected from any of the above methods, they are taken for further testing either in the field or in the laboratory. Some of the in-situ tests include: standard penetration test (SPT), cone penetrometer testing, dynamic cone testing, field vane shear test, dilatometer testing, seismic piezocone penetrometer probe, full flow penetrometers, helical probe test, etc.
The test carried out in this question is that of determining the relationship between moisture content and permeability. Moisture content has a direct effect on permeability (Abdullah, et al., 2011), compressibility and shear strength of soil (Tanzen, et al., 2016).
The permeability is determined using constant head method. Coefficient of permeability, K, is calculated using the following formula:
Where q = discharge, L = length of oil sample, A = cross-sectional area of soil sample and h = constant head that is causing the flow.
The equipment needed are: permeameter mould with a removable extension stand and detachable base plate, compacting equipment, drainage cap, drainage bade, glass cylinder, stopwatch and meter scale. An oven-dried sample of soil was obtained and its initial moisture content determined. The sample was put in an air tight container and water added to attain a certain moisture content. The soil was mixed thoroughly, the empty permeameter mould was weighed, its inside was greased then it was clamped between the extension stand and compaction base plate. The assembly was placed on a solid base, filled with soil then it was compacted thoroughly. After compacting, excess soil was removed then the weight of the permeameter mould plus the soil sample was measured. The constant head reservoir as connected to the specimen using the top inlet. The bottom outlet was opened then water was allowed to flow through the sample steadily. The amount of water released from the bottom outlet was collected. This test was repeated at different moisture content levels (varied values of h). The results obtained were as shown in the table and graph below
|
Water content (%) |
Penetration (cm) |
|
5 |
1 |
|
7 |
3 |
|
9 |
4 |
|
10 |
5 |
|
15 |
3.333 |
|
20 |
2.5 |
|
25 |
2 |
|
30 |
1.667 |
|
35 |
1.429 |
|
40 |
1.25 |
|
45 |
1.111 |
|
50 |
1 |
|
55 |
0.909 |
|
60 |
0.833 |
|
65 |
0.769 |
|
70 |
0.714 |
The graph above shows that penetration increased initially with increase in water content. This was as a result of the soil changing from solid to liquid state. After reaching a certain point, penetration started reducing with increase in water content. This is because the air voids present in the soil became filled with the water leaving no space for penetration.
Shear strength is one of the most important geotechnical design parameters. The shear strength is derived from two parameters: friction angle and cohesion (Eslami & Mohammadi, 2015). These shear strength parameters are determined from geotechnical laboratory data. The most common method used for determining these parameters is cone penetration test (CPT) (Motaghedi & Eslami, 2014). The results obtained from CPT are usually used to draw the Mohr’s circle, from which the values of friction angle and cohesion are determined. Another effective and easy to perform test is the direct shear test. The results obtained from this test are normal stress (σ) and shear stress (τ), which are calculated as follows:
The results analyzed in this question were obtained from direct shear tests that were done on two different soil samples. The first set of results are as shown in the table below
|
Shear stress, τ (Pa) |
Normal stress, σ (Pa) |
|
20 |
0 |
|
30 |
5 |
|
40 |
10 |
|
50 |
15 |
|
60 |
20 |
|
70 |
25 |
|
80 |
30 |
|
90 |
35 |
|
100 |
40 |
|
110 |
45 |
|
120 |
50 |
From the graph above, the value of cohesion is the point where the straight line cuts the y-axis. In this case, the value of cohesion is 20 Pa. This means that the soil tested was cohesive soil such as clay, loam or silt.
Friction angle = tan-1 = tan-1 (2) = 63.4°
The second set of results are as shown below
|
Shear stress, τ (Pa) |
Normal stress, σ (Pa) |
|
0 |
0 |
|
15 |
8 |
|
22 |
12 |
|
28 |
15 |
|
40 |
22 |
|
52 |
29 |
|
62 |
35 |
|
70 |
40 |
|
78 |
45 |
|
94 |
55 |
|
102 |
60 |
From the graph above, the value of cohesion is the point where the straight line cuts the y-axis. In this case, the value of cohesion is 0 Pa. This means that the soil tested was non-cohesive soil such as sand or gravel.
Friction angle = tan-1 = tan-1 (1.667) = 59°
From the laboratory data and result above, it was concluded that the soil in the first set of data was more dense than that of the second set of data. This is because more dense soils have higher friction angles than loose soils (Geotechdata.info, 2013).
Shear strength, S, is determined as follows: S = C + σ tan φ (where c = cohesion, σ = normal stress ad φ = friction angle).
This test is used for determining the flow of water through granular or permeable soils such as gravel and sand. There are two main permeability tests: falling head permeability test and constant head permeability test (Sandoval, et al., 2017). The test demonstrated in this question is constant head permeability test. The main objective of this test is to determine the coefficient of permeability. The equipment used in this test are: permeameter mould, drainage base, dummy plate, detachable chair, drainage cap, compaction device (tamper), water supply reservoir with a constant head, collecting chamber with constant head, vacuum pump, thermometer, stopwatch, filter paper, large funnel and weighing balance.
The mould’s collar is removed then its internal dimensions are measured. The weight of the mould together with dummy plate is recorded. Some grease is applied on the inside surface of the mould. The mould is clamped between the extension collar and base plate then the assembly is placed on a solid base. About 1.5kg of the soil sample is put in the mould and compacted thoroughly (using the tamper) at the desired dry density. The base plate and collar are removed. The excess soil is trimmed to ensure that it is level with the mould’s top. The outside of the dummy plate and mould are cleaned then mass of the soil sample remaining in the mould is measured. A small sample of the soil is taken and put in a container to find its water content. The porous disc is saturated then placed on the drainage base. A filter paper is kept on the porous disc, the dummy plate is removed, a washer is put on the drainage base then the mould is placed on it. The mould’s edges are cleaned then grease is applied in its grooves. A porous disc and filter paper are placed then washers are used to fix the drainage cap. The drainage base’s outlet is connected to the water reservoir then the soil sample is saturated by allowing water to flow through it upwards. The empty part of the mould is filled with the required amount of water with the soil ample remaining undisturbed. The reservoir is disconnected from the outlet then the constant head reservoir is connected to the inlet of the drainage cap. The stop cock is opened and water is allowed to flow downward for any air present to be removed. The stop cock is closed and flow of water through the soil sample is allowed for some time until attaining a steady state. The stopwatch is started and water coming through the base is collected in a measuring flask at specific time intervals. This is repeated 3 times, ensuring that the amount of water collected during each time is the same. The head difference (h) is measured between the base’s outlet and constant head reservoir. The test is repeated at different time intervals. The setup of the test is as shown in the figure below
The coefficient of permeability is then calculated using the following equation:
Where Q = total volume of water collected (quantity of flow), L = length of soil sample, A = cross-sectional area of the mould, h = head that is causing the flow (hydraulic head) and t = time interval.
This test is performed so as to determine the consolidation properties (i.e. rate and magnitude) of soil or settlement of a structure (Xie, et al., 2012). Consolidation is a time-dependent process that results to consolidation settlement (Liu, et al., 2009). This process is very important when designing a structure as it helps in preventing excessive deformation (Wang, et al., 2015). The essential soil properties that are calculated from a one dimensional consolidation test are: compression index (Cc), pre-consolidation stress (σp), recompression index (Cr) and coefficient of consolidation (Cv) (Singh, et al., 2014). The most common one dimensional consolidation test is the oedometer test. This test is carried out by applying loads of different magnitude to a soil specimen then determining how the soil responds to deformation (Lovisa & Sivakugan, 2015). The results obtained from the test is then used for predicting the soil’s response in the field when subjected to different effective stresses. There are different types of oedometer tests. The most common type (which is the one discussed in this question) is the incremental loading test. This test falls under the category of fixed-ring consolidometer (the other one is floating ring consolidometer). The test basically involves placing the soil sample in the fixed ring, applying load on the sample, measuring height change and repeating the process with new load. The setup of the test is as shown in the figure below.
The test starts by measuring the consolidation ring/cutter’s internal diameter, height and mass. A soil specimen is prepared by trimming it to the right size and placing it in the fixed ring. The mass of soil and ring is determined. Some excess soil is collected for determination of moisture content. The larger or lower porous stone at the consolidometer’s base is saturated then the ring and soil specimen are placed on the upper disk or porous stone. It is assumed that specific gravity (Gs) is 2.7. Vertical static loads are applied on the soil specimen and their corresponding settlement recorded. The change in the specimen’s thickness is recorded in each loading increment. A load is increased after reaching equilibrium, that is, when there is no change in specimen’s thickness. During each increment, the load is doubled. When maximum load is attained and full consolidation attained, the specimen is unloaded then its swelling, moisture content and thickness are measured and recorded. The results obtained from the test are then used to calculate the various consolidation properties of the soil specimen: Cc, σp, Cr and Cv.
The data collected from the test is used to plot a graph of vertical displacement (inches) against logarithm of time (min). From this graph, t50 and t90 are determined. The following calculations are also done:
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