Classification Of Rocks, Soils And Aggregate In Construction Industry

Petrographic classification of igneous rocks

Aggregates are types of small-sized rock materials that are commonly used in construction projects. They include different kinds of rock such as sand, geosynthetic, crushed stone, gravel, etc. One of the reasons why aggregates are used in the construction industry is for economic benefits by reducing cost of concrete. In most cases, aggregates are used for constructing sub bases, where they are not mixed with water and cement. Characteristics of aggregates also have direct influence of the properties of concrete mix formed. These characteristics are dependent on aggregate classification.

Aggregates can be classified based on different parameters, including: size (fine, coarse and single size aggregates), shape (angular, flaky, rounded and irregular aggregates), unit weight (light, usual and heavy weight aggregates) and geological origin (natural and artificial aggregates).

Aggregate characteristics are influenced by the following properties: bulk density, absorption and porosity, fine aggregate bulking, specific gravity, particle texture and shape, moisture content and strength of aggregate.  

Rocks contain different minerals. But besides the minerals, other characteristics of rocks such as structure and form depend on formation mode. There are 3 main kinds of rocks that are widely used in construction industry. These are: igneous rocks, metamorphic rocks and sedimentary rocks. The following are some of the characteristics of these rocks:

Characteristics Save Time On Research and Writing Hire a Pro to Write You a 100% Plagiarism-Free Paper. Get My Paper	Description
Silica content	Silica content in these rocks ranges between 40 and 80%. The most important elements are iron and magnesium
Inappropriately eroded	The rocks can resist erosion but with time, they start weathering at a relatively slow rate
Non porous	These rocks does not allow infiltration, penetration or permeation of water
Lack of fossils	The rocks do not contain any fossils
Crystalline & non-crystalline	The rocks are both crystalline and non-crystalline. Crystalline igneous rocks are formed when magma cools down slowly within the empty spaces and crystallizes before coming out. On the other hand, the rock is no-crystalline if the magma rising from the cracks to the surface does not get adequate time to crystallize inside the empty spaces.

These rocks have several distinct characteristics from those of igneous rocks. Their formation usually takes place in marine environments, such as shallow seas and lakes.

Characteristics	Description
Porosity	Their different particle sizes make them permeable. The size of particles can be small, medium or large, thus allowing infiltration of water at different rates.
Stratification	The rock is made up of numerous layers known as strata. During their formation, each layer is created one at a time, a process known as stratification.
Prompt erosion	The rocks are weathered and therefore they erode very quickly in comparison with other types of rocks
Fossilization	The rocks contain fossils such as plant debris and dead animals. Archeologists use these fossils for determining the rocks’ age and obtaining other vital information about evolution of animal and plant life during that time.
Imprints and marks	These rocks contain imprints and marks that are caused by sea waves. Typically, these rocks are at the shoreline hence they get hit by sea waves that create imprints and marks on the rocks. The imprints and marks can also be referred to as ripple marks.

Characteristics	Description
Hot magma	During their formation, the rocks rise at an extremely high temperature
Geothermal heat	Heat below the earth’s surface influences metamorphism and formation of metamorphic rocks. The geothermal heat is usually very high
Thermal metamorphism	The entire metamorphism process is passed by heat in different ways.
Dynamic metamorphism	Geodynamic forces present on earth put a lot of pressure on rocks found on the earth’s surface. These rocks become distorted, causing an increase in heat. Presence of any liquids such as water ten causes the rocks to restructure and form new aggregates called metamorphic rocks.
Mutual friction	Mutual friction resulting from movement causes these rocks to produce heat
Hot liquids, vapors and gases	Hot mineralizers like vapors create metamorphism. The time it takes for metamorphism to complete depends on the nature and quantity of the mineralizers.

Use of rocks as a construction material started thousands of years ago. Until today, rocks are still widely used as construction materials because of their superior properties such as aesthetics, strength and durability. Their aesthetic value makes them suitable for decorative design elements such as monuments, facings and memorials. There are 3 main types of rocks that are commonly used as construction materials: igneous rocks, metamorphic rocks and sedimentary rocks.

Igneous rocks are those formed when molten magma from underneath the earth’s surface cools. Obsidian is an example of an igneous rock. Sedimentary rocks are formed from plant debris and dead animals that become compacted over the years. Sandstone is one example of a sedimentary rock. Metamorphic rocks are existing rocks on the earth’s surface that have been transformed by high temperature and pressure levels. An example of a metamorphic rock is marble.

Rocks can be used in construction for building or erecting structures such as walls, buildings, bridges, dams, roads, etc. Extraction of these rocks is usually done in quarry sites before they are transported to the factory for processing. There are different types of construction methods used with rocks. One of these in referred to as traditional stone masonry construction method. This is a conventional method and is rarely used nowadays because it requires a lot of man and labor hours. Additionally, a lot of money is required for quarrying the materials and transporting them from one location to another.

Classification tests to current codes of practice

Nevertheless, traditional stone masonry method has found effective use in modern days for making decorative finishes. Using this method, thin pieces are constructed using selected rocks such as slate. The pieces are then glued on concrete block walls, a technique called stone cladding. This is more efficient as it does not require construction of the entire structure using a large number of stones.

Bowen Series refers to an experiment that investigates how minerals are formed from magma when it is cooling. The experiment was created by Norman Bowen, a petrologist, when he performed numerous melting tests in early 1900s so as to support his granite theory. According to his investigations, he found that when basaltic melted and cooled down slowly, solidification of minerals started and they transformed into crystals. This demonstrated the formation of igneous rocks from magma.

Rocks are materials comprising of different minerals. These materials are slid, hard and durable, and are formed as the earth’s outer crust. As stated before, there are three groups of rocks: igneous rocks, metamorphic rocks and sedimentary rocks.

Igneous rocks: these rocks are formed when molten magma solidifies and hardens. They are produced on the surface of the earth as lava then become igneous rock after hardening naturally. Igneous rocks can also be created below the surface of the earth. There are two types of igneous rocks: extrusive and intrusive igneous rocks.

Extrusive igneous rocks: these are types of igneous rocks formed and found on the earth’s surface. The rocks are produced from lava then they cool to form extrusive igneous rocks. Solidification of these rocks into igneous extrusive rocks is quicker. The following are a few examples of extrusive igneous rocks: obsidian, andesite basalt and scoria.  

Extrusive igneous rocks can be formed below or on earth’s surface. When igneous rocks form on the surface of the earth, they do so through volcanic eruptions. In this process, eruption of the volcano produces lava that cools down above the surface of the earth before solidifying to form igneous rocks. Formation of igneous rocks below the surface of the earth occurs when magma below the earth’s surface is trapped in small empty areas and start cooling down slowly since there is no space where they can flow to. After cooling, the magma transforms into igneous rocks.

Intrusive igneous rocks: these are types of igneous rocks that are formed from magma that is found below the earth’s surface. The rocks are formed when magma forces itself through the earth’s cracks and flows into rock beds. The solidification process of intrusive igneous rocks is slower than that of extrusive igneous rocks. Examples of intrusive igneous rocks are diorite, gabbro and pegmatite.

Particulate nature and properties of soils

Intrusive igneous rocks are formed by cooling down of magma after which it solidifies to form the rock below the surface of the earth. The rock usually has a rough texture and it can be easily identified using a naked eye, without necessarily having to perform tests. Identification can be done by looking at the rock’s size and shape.

Igneous rocks are largely used in construction for making decorative design elements such as monuments, facings and memorials. Granite is one example of igneous rocks. This rock can be used for making tiles, facing stones, paving stones, slabs, and curbs, among others.

Sedimentary rocks: these rocks comprise of animal, plant and rock remains. The rocks are formed and found near the surface of the earth. Different types of sedimentary rocks are typically fragmented into small pieces known as sediments. These sediments can be processed to produce sedimentary rocks. One process technique is compressing the small sediments together to form one solid rock.

There are three main types of sedimentary rocks, which depend on their composition and mode of formation: organic sedimentary rocks, clastic sedimentary rocks and chemical sedimentary rocks.

Organic sedimentary rocks: these are rocks formed from dead particles such as dead plants and animals. An example of organic sedimentary rock is limestone.

Clastic sedimentary rocks: these are rocks formed through mechanical weathering. This occurs when rocks are exposed to weather and because of this collision, their surfaces start weathering. Examples of clastic sedimentary rocks are conglomerate and sandstone.

Chemical sedimentary rocks: these are rocks formed as a result of mineral particles present in solution becoming supersaturated leading to sudden formation of an inorganic solid. Examples of chemical sedimentary rocks are salt and limestone.

Formation of sedimentary rocks occurs when these rocks get buried below the surface of the earth. Being buried beneath the surface of the earth means that the rocks are below other sediment layers hence the temperature and pressure levels rise. In this process, the sediments become compacted after which they solidify to form sedimentary rocks.

In construction industry, sedimentary rocks are usually used as very valuable resources. An example of such a resource is chalk, which can be used for making cement.

Metamorphic rocks: these are rocks comprising of other rocks that have been altered or transformed due to high pressure and heat levels. In other words, metamorphic rocks are existing rocks transformed into metamorphic rocks due to temperature and pressure levels.

Influence of seepage on effective stress

There are two main types of metamorphic rocks: foliated metamorphic rocks and non-foliated metamorphic rocks.

Foliated metamorphic rocks: these are rocks that are identified by their lined or banded characteristic (as shown in Figure 1 below), which is created as a result of high pressure and heat levels. Examples of foliated metamorphic rocks are phyllite and slate.

Figure 1: An example of foliated metamorphic rock (slate)

Non-foliated metamorphic rocks: these rocks lack a lined or banded characteristic, as shown in Figure 2 below. Examples of these rocks are amphibolite and marble.

Figure 2: An example of foliated metamorphic rock (amphibolite)

Figure 3 below is an example of a metamorphic rock called gneiss rock. In its earlier life span before being transformed into a metamorphic rock, this rock may have been an igneous rock such as granite. However, it got altered by high pressure and temperature levels, turning it into gneiss rock. The image in Figure 3 below shows how the rock’s grains were altered as a result of high pressure and temperature levels.

Figure 3: An example of metamorphic rock (gneiss rock)

In most cases, metamorphic rocks are used in construction for finishing decorative design elements. For example, slate, a foliated metamorphic rock, is usually used for making flooring and roofing units because it is more durable and eye-catching.

An aggregate is basically a mixture of sand, gravel and rocks. These materials are mixed to form one composite materials such as concrete and asphalt. Aggregate materials are usually sourced from quarry zones in different parts of the world.

The main reason for the wide use of aggregates in construction is because of their superior reinforcement that adds significant strength to the composite materials or components such as retaining walls and foundations. Aggregates are used so as to improve stability of the structure, such as such as roads and foundations, where the aggregates can be used as base materials below the foundations. In this kind of projects, aggregates are used to prevent different settlement of the structure being constructed.

Before starting to construct a foundation, the first task is to collect suitable materials and resources. Most foundations comprise of aggregates i.e. a mixture of sand and rocks, combined with water and cement and allowed to solidify so as to create a foundation. Therefore the structure of the foundation is made by mixing different types of rocks and soils.

It is very important to know the type of soil on which a structure is being constructed. This is because the type of soil has direct impact on the structure’s foundation and overall stability. Thus is it important to know the specific ground conditions so as to select the most appropriate foundation solution. The best way to do so is to contact the local authority. Most authorities have building control departments that share information on different types of soils within particular areas. This helps contractors to know the most suitable type of foundation for a particular structure in a specific area.

Characteristics and various uses of common rocks in construction

Soils are broadly classified depending on their particle sizes. Based on this system, soils can be classified as very coarse, coarse and fine soils, as shown in Table 1 below

Very coarse soils	BOULDERS	> 200 mm
COBBLES	60 – 200 mm
Coarse soils	G GRAVEL	coarse	20 – 60 mm
medium	6 – 20 mm
fine	2 – 6 mm
S SAND	coarse	0.6 – 2.0 mm
medium	0.2 – 0.6 mm
fine	0.06 – 0.2 mm
Fine soils	M SILT	coarse	0.02 – 0.06 mm
medium	0.006 – 0.02 mm
fine	0.002 – 0.006 mm
C  CLAY	< 0.002 mm

Table 1: Grain size range of soils

The three main types of soils based on their characteristics are: clay soils (small grains), silt soils (medium grains) and sand (large grains). Clay particles are usually greasy and sticky when moist and become hard as they dry. It is difficult to clean off clay. Silt particles are dusty and dry, and can be easily brushed or cleaned off. Sand particles are grained and can be seen by the naked eye.

Table 2 below shows different types of soils and their parent rocks

Type of soil	Type of parent rock
Basalt	Igneous
Granite	Igneous
Limestone	Sedimentary
Carboniferous	Sedimentary
Chalk	Sedimentary
Sandstone	Sedimentary
Clay	Sedimentary
Slate	Metamorphic

Table 2: Types of soils and their parent rocks

Limestone is usually found, extracted and produced from marine environments such as marine water. Several decades ago, limestone was a common material in construction of structures. However, use of this material in the construction industry has decreased as is now used for building foundations. Limestone can be extracted from quarry sites and directly used as base materials. Some areas also have low graded limestone that contains some clay. This kind of limestone can be processed further to form cement, which can be used to make concrete for building foundations and other structures.

Basalt can be obtained nearby lava highlands in form of grained and textured volcanic rocks. These rocks are formed as a result of continuous cooling down of molten lava from high temperatures. Some basalts were produced millions of years back. Basalt can be used for making foundations, floorings and window sills. The basalt can also be used in highway construction, where it is crushed into powder that can be converted into a fibre that is very resistant to heat. When basalt is used to make cement mixture, it adds significant strength to the concrete, which can be used to make foundations or as a fire proof coating on gypsum boards.

There are different sizes of aggregates and each is suitable for particular uses. Producers can supply ready mixed aggregates though this is only suitable for small projects such as repair works. Contractors are usually recommended to mix sands and stones by themselves because this allows them to vary mortar proportions to suit their intended use. Nevertheless, the type of aggregate used still depend on the size and shape of the material.

Classification of aggregates based on different parameters

Aggregates can be differentiated by their roughness, where grinded stones and gravels are passed through sieves of different sizes. This helps in choosing the maximum size of the rock that is suitable for the project at hand.     

The type of foundation usually relies on the type of soil. Below are some of the different types of soils:

Sand and gravel: usually, strip foundations are created using sand and gravel sub soils of depth up to 700 mm and which have sufficient bearing capacity. The bearing capacity of soil usually gets halved if the level of water table is quite high. This is why foundations should be formed as high as possible. Therefore when using gravel and sand to make foundations, reinforced wide strip and shallow foundations are the best types of foundations for this soils. On the other hand, gravel is also held together properly by sand when it is damped and compacted even though trenches may separate. Because of this, sheet piling is usually used (as shown in Figure 4 below) so as to enhance stability of the ground in trenches while waiting for the concrete to be poured.

Figure 4: Sheet piling

Rock: rocks such as limestone, sandstone and granite have very high bearing capacity. Before building, the rock must be stripped and flattened. The rocks are usually impermeable hence the topsoil must have a drainage system so as to remove rain or surface water.

Clay: it is estimated that 200 mm top layer of clay is usually washed off as a result of crust movement of the earth due to natural occurrences and growth and shrinkage depending on moisture content. This means that if clay has to be used for creating a foundation, the foundation must be excavated to a depth where the clay’s dampness or moisture content remains constant. When clay is used for concreting foundations, a heave is usually used to guard the trench. This is done by lining the trench using a compressible layer like clayboard, as shown in Figure 5 below

Figure 5: Clayboard

Loose waterlogged sand and peat: this is a poor subsoil but it can be used to make strip foundations if the peat is uncovered so as to find an appropriate load bearing ground depth of 1.5 m. Peat is an organic material that is relatively soft. They are usually formed below several deposits of firm soil. It is not recommended to build on organic soils but if it has to be done then reinforced raft foundation should be used, as shown in Figure 6 below

Intrusive and extrusive igneous rocks

Figure 6: Reinforced raft foundation

Firm clay on top of soft clay: this type of soil is suitable for conventional strip foundations. However, digging must not be very deep or else it can amplify stress on the softer clay below. The best way to resolve this is issue is by digging wide strip foundations and reinforcing them with steel. In most cases, an engineered foundation may be needed for this type of soil.

Strip foundation: the 2 main types of strip foundation are: wide and deep strip foundations. Wide strip foundation is usually used in areas where the bearing capacity of the soil is low or when the soil is soft. The foundation spreads or distributes load over a wider area comprising of reinforced steel that reduces the loading. Deep strip foundation is built at a relatively low level so as to reach the soil with an appropriate bearing capacity. A wider and deeper trench is then dug so as to create more work space. Concrete is usually poured at a lower level of the strip foundation then masonry walls are constructed to the level of the ground.

Raft foundation: it is usually used in areas with weak soils such as peat and clay. A raft foundation is required when the soil requires a large bearing area and using wide strip foundations would be costly because the foundations will have to be spread out broadly. Therefore it becomes cheaper if concrete is poured once to make a big reinforced concrete slab as the foundation.

Rock material: this is the whole rock portion. Rock materials have different densities, which are influenced by the rock material’s porosity. One of the rock material’s most important mechanical property is compressive strength, which is used for the design, modelling and analysis of structures.

Rock mass: in the context of civil engineering, rock mass is a distinct material from other kinds of structural resources that are used in construction. The material is usually different and discontinuous but it is among the materials found on the surface of the earth that are widely used in construction. There are different classification systems of rock mass, which are used for varied design analyses and engineering stability. These classification systems are usually determined by actual relationships between engineering uses and parameters of rock mass such as foundation and tunnels.  

In general, the difference between rock mass and rock material is that the former refers to the volume of the rock while the latter refers to the composition of the rock.

Bowen Series of instability

Practically, it is not easy to describe rock masses and ground conditions because engineers prefer using calculations instead of adjectives. This is what makes rock mechanics and geological engineering very valuable, including tunneling methods and classification systems. In 1946, Terzaghi created a steadfast system that can be used to support tunnels. The system was developed in the U.S. and it was mainly about systems used for classifying tunnel supports. The system provides relatively accurate approximations of loads on tunnel supports based on steel used. The system later became a widespread support classification system.

In most cases, RQD is used as a guide for describing the quality of fractured condition of the rock mass. Civil engineers were among the first users of the RQD system, which has continued to be developed and improved over the years. Today, RQD system is used in mining, engineering geology and geotechnical engineering.

Basically, RQD is used for measuring jointing and degree of fractures present in rock mass, as a % of drill core over a length of at least 10 cm. As shown in Table 3 below, rock masses with ROD less than 50% are considered to be of poor quality while those with over 75% are said to be of good quality.

RQD	Rock mass quality
<25%	very poor
25-50%	poor
50-75%	fair
75-90%	good
90-100%	excellent

Table 3: RQD classification index

RQD is very useful in estimating rock tunnels’ supports. The system is a basic element in majority of classification systems for rock mass, such as rock mass number, slope mass rating (SMR), rock tunnel quality Q-system, and rock mass rating (RMN), among others. There are also different definitions of RQD. The most common definition of RQD is that developed by D.U. Deere in 1967. According to his definition, RQD is a system used to provide a reliable estimate of rock mass quality obtained from drill core logs. Only solid core pieces measuring more than 100 mm are used.

Figure 7 below shows the correct method used for calculating RQD and measuring core pieces’ length.

Figure 7: Method used for measuring and calculating RQD

When analyzing the type of support needed for a tunnel, it is important to consider the fact that RQD can give a wrong value if the joints being evaluated contain thin clay fillings or weathered materials. Additionally, RQD system does not take joint orientation’s direct amount.

In geotechnical engineering, discontinuities occurs when a surface of a material gets altered as a result change of the soil or rock mass’ physical or chemical characteristics. Examples of rock discontinuities are jointing and fracture. Fracturing entails dividing the rock into several pieces or fragments. The main cause of fractures is stress that occurs in rock strength, which reduces cohesion of the rock. Jointing or fracture happens between mechanical and integral discontinuities.

Differences between types of rocks

Rock discontinuities can occur several times with unchanged mechanical properties in one discontinuity or a sequence of discontinuities. The discontinuities make the soil or rock mass to become anisotropic thus affecting the rock material’s engineering performance. Anisotropic means that physical properties of the rock material are different from measured values of the same rock material.

Soil is the earth’s top layer where plants grow, structures are constructed, etc. Types of soils include sandy, silt and peat soils.

Soil description is used to describe and show the soil’s aesthetics and physical nature. Description can be done in a lab on a soil sample obtained from the field or it can be in situ.

Soil classification entails separating soils into different classes or groups, with each group exhibiting similar behavior and characteristics. Classification of soil is largely based on the soil’s mechanical properties, including strength, stiffness, permeability, etc. Soil classification is very important as it helps geotechnical engineers to get very useful information about the soil’s engineering properties and its suitable applications.

There are different methods that can be used to classify soils by identifying the soil’s particle sizes. One of these methods is sieve analysis. This is the method that will be used in this report. In this method, grain sizes of soils are controlled and separated between different sizes. This is the most common soil classification method. The data is usually obtained from distribution curves of grain sizes used in filters’ aesthetics for dam constructions. The method is also used for determining suitability of soil for various construction projects, such as road construction. Information obtained from sieve analysis can also be used for predicting water movement through the soil.    

Equipment needed:

The equipment needed to perform this test are: sieves, oven, mechanical sieve shaker, weighing balance with an accuracy of 0.01g, pestle and mortar.

Procedure:

About 500g of oven dried soil sample was taken and crushed using mortar and pestle so as to disintegrate the soil particles but not to crush them. The mass of the soil sample was precisely controlled to avoid losses. The 10 sieves were prepared and arranged in a descending order starting with the one with the largest opening sizes (lower number) being on top while the one with the smallest opening size (higher number) being at the bottom. The last sieve was number 10 then a pan was placed underneath it for collecting the soil passing through the 10^th sieve. After collecting data, particle size distribution curves for the two soil samples were plotted then used in conjunction with the table in Figure 8 below to identify the various types of soils that were being investigated

Figure 8: Table of particle size distribution

The curves in Figure 8 below show cumulative particle size distribution of two soil samples investigated in this test.

Figure 8: Cumulative particle size distribution

In this case, soil 1 is represented by sample A while soil 2 is represented by sample B. The two soil samples are relatively uniform even though sample B appears to be more uniform that sample A, as shown by the curves in Figure 8 above. Sample A contains largely of sand and a small percentage of coarse silt fine gravel and therefore can be said to be less cohesive. Sample B comprises largely of coarse sand and a small percentage of fine gravel hence it can be said to be more cohesive. Therefore sample A is fine sand soil whereas sample B is a coarse sand soil.

Figure 9 below shows a three phase diagram of soil. The diagram shows that the soil sample has 3 different components: air, solid and water, hence the name three phase diagram.

Figure 9: Three phase diagram of soil

The soil can also comprise of two elements: water and solid, or air and solid, as shown in Figure 10 and 11 below respectively. These are two phase diagrams of soil.

Figure 10: Two phase diagram (water and solid)

Figure 11: Two phase diagram (air and solid)

Soil mass is typically a three phase diagram since it comprises of solids, water and air. The area that is covered by water and air is referred to as voids. The void can be filled totally by water and air or by water only. If the voids are partially packed with water and air, the soil mass is said to be partially saturated but when the voids are totally packed with water, then the soil mass is said to be fully saturated. A soil sample is said to be dry if its voids are packed with air only. The soil’s three phase diagram does not have separate spaces but are concealed between solid soil particles that create a complex material. Therefore 9, 10 and 11 above shows a

Void volume:

Density of soil’s solids = 2650 kg/m³ 

 ≈ 0.6

Therefore void volume = 1 – 0.6 = 0.4

Water percentage:

Water percentage = difference between weight of soil sample before and after drying = 199 g – 188 g = 11 g

Void volume

= 11/188 = 0.0585 ≈ 0.06

Volume of water:  

= 0.06/1000 = 6 x 10^-5 m³

Degree of saturation:

The features or factors that contribute to cohesion development of fine grained soils like clay is their small-sized soil particles and mutual attraction found between soil particles. The shear strength and friction found between adjacent soil particles assists in creating strong cohesion between adjacent soil particles. Also, the soil is not able to move or deform because cohesion level increases as a result of occurrence of merging between soil particles. 

Figure 12 below is a free body diagram of an isolated soil layer from a soil profile. 

Figure 12: Free body diagram of an isolated soil layer

Total stress is the summative stress applied on each of the free body’s outer layer. This total stress must balance with the amount of external forces acting on the self-weight of the free body and the body itself. Therefore total stress is the amount of stress applied on the outside or external surface of the soil layer.

Effective stress is the measure of the manner in which the stress applied on the external surface of the soil is transferred within the soil that is inside the soil layer or free body. Therefore effective stress is the stress resulting from contact forces found between soil particles.

Total stress is the amount of vertical stress at a particular point below the ground surface. In other words, this is the stress resulting from weight of all items or particles that are on top of the surface, including soil and water. Total stress is calculated from the soil’s unit weight.

Effective stress is the stress developed when a load is applied on the soil then it is carried or supported by the water present in the pores or solid grains of the soil. This increases pressure within porewater thus causing drainage. Soil permeability is major parameter affecting rate of drainage. Thereafter, the load is transferred to the soil’s solid grains.

Pore pressure refers to pressure inside porewater, which is the water present in the soil’s pores. The magnitude or degree of pore pressure depends on seepage flow conditions and the water table depth.

Shear strength develops when neighboring soil particles merge and separate. In some cases, shear strength is seen as the amount of physical stress that can be supported by soil. If volumetric expansion occurs, the density of soil decreases thus reducing strength of soil. Therefore soil saturation reduces shear strength.

Fine sand (a coarse grained soil) has different chemical, physical and mechanical properties from those of clay. Clay material has a large surface area and therefore when it is contact with water for a longer period of time, it absorbs the water. The actual holding capacity of the soil depends on its particle size or surface area. On the other hand, fine sand has a low water holding capacity. The size of fine sand particles usually range between 0.063 mm and 0.2mm hence water can easily and quickly escape from these particles. However, clay has smaller soil particles and therefore its surface area is larger than that of fine sand. Hence clay under a heavy foundation will absorb and hold more water for longer than fine sand.

Total stress:

Total stress = effective stress x pore pressure

Total stress (sand) = 19 x 3 = 57 kN/m²

 Total stress (clay) = 20 x 1 = 20 kN/m²

Total stress (water) = 20 x 2 = 40 kN/m²

Total stress = 57 + 20 + 40 = 117 kN/m²

Effective stress:

Effective stress (sand) = 19 x 3 = 57 kN/m²

Effective stress (clay) = 20 x 1 = 20 kN/m²

Effective stress (water) = (20 – 10) x 2 = 20 kN/m²

Effective stress = 57 + 20 + 20 = 97 kN/m²

Effective stress after lowering water table:

Effective stress (sand) = 19 x 3 = 57 kN/m²

Effective stress (clay) = 20 x 3 = 60 kN/m²

Effective stress = 57 + 60 = 117 kN/m²

Pore pressure in soil particles usually changes depending on the level of water table and seepage flow conditions. Soil saturation causes negative pore pressure. The negativity of pore pressure is also dependent on different sizes and types of grains. Water table is observed when the water’s natural static level becomes constant or is maintained. Therefore if seepage flow is not maintained, water table is expected to be parallel. This means that the pore pressure at that point on the water table is zero. Figure 13 below demonstrates how pore pressure changes below the water table

Figure 13: Changing pore pressure below water table

Consolidated drain (CD)

This test involves shearing and consolidating the soil sample slowly so as to give pore pressures adequate time to build up gradually by preventing water from evaporating. By controlling the strain, it is possible to sustain axial deformation rate. CD test allows the sample and pore pressure to adjust to the neighboring stresses thus becoming stronger. However, the test requires a lot of time for the sample and pore pressure to adjust. For instance, samples with low permeability will need more time before their strain can adjust to surrounding stress levels.

Consolidated Undrained (CU)

This does not allow the sample to be drained. Shear quality of the sample is measured when the sample is in undrained state, where the sample is apparently fully saturated. Nevertheless, consolidated drained strength of the sample can be estimated by measuring its pore pressures.

Unconsolidated Undrained (UU)

This test involves applying loads at a faster pace hence the sample has not time to consolidate during the course of the test. In UU test, compression of the sample is done at a normal rate.

Permeability means the rate at which the soil allows water to flow through it. Rocks are impermeable since they do not allow flow of water through them but soils are generally pervious because they allow easy flow of water through them, such as gravel. However, some soils are impermeable, such as clay.

The test methods compared in this report are constant head and falling head permeability tests.

Equipment:

The equipment needed for this test are: weighing balance with accuracy of 0.1 g, vacuum pump, filter paper, constant head tank, and permeability mould (effective height of 27.3 mm and internal diameter of 100 mm).

The setup of the experiment is as shown in Figure 14 below

Figure 14: setup of constant head permeability test

Procedure:

The test starts by measuring the mould’s internal dimensions followed by applying grease on its internal surface. About 2.5 kg of soil is taken from the mixed damp soil inside the mould. A suitable compaction tool is used to compact the wet soil at a desired dry density. After that, the collar plate and base are removed, excess soil is trimmed until the soil level is equal to the level of the mould then the external surface of the mould is cleaned. The mass of the soil inside the mould is then measured. A small sample of soil inside the container is taken and put aside to be used for determining water content. The next step is to saturate the permeable rock. The permeable rock is placed on the drainage base with the filter paper placed on the rock. The mould containing the soil is placed on the drainage base then the permeable rock with the filter paper are placed on top of the sample. The inlet cap of the drainage is then connected to the constant head tank. The valve is opened so that water can run down to remove all air particles present then the valve is closed. The valve is then opened and a timer started simultaneously. All data about water flowing from the base into the measuring jar is collected at appropriate time interval. Lastly, the difference of head between outlet of the constant head tank and base tank is measured.

Equipment

The equipment needed to perform this test include: weighing balance with accuracy of 0.1 g, vacuum pump, filter paper, constant head tank, permeability mould (effective height of 27.3 mm and internal diameter of 100 mm) and permeameter.

Procedure

This test starts by placing the remoulded soil specimen into the permeameter and saturating it there. The permeameter mould is then left at the bottom tank, which is then filled with water until the water reaches the outlet. The mould’s water inlet nozzle is connected to the stand pipe filled with water. The setup is left untouched until when the water is steady. After that, the stand pipe valve is opened and the timer started simultaneously then the time taken for the head to fall from h₁ to h₂ is recorded. So get more accurate results, the test is repeated at least twice.

The difference between the two tests:

The difference between these two tests is that falling head permeability test is used for testing permeability of soils that are less permeable such as clay while constant head permeability test is used for testing permeability of soils that are more permeable such as sand.

Permeability of soil is affected by a variety of factors, including: void ratio, water impurities, soil mass structure, soil particles shape, particle size, and pore fluid properties.

When soil is compacted, its engineering properties tend to be different hence its behavior will also be different. Therefore compaction usually reduces permeability of soil. Generally, compaction affects the following five engineering properties of soil: permeability, pore pressure, compressibility, shrinkage & swelling, and stress-strain relationship.

Equipment

The equipment needed to perform this test include: mould (with mould body, collar, base plate and guide sleeve), water and soil sample. The test determines the amount of air that can be compressed from the soil using heavy compacting devices like a heavy roller. The test also determines optimum moisture content that provides maximum compaction. The test is usually used for determining the relationship between a compacted soil’s moisture content and density.

Procedure

This test is performed in a laboratory so as to regulate optimal water content for the soil to attain its maximum dry density. The test requires that soil samples ware compacted at predetermined water content, inside a mould using standard compaction energy. The mould used in this test is 4 inches in diameter. Three different soil layers are compacted, one at a time. Each layer is compacted 25 times using a hammer until it falls to about 12 inches. The test starts by air drying the soil sample then dividing it into 6 specimens. Water content of these specimens can be changed by simply adding water to the specimen. After compacted the soil, it is placed in the proctor mould in 3 layers with each layer being compacted 25 times with a hammer. But before a subsequent layer of soil is added, the top layer’s surface is scratched to ensure that the soil is spread constantly. At the end of the test, the soil specimen is removed and dried then dry density and water content of the soil specimen is regulated for every test. The results obtained are used to plot a curve for dry density. The curves are then used to determine optimal water content at which dry density is highest. 

Different methods can be used to obtain undisturbed soil samples for analysis so as to establish the suitability of a construction site for the proposed construction work or for any further studies. The two methods that will be discussed here are: boreholes and trial pit.

Boreholes

This is one of the commonest methods of obtained undisturbed soil samples. Various in-situ testing methods such as borehole vane testing, packer testing and permeability testing can be performed in boreholes. Engineers take undisturbed soil samples from the boreholes for inspection and making a log then proceed to perform various lab tests.

There are various borehole methods including rotary boring, auger boring, wash boring, auger & shell boring and precaution boring.

Rotary boring is a quick process where a hole is made in rocks or soil using a rotating boring device called drill rod. A drill is fixed to the drill rod’s lower part and the entire device is always in contact with the hole’s base.

Auger boring can be operated manually or mechanically. The method is used to obtain samples of cohesive soils and soft soils above the water table. There are 2 kinds of auger hole drillers: spiral auger and post hole auger, as shown in Figure 15 below

Figure 15: Spiral auger and post hole auger

Wash boring is easily used in all types of soils but is unable to penetrate boulders or rocks.

Auger & shell boring is suitable for all kinds of clay soils. The equipment used in this method comprises of a shell that is able to penetrate very stiff and hard clay. The method is used to make very deep borings. It can also be mechanized and dig up to 50 m deep.

Precaution boring can be used in all types of soils. However, water has to be added in the hole as the hole is being bored. In this method, heavy chisels or drill rod continuously breaks down soil and rock formations.

Trial pit

This method is more efficient because it is suitable for different types of soils. It is most suitable when analyzing sites for low rise construction projects. There are different methods of carrying out trial pitting, including manual digging of pits and digging trenches using machines. An excavation plant can dig up to 4.5 m deep depending on soil conditions. Trial pit methods are usually conducted by engineers who are experienced in geology and soil mechanics. Most tests can also be performed in trial pits, including in-situ strength testing and soak away testing. When machines are used, shallow trenches of up to 6 m deep can be dug.

There are 3 kinds of soil samples: altered, disturbed and undisturbed soils.

Altered soil: when a trench is being bored, there is a possibility of the soil being changed as a result of contamination or mixing of the soil. For example, using one boring on different sites without cleaning it will contaminate the soil. The fluid present in the drills can also contaminate the soil sample, especially when the method being used is wash type boring. Soil also get altered if it gets densified by over pushing or over driving. The process is likely to squash water from the soil. Additionally, soil that experiences a change in their moisture content is considered to be altered because soil drilling may produce heat, and the moisture content can also be changed by the drilling fluid.

Disturbed soil: this is soil that has changed in the process of sampling. An example of a disturbed soil sample is the one obtained from driven samplers like unbroken soil pieces found in an auger bucket. In most cases, this kind of soil is slightly disturbed and therefore lab testing can still be performed on them. Disturbed soil samples are suitable for strength and consolidation tests.  

Undisturbed soil: it may not possible to extract a perfect soil sample from the ground but a theory has been established to make sure that the sample is undisturbed. To collect an undisturbed soil sample, sharp cutting ends are driven into the ground then this walled tubes are slowly pushed into the soil. However, this method must be performed by a professional operator. The method is difficult for obtaining soils with big gravels, which requires a slotted casing.

In-situ testing methods include: standard penetration testing (SPT), well test, cone penetration testing (CPT) and dynamic cone penetrometer (DCP). The method discussed here is SPT. This is a simple and less expensive in-situ testing method and provides very useful information of the soil conditions. In this method, a sample tube is put at the bottom of a borehole then a heavy hammer is plummeted on top of the sample tube repetitively for 30 inches. The number of hammer strikes needed to push the sample tube 6 inches must be recorded. After that, the tube is taken from the borehole then the sample soil in the tube is removed. Thereafter, the borehole is dug deeper and then the test is repeated so as to get more accurate results. This test helps in obtaining information used to have a better understanding of the type of soil that is underground. The method is suitable for use in areas that cannot be easily accessed by vehicles.

Examples of lab testing methods include: constant head permeability test, falling head permeability test, triaxial compression test, and triaxial permeability test, among others. The method discussed here is falling head permeability test. This method entails passing water through a short soil sample that is connected to a standpipe used for providing the water head. With this kind of setup, it is possible to obtain the amount of water flowing through the soil sample. An oedometer cell can also be used to perform this test.

Before measuring water flow, de-aired water must be filled in the standpipe to a particular level and the soil sample has to be saturated. After doing this, water is allowed to flow through the soil sample. The flow continues until the head in the standpipe reaches a particular limit. The time taken for the water in the standpipe to drop from upper to lower level must be recorded. The method is repeated several times so as to get more accurate results.

Results obtained from the in-situ test and laboratory test are difference because the former is performed in an outside environment while the latter is done in an internal environment. This means that soil samples used in laboratory testing may be disturbed while those used in in-situ testing are likely to be undisturbed.

Vane test: this is an in-situ testing method used for estimating undrained shear strength of clay soil that is saturated clay without having to disturb or alter the soil. This test is quite simple and fast but the results obtain may not be correct or accurate if the clay soil tested contains silt or sand. With proper supervision, vane test can also be performed in the lab on undisturbed soil sample.  

This test can be performed either on land surface or at the bottom of a pit or borehole. The apparatus used in the test encompassing a vertical steel rod, containing 4 thin stainless steel vanes fixed at the bottom of it. Figure 16 below is an example of a vane test apparatus

Figure 16: Vane test apparatus

Vane test is can also be used to obtain information about the soil’s sensitivity. One of the disadvantages of vane test is that it cannot be used to test clay soil containing silt or sand, or fissured clay. Also, results obtained from this test will not be accurate if failure envelope of the soil tested is not parallel.

Shear box test: this is also called direct shear test. The test can be performed in the lab or on field. Geotechnical engineers use this test to determine and obtain information about discontinuities in soil masses and rocks, including shear strength properties. This test is performed on a soil sample that is placed in a shear box separated into 2 halves along the middle of the parallel plane. The separated planes are connected by locking pins. Figure 17 below is an example of a shear box test apparatus

Figure 17: Shear box apparatus

Shear box test is most suitable when performing drained shear tests on cohesion less soils. The process of obtaining the soil sample is very easy. The method is also quick because sample thickness is small hence drainage is fast. In addition, the shear box apparatus is very cheap. However, this method also has some disadvantages. The first disadvantage is that it is very difficult to control drainage conditions when performing this test. Another disadvantage is that it is only possible to see stress conditions at failures. Lastly, area of shear reduces slowly as the test continues. 

Volume of soil = 134 cm³, mass of dry soil = 221 g, mass of wet soil = 230 g, density of soil = 2650 kg/m³

Void ratio, e

 = 4%

When soil is saturated, S_r = 1.0, assuming that the soil particles have a specific gravity, G_s, of 2.7

e = moisture context x specific gravity = 0.04 x 2.7 = 0.108

Percentage/degree of saturation, S_r

 = 72%

Porosity

 = 0.0148

Atterberg limit test entails measuring different water contents of the soil, including liquid limit, shrinkage limit and plastic limit. When the soil has the capacity to absorb a lot of water, the soil will exhibit very different behavior and consistency. Atterberg test can be performed on soils with the following conditions or states, depending on their moisture content: solid state, semi-solid state, liquid state and plastic state.

The soil’s consistency and behavior will be different in each condition. The boundary between two conditions can also be different depending on how the soil’s behavior changes. The method can be used for differentiating different types of silts and clays.

Shrinkage limit is found between semi-solid state and plastic state while liquid limit (LL) is found between plastic state and liquid state. Therefore if a soil sample is in liquid state then dried, it transforms from LL to semi-solid state then to plastic limit (PL), shrinkage limit and then to plastic state after which it reaches the solid stage. PL, LL and shrinkage limit are expressed in terms of water content. These limits are where soil changes from liquid state to plastic state.

The difference between LL and PL is known as plasticity index (PI). LL is basically the moisture content at which soil changes from liquid state to plastic state. PL is also used for soil classification.

The completed table is as shown in Table 4 below

Amount of penetration (mm)	Weight of sample (g)	Weight of sample after drying (g)	Moisture content (%)
5.3	12.20	11.35	7.5
9.8	10.90	9.82	10.99
15.9	11.15	9.18	21.45
21.8	10.05	7.89	27.37

Table 4: Completed table

Equipment

The equipment needed for this test are: oven, cone penetrometer, sieve and balance. The soil sample has to be prepared first by obtaining it from the field and drying it in the oven at a constant temperature of 60 °C.

Procedure

The soil sample is passed through 425 micron IS sieve then about 1.20N of the sample is taken. The soil is mixed with purified water to create a uniform paste. The wet soil paste is transferred to a cylindrical cup inside the penetrometer apparatus, ensuring that there is no air trapped in the soil. The soil paste is levelled at its top then placed at the cone penetrometer’s bottom. After that, the cone penetrometer apparatus is changed so that the soil paste inside the cup is in contact with the cone point. This is followed by releasing the perpendicular clamp for the cone to slump the paste for about 5 seconds then the value measured is recorded to nearest mm. This process is repeated several times to obtain at least 4 sets of penetration values ranging between 14 mm and 28 mm. Results obtained are used to plot a graph of water content against cone penetration. A straight line is then drawn then LL is obtained as moisture content equivalent to cone penetration of 20 mm.

Accuracy of LL is not dependent on the operator’s skills like it is for PL. In most cases, the operator conducting PL test should be very experienced and the test is not quite reliable. 

The completed table is as shown in Table 5 below

Area of cross section = πd²/ 4 = (π x 38²)/ 4 = 1134.11 mm²

Major principal axis stress =

Pressure in the Lucite cell, kPa	Axial Loading, kN	Vertical Deformation, mm	Area of cross section of sample, mm2	Major Principal Axis Stress, kPa
100	75	3.9	1134.11	71.15
250	155	4.6	1134.11	108.04
400	235	6.2	1134.11	215.83

Table 5: Completed table

The three Mohr’s circle are as shown in Figure 18 below

Figure 18: Mohr’s circles

The main difference is that shear box test is completed more quickly than triaxial test and it does not include weakest plane’s strength while triaxial test does. Triaxial test can also be used to perform drained and undrained tests and also to measure soil samples’ pore pressures.

The graph of shear strength against applied pressure is as shown in Figure 19 below

Figure 19: Graph of shear strength against applied force

Difference between reading at the top and bottom = 230 mm – 20 mm = 210 mm

Cross sectional area, A = πR² = π x 32² = 3217 mm²

Average of Q = (45+52+57+46+43)/ 5 = 48.6 ml/ 120 s = 0.405 mm/s

This is a geotechnical test used to measure properties of soil compaction. In this test, different types of loads are applied on the soil sample so as to impose pressure. The deformation response of the soil is then measured.

This is lab test used to determine soil’s optimal moisture content so as to understand when the soil attains its maximum dry density.

This is a penetration test used to investigate the soil’s mechanical strength or that of natural ground.

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https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=what+is+a+proctor+test (Accessed on June 6, 2017)

https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=what%20is%20a%20california%20bearing%20ratio%20test (Accessed on June 6, 2017)

https://environment.uwe.ac.uk/geocal/SoilMech/stresses/stresses.htm#STRESSTOTAL (Accessed on June 6, 2017)

https://osp.mans.edu.eg/geotechnical/Ch1%20B.htm (Accessed on June 6, 2017)