Chapter 1 - Site Investigation PDF

Construction Technology I Module Code: ECE 10207 Chapter 01 Site Investigation Dr. Sitesh Kumar Singh Introduction to Site Investigation 1. Rocks and Classification of rocks 2. Soils and Soil classification 3. Site Investigation  1.Rocks and Classification of rocks What a...

Construction Technology I Module Code: ECE 10207 Chapter 01 Site Investigation Dr. Sitesh Kumar Singh Introduction to Site Investigation 1. Rocks and Classification of rocks 2. Soils and Soil classification 3. Site Investigation  1.Rocks and Classification of rocks What are rocks? A rock is a naturally formed, consolidated material usually composed of grains of one or more minerals. How rocks can be classified? There are three different ways rock can be created on earth and thus there are three main classifications of rock, based on their formations – Igneous: Volcanic and plutonic rocks – Sedimentary, and – Metamorphic. Igneous Rocks:(derived from the Latin word ignis meaning fire) (formed through the cooling and solidification of magma or lava) Volcanic Rocks Form at the Earth's surface. Cool and crystallized from magma which has spilled out onto the surface at a volcano. Magma is more familiarly known as LAVA Plutonic Rocks Magma that cools and crystallizes beneath the Earth's surface. Portion of the magma that never makes it to the surface. Plutonic rock to become exposed at the surface, it must be tectonically uplifted and the overlying material must be removed by erosion. Example: Basalt, Granite  Sedimentary rocks These are types of rocks that are formed by the deposition of material at the earth's surface and within bodies of water. Example: Sand stone, Lime stone. The general term for the process of accumulation is DEPOSITION. The material consists of the products of weathering and erosion, and other materials available at the surface of the Earth, such as organic material. The process by which this otherwise unconsolidated material becomes solidified into rock is variously referred to LITHIFICATION (literally turned into rock), DIAGENESIS or CEMENTATION.  Metamorphic rocks These rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". Metamorphism is the transformation of rock by temperature and pressure. The original rock is subjected to heat and pressure, causing deep physical /chemical change. Metamorphic rocks are produced by transformation of Sedimentary and Igneous rocks and by the further alteration of other metamorphic rocks. Example: Quartzite, Slate  Igneous Rocks Basalt Granite Sedimentary Rocks Sand stone Lime stone Metamorphic Rocks Slate Quartzite How different types of rocks were formed? Rock / Geological cycle is continuous chemical and/or physical processes (weathering) that transform rocks from one type to another. Weathering The rocks from the earth's surface are eroded and weathered all the time by the natural forces such as the sun, the water, the rain and the ice etc. There are mainly three weathering processes: i. Physical weathering, ii. Chemical weathering and iii. Biological Weathering. (i) Physical Weathering involves physically breaking rocks into fragments without changing the chemical make-up of the minerals within them. four main sources of physical weathering: Water, Wind, temperature & Waves. Gravel and Sand are products of physical weathering. (ii) Chemical weathering takes place when at least some of the rock’s minerals are changed into different substances. Chemical processes include rain, acid etching by plants, oxidizing, and demineralizing by water. Example: Clay (iii) Biological weathering is mainly brought about by the action of living organisms including the tree roots. Powerful plant roots grow into rock cracks and cause fractures.  2. Soils and Soil classification Soils are soft loose and un-cemented deposits have lower bearing capacity (less than 400 kN/m2) than rocks and easier to excavate.  SOIL CLASSIFICATION (a) Classification of soil based on Origin (i) Residual soil: A soil formed by the weathering at the same place as parent rock. It is therefore chemically similar to that bedrock. It will also contain organic material derived from plants and animals living in the area. Example: Clay (ii) Transported soil: A soil that has been transported (eroded) and then deposited in a region where it was not originally formed. Transported soils are the result of erosion and deposition by wind, water, and ice. Transported soils may have properties very different from the parent rock. Most soils are transported.  (b) Classification of soil based on grain size Soil Type Rock/Boulder Gravel Sand Silt Clay Grain More than 200 More than 2 2 to 0.06 – 0.002 Less than size mm mm 0.06 mm mm 0.002 mm (c) Classification of soil based on cohesion (i) Cohesive soils These type of soils have a higher water content and a natural tendency to 'ball' together when squeezed. Under pressure from loads these types of soils settle and consolidate over a number of years. Example: Clay, Silt (ii) Non-cohesive soils These are granular by nature. In this type soils under load, settlement normally occurs quite quickly. Example: Gravel, Sand  Stages In Construction Activities 1. Site and soil investigation 2. Site clearing and setting out 3. Construction of building elements 4. Finishing works Introduction to Site Investigation  Importance of To assess the Before construction general suitability site work can go ahead. of the site with the investigation: proposed work. To reduce the risks To help produce a It helps to avoid any associated with the design which is unforeseen works. financing of a adequate and project. economic. To help overcome possible To predict possible difficulties and delays that changes that may To maximize may arise during construction period due to occur / cause of all potential of the ground and other local changes in site site. conditions. condition.  To determine the type of foundation To make required for the recommendations proposed project WHY? at the site, i.e. regarding the safe bearing capacity or shallow foundation pile load capacity. or deep foundation.  Attempt at understanding the subsurface conditions such as: Soil and rock profile Geological features of the region Position and variation of ground WHAT? water table Physical properties of soil and rock Contamination, if any General data of adjacent structures, hydrological data, topography, soil maps, seismicity, etc.  Definition Site investigation is: The process of determining the layers of natural soil deposits that will underlie a proposed structure and their physical properties.  PURPOSE To determine the site’s suitability for building and the nature and the extent of preliminary work that will be needed Why it is important? It will shows the detailed to many physical aspect such as subsoil composition, demolition and the legal aspect such as planning permission, right of access and preservation order  Scope of site investigations To determine the sequence, Thickness and lateral extent of soil strata and, where appropriate, the level of bed rock  The purpose (objectives) of a soil investigation program 1. Selection of the type and the depth of foundation suitable for a given structure. 2. Evaluation of the load-bearing capacity of the foundation. 3. Estimation of the probable settlement of a structure. 4. Determination of potential foundation problems (for example, expansive soil, collapsible soil, and so on). 5. Establishment of ground water table. 6. Prediction of lateral earth pressure for structures like retaining walls, sheet pile etc.. 7. Establishment of construction methods.  DESCRIPTION A combination process which range from looking at published information such as maps to arrange laboratory test on the soil  Information required from a site investigation a. Information affecting the design of the structure: shear strength and compressibility of the soil b. Information affecting the construction of the works: the extent and properties of material to be excavated, or to be used for fill or for road bases or concrete aggregates 29 c. Information on ground water conditions: the level and seasonal variation of the water table, the pressures in the soil water, and the permeability of the soil. 30  INFORMATION/ISSUES TO CHECK Site Availability Accessibility Location of space Ground Services composition  Planning an investigation In the earlier stages of an investigation, the information available is often inadequate to allow a detailed plan to be made. The investigation must therefore proceed in 3 stages: 1. Desk study 2. Site reconnaissance 3. Detail Examination of Tests and Programs 32  Site investigation 1.Desk study ✔Structure Data building, road, wall, etc. Type - storeys, loads, materials, etc ✔Known Soil Data Your own knowledge of the area Geologic or other maps Evidence of groundwater levels and flooding Aerial photos  2. Site reconnaissance Before you move any equipment to site, visit the site. A visual inspection can tell you a lot: Site access existing structures existing services evidence of old structures non-engineered fill topography condition of nearby structures Data to be collected during site reconnaissance Details of access roads Distance to building materials/plant suppliers market Boundary hedges and /or fencing Property boundary lines and location of site Existing buildings/structures Existing trees Trees and buildings on adjacent site Details of above ground obstructions such as transmission lines Contours and site levels – sloping, undulating or flat Planning or similar restrictions on proposed building or structure Full data as to type, size, depth and location of all services such as gas, water, drains, Sewers, electricity, telephone services 3. Soil Investigation Methods of site investigation... Factors which affect the selection of methods for site exploration 1.Geological nature of site The method depends on the nature of soil/rock at site 2.Topographical nature of the site Topographical nature of the site such as whether site is plane, sloping or hilly and the accessibility to the site influence the selection of method for site investigation 3. Type of information required The site investigation method depends on what type of data is required to be collected. 4. Cost and time Cost and time available for exploration also influence the selection of method. Methods of Soil Investigation A trial pit is an excavation of ground in order TRIAL PIT to study or sample the composition and structure of the subsurface. Especially useful for gravelly soil where boreholes may be difficult usually dug during a site investigation, a soil survey or a geological survey. Trial pits are dug before the construction. Permits visual inspection of subsurface conditions in natural state They are dug to determine the geology and the water table of that site. Sampling/testing done on exposed surfaces. Trial pits are usually between 1 and 4 metres deep, and are dug either by hand or using a mechanical digger. Building and construction regulations clearly state that any trial pits that concede deeper than 1.2 metres should be secured against structural collapse, if they are to be entered by people.  BOREHOLES In general, a machine used to drill holes is called a drill rig (generally power driven, but may be hand driven). A winch is provided to raise and lower the drilling tools into the hole. Hand Operated Augers The hand auger is a very simple hand tool used for drilling into soft soil. The depths of the holes are normally limited to 10 m. These augers are generally suitable for all types of soil above the water table but suitable only in clayey soil below the water table. A string of drill rods is used for advancing the boring. The diameters of the holes normally vary from 10 to 20 cm. Power Driven Augers ⮚ used in all types of soil including sandy soils below the water table but is not suitable if the soil is mixed with gravel, cobbles etc. ⮚ The central stem of the auger flight may be hollow or solid. ⮚ Holes may be drilled by this method rapidly to depths of 60 m or more. Auger Boring Wash boring a very convenient method provided the soil is sand, silt or clay (loose type of soil). To start with, the hole is advanced a short depth by auger and then a casing pipe is pushed to prevent the sides from caving in. The hole is then continued by the use of a chopping bit fixed at the end of a string of hollow drill rods. A stream of water under pressure is forced through the rod and the bit into the hole, which loosens the soil as the water flows up around the pipe. The loosened soil in suspension in water is discharged into a tub.  4. Laboratory testing of samples At the end of all subsoil exploration programs, the soil and/or rock specimens collected from the field are subjected to visual observation and appropriate laboratory testing. Tests on disturbed and undisturbed samples submitted from the site team, tests on soil for classification, quality, permeability, shear strength and compressibility tests on rock cores and samples for strength and durability, tests on constructional materials, tests on ground water. Disturbed and undisturbed soil samples Disturbed soil sample In the disturbed soil sample the natural structure and properties of the subsoil are disturbed. Example: Soil samples obtained from a trial pit will be a disturbed sample. Undisturbed soil sample In the undisturbed soil sample the natural structure and properties of the subsoil are not disturbed. Example: Soil samples taken using coring tools will be undisturbed samples.  5- In-Situ Testing Probing or sounding methods: Drive a pipe or rod into the soil. Measure the resistance offered by the soil. Ex. CPT (cone penetration test), SPT (standard penetration test) Geophysical methods: Seismic refraction method Electrical resistivity method. Standard Penetration Test (SPT) – Field Test SPT procedure and equipment follow ASTM D 1586-84, "Standard Method for Penetration Test and Split Barrel Sampling of Soils". The resistance of soils is represented by the N-value. Number of blows (N value) required to drive split barrel sampler into the soil layer 30 cm by a standard mass ( 63.5 Kg) dropped a specific distance ( 75 cm).The test results are recorded in the boring log In-Situ Testing: Standard penetration test (SPT) In-Situ Testing: Cone penetration test (CPT) cone is driven through the soil and the number of blows for each foot recorded. Indicates the depth of fill or the depth to layer changes Continuous readings of stress, friction, pressure In-Situ Testing: Geophysical methods. Two main types: 1. Electrical resistivity method Based on measurement of changes in resistivity of various soils 2. Seismic methods: Seismic refraction method Based on the fact that seismic waves have different velocities in different types of soils or rocks Also waves are refracted when they cross boundary between different soil types  4. Report Writing After the compilation of all the required information, a soil exploration report is prepared for the use of the design office and for reference during future construction work. The report should include the location of all the boreholes, trial pits, other excavations and their logs. The soil and rock descriptions should be as defined in BS5930 and should contain the information Obtains the bearing capacity of the soil