Site Investigations PDF

Site Investigation  Definition. A site investigation simply is the process of the collection of information, the appraisal of data, assessment, and reporting without which the hazards in the ground beneath the site cannot be known. Site Investigation  Site investigations are performed to obta...

Site Investigation  Definition. A site investigation simply is the process of the collection of information, the appraisal of data, assessment, and reporting without which the hazards in the ground beneath the site cannot be known. Site Investigation  Site investigations are performed to obtain information on the physical properties of soil and rock around a site to design earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions. This type of investigation is called a site investigation  Additionally site investigations are also used:  To measure the thermal resistivity of the soils or backfill materials required for underground transmission lines, oil and gas pipelines, radioactive waste disposal, and solar thermal storage facilities.  A geotechnical investigation will include surface exploration and subsurface exploration of a site.  Sometimes Geophysical methods are used for collecting data about sites. Surface Exploration  Surface exploration includes  Geologic mapping, geophysical methods and photogrametry.  Or it can be as simple as a geotechnical professional walking around on the site to observe the physical conditions at the site.  To obtain information about the soil conditions below the surface, some form of subsurface exploration is required. Objectives of site investigation ❖The objective of a site investigation is to gather the information needed to carry out the risk assessment, , in order to be in a position to assess the presence and significance of contamination of land. ❖Samples of soil can be taken at various depths using a number of pieces of equipment – ranging from simple hand tools to rig mounted drilling systems ❖In addition to sampling of soils, the investigation may include the sampling of water and gases that may be present in the ground. ❖The information gathered enables the risk assessment to be carried out to conclusions in which an acceptable degree of confidence can be placed. ❖At any stage of an investigation, the overall objectives will be to characterize the contaminants present and to identify pathways and receptors for the purposes of the risk assessment. ❖The information required to carry out the risk assessment to a robust conclusion should be identified before designing or planning an investigation. The objectives of a site investigation will vary, depending upon the stage in the process that has been reached, and the underlying intentions for the land involved. Stages of site investigation ❖The process of site investigation can be separated into following stages. ❖ Objectives of site investigation. Desk study. Site Reconnaissance. Ground investigation Trial Pits Boreholes Sampling ▪ Reporting Desk Study  The desk study is work taken up prior to commencing the work on site and the Ground Investigation.  It should always be the first stage of the Site Investigation and is used to plan the Ground Investigation.  The work involves researching the site to gain as much information as possible, both geological and historical.  Geological Maps and memoirs are probably the most important source of information as these give an excellent indication of the sort of ground.  Ariel Photography is another extremely useful source of information.  Records of Previous Site Investigation reports are also helpful in a desk study.  Services records are also an essential part of the desk study. Desk Study of Maps Desk Study of Aerial Photographs Site Reconnaissance  The Site Reconnaissance phase of a site investigation is normally in the form of a walk over survey of the site.  Information should be collected on the overall site layout, topography, basic geology.  Local conditions should be examined such as climate, stream flows, ground water conditions, site utilization related to weather and time of year.  Possible photographic records should be kept.  Important evidences to look for are as follows:  Hydrogeology: Wet marshy ground, springs or seepage, ponds or streams and wells.  Slope Instability: The signs of slope instability show a bent trees, hummocks on the grounds and displaced fences or drains  Mining: The presence of mining is often signs of subsidence and possibly disused mine shafts. Open cast mining is indicated by diverted streams replaced or removed fence/hedge lines.  Access: It is essential that access to the site can be easily obtained. Possible problems include low overhead cables and watercourses Ground Investigation  Ground investigation is taken to be that other than the information available from the walk over survey as discussed previously. Trial Pits Trial pits are shallow excavations going down to a depth no greater 6m. The trial pit as such is used extensively at the surface for block sampling and detection of services prior to borehole excavation. Trial Pits Types of Samples from Trial Pits  Three types of sample can be taken from a trial pit:  Disturbed Sample - Samples where the soils in-situ properties are not retained.  Block Sample - A sample that is not undisturbed but retains some in-situ properties.  Push in tube sample - Tube samples of the soil in a trial pit. Boreholes  A borehole is used to determine the nature of the ground (usually below 6m depth) in a qualitative manner and then recover undisturbed samples for quantitative examination. Where this is not possible, for in gravelly soils below the water table, in-situ testing methods are used.  Obviously the information gained from a borehole is an extremely limited picture of the subsurface structure. It is therefore essential to compare the results obtained with those that could have been expected from the desk study. The greater the number of boreholes the more certain it is possible to be of the correlation and thus to trust in the results. Sampling  SPT test: This is a measure of the density of the soil  Core Sample: Core samples must be sealed with paraffin to maintain the water conditions and then end sealed to prevent physical interference.  Bulk Samples: Usually taken from trial pits or in soils where there is little or no cohesion. Reporting  The Site Investigation report for a highway design scheme should answer all the questions set out in the planning phase of the Investigation. This should include an assessment of the viability of the proposed route and indication of any alternatives.  Included in the report should be a location of all the boreholes, trial pits, other excavations and their logs. These logs should give as much information as possible on the soil and rock structure as it is possible to obtain. Soil Sampling  Borings come in two main varieties, large-diameter and small-diameter.  Large-diameter borings are rarely used due to safety concerns and expense, but are sometimes used to allow a geologist or engineer to visually and manually examine the soil and rock stratigraphy in-situ.  Small-diameter borings are frequently used to allow a geologist or engineer to examine soil or rock cuttings or to retrieve samples at depth using Soil Samplers and to perform in place soil tests.  Soil Samples are often categorized as being either ‘Undisturbed Samples’ or ‘Disturbed Samples’.  However ‘Undisturbed samples’ are not truly undisturbed.  An undisturbed sample is one where the condition of the soil in the sample is close enough to the conditions of the soil in-situ to allow tests of structural properties of the soil to be used to approximate the properties of the soil in-situ.  A Disturbed sample is one in which the structure of the soil has been changed sufficiently that tests of structural properties of the soil will not be representative of in-situ conditions, and only properties of the soil grains (e.g., grain size distribution, Atterberg Limits and possibly the water content) can be accurately determined. Off-shore Soil Samples  Offshore soil collection introduces many difficult variables.  In shallow water, work can be done off a barge.  In deeper water a ship will be required.  Deepwater soil samplers are normally variants of Kullenberg-type samplers, a modification on a basic gravity corer using a piston (Lunne and Long, 2006).  Seabed samplers are also available, which push the collection tube slowly into the soil. Soil Samplers  Soil samples are taken using a variety of samplers; some provide only disturbed samples, while others can provide relatively undisturbed samples. Shovel; Samples can be obtained by digging out soil from the site. Samples taken this way are disturbed samples. Trial Pits; are relatively small hand or machine excavated tranches used to determine groundwater levels and take disturbed samples from. Shovel Trial Pits  Hand/Machine Driven Auger. This sampler typically consists of a short cylinder with a cutting edge attached to a rod and handle. The sampler is advanced by a combination of rotation and downward force. Samples taken this way are disturbed samples. Continuous Flight Auger. A method of sampling using an auger as a corkscrew. The auger is screwed into the ground then lifted out. Soil is retained on the blades of the auger and kept for testing. The soil sampled this way is considered disturbed. Hand Driven Auger Machine Driven Auger  Shelby tube; the thin walled seamless steel tubes are called shelby tubes. It is generally used to take undisturbed samples for routine site investigation works. Piston sampler; when soils are much sensitive to disturbance or soil consist of silts and silty sands, which have some cohesion piston sampler is preferred. It ensure the availability of samples of first class quality. Shelby Tubes  Scraper bucket; when soil deposits are sand mixed with pebbles, obtaining samples by split spoon may not be possible with a spring core catcher and a scraper bucket may be used.  The scraper bucket has a driving point and can be attached to a drilling rod. The sampler is driven down into the soil and rotated and the scrapings from the side fall into bucket. Standard sampler(Split Spoon Sampler); has an inside diameter of 35mm and an outside diameter of 50mm.  These are used for visual examination and for classification test. Split Spoon Samplers Dam Site Investigations  The primary purpose of geological site investigations for a dam project is provide the information that dam designers require in order to design a safe dam structure and to be able to estimate with reasonable accuracy how much the dam is going to cost.  The aim of the dam designers is to build the dam for the lowest cost consistent with currently accepted standards of safety. Investigation at dam site  The following investigation should be done at the dam site are as:  Regional Geological Mapping  Dam Axis Survey (Left/Right)  Landslide Possibility Survey  Environmental Impact Dam Site Investigation Methods  Geological mapping of surface rock outcrops.  Geophysical surveys; Seismic refraction is often used to determine depth of overburden.  Excavation of trenches and pits using bulldozers, backhoes etc.  Diamond core drilling; As usually carried out this method recovers an undisturbed, cylindrical sample (a core sample) about 50 mm in diameter from depths of a few meters to hundreds of meters, if necessary.  Other types of drilling which recover disturbed samples may also be used in some circumstances. Site Investigation For Bridges Introduction  Site investigation is done to assist in the planning cost investigation, conduct and reporting of investigation at bridge sites. Objectives  Describe stages of work to be followed for a bridge site investigation.  Describe information required to design bridge foundations.