Site investigation 8.1 Site investigation objectives The standard lists the objectives of site investigations as follow: (a) To assess the general suitability of the site and environs for the proposed works (b) To enable an adequate and economic design to be prepared, including the design of temporary works (c) To plan the best method of construction; to foresee and provide against difficulties and delays that may arise during construction due to ground and other local conditions (d) To determine the changes that may arise in the ground and environmental conditions, either naturally or as a result of the works, and the effect of such changes on the work, on adjacent work, and on the environment in general (e) Where alternatives exist, to advise on the relative suitability of different sites, or different parts of the same site In Civil Engineering, site investigations are necessary (i) when reporting on the safety of existing works (ii) where alteration to existing works is being planned (iii) when investigating cases where failure has occurred and (iv) when looking for sources of constructional materials In most projects, it is advisable that site investigation is carried out and finished before the design stage of the project. There might be some overlap in the case of major works and where in-situ testing is required. 8.2 Site investigation stages There are several stages involved in site investigation- (1) DESK STUDY-the collection of information related to the site e.g. maps, drawings, geological maps, etc (2) SITE RECONNAISSANCE-examination of the site by appropriate experts e.g. geology, land surveyor, soil engineer, hydrologist, etc. (3) DETAILED SITE EXPLORATION AND SAMPLING-(a) investigating of detailed geology and subsurface soil conditions using trial pits, surface surveys, boreholes, geophysical methods; (b) survey of groundwater; (c) examination of existing and adjacent structures for signs of cracking or settlement; (d) location of underground structures or cavities, services, buried pipes, etc; and (e) provision of samples for further examination and laboratory testing (4) LABORATORY TESTING OF SAMPLES-soil/rock testing (5) IN-SITU TESTING-testing on the site either prior to or during the construction process (6) REPORTING RESULTS-details of geological study, including structures, stratigraphy and mapping; results of borings, etc.; comments and recommendations relating to the design and construction of the proposed works; recommendations relating to further investigating or testing, and to ongoing or post- completion monitoring. It should be noted, however, that the extent and scope on a site investigation is partly dependent on the nature of the site and the type of the structure to be constructed. 8.3 Site exploration Site exploration refers to sub-surface investigation of rocks and soils, and is aimed at building up a three-dimensional picture of the site, which extends laterally and vertically to include all of the strata likely to be affected by the changes in loading, etc., brought about by the proposed construction. 8.4 Site exploration methods The choice of site exploration method depends on (a) GEOLOGICAL NATURE OF THE SITE e.g. clay soils-boreholes are suitable sandy soils-lined boreholes and special sampling equipment may be required firm/compact soils or rocks above water table-trial pits are practicable (b) TOPOGRAPHICAL NATURE OF THE SITE: the terrain and access to the site are very important in relation to the exploration equipment to be used (c) TYPE OF INFORMATION REQUIRED: the main information required includes the nature and sequence of sub-surface rocks/soils, but specialist information may be required i.e. details of joint geometry, ground water flows, presence of buried features (either natural or man-made), location of previous failure surface, etc. (d) COST AND TIME: the cost and time of site exploration increase with the increasing depth of exploration 8.4.1 TRIAL PITS Trial pits are suitable for cohesive soils and soft rocks can easily be dug by hand or with a mechanical excavator. Their main advantage is that they expose the succession of strata for easy visual examination. The main disadvantage is that they are limited to depths of 2-3cm. The sides of the trial pits must never be assumed to be stable as they may collapse and cause deaths on the construction sites. The sides must be supported at all times if the pits are to be entered. 8.4.2 HEADINGS OR ADITS These are excavated almost horizontally, either from the surface steeply inclined ground, from cliff or quarry face, or from bottom of shafts. The disadvantage of headings is that their driving cost is very high and in addition the operation is difficult in loose ground or below water table. Headings are therefore not very commonly used, except for investigations, such as pilot tunnels, mineral exploration surveys or where other methods are unsuitable. 8.4.3 HAND AUGER The hand auger (also known as a Post-hole or Iwan-auger) is a very simple hand-tool used for drilling into soft soils down to a maximum of 5-6cm. 8.4.4 PERCUSSION BORING The most common method of percussion boring is the shell or bailer method. The percussion boring equipment is relatively simple, portable and in soft to very firm soils free from boulders or cobbles a good rate of progress can be maintained. The most usual borehole diameter is 150 mm, but others up to 300 mm can be drilled; the maximum depth of exploration, although dependent on soil type to some extent, is around 50-60 m. The clay-cutter is particularly used for clay soils in dry boreholes. A clay-auger is occasionally used; this is rotated by hand to advance the drilling; it is also used to clean the bottom of the hole before sampling. The sand shell or bailer is used for boring in sand and gravels. In compact cohesionless soils, or where boulders or cobbles are encountered, the chiesel is used to break up hard materials; fragments and slurry are then removed using the bailer. In wet conditions and in loose soils, and for very deep holes, a casing must be installed near the surface. This usually consists of steel tubes, screwed together in as many lengths as appropriate, and jacked or knocked into the drilled hole as drilling proceeds. They can be hauled out after completion of drilling or left in place if further observations are required. 8.4.5 ROTARY AUGERS Power operated rotary augers mounted on vehicles provide an extremely portable and versatile method of drilling. Rotary augers are mostly used for exploration purposes and in pilling operations. They are available in various sizes and types; from small units mounted on ordinary tractors, to larger machines capable of drilling holes to depths of 50 m and, with certain types of auger, up to 2 m in diameter. Augers are usually referred to as flight-augers or bucket augers. (a) (i) Short flight augers consist of a short-length helical blade attached to the shaft, with cutter teeth and a small diameter pilot bit at the lower end. Short flight augers are available in diameters between 75 and 500 mm, and with some special devices as large as 1 m. For some larger auger the helix comprises only one revolution-these are called single flight augers (ii) Continuous flight augers are used to enable the cut soil to rise up the spiral to the top of the hole, thus removing the necessity to lift and clear the tool a number of times; as the drilling advances, additional spiral sections are added. (b) Bucket augers consist of an open-top metal cylinder with cutters mounted on a baseplate. As a soil is cut it passes into the bucket that is then raised and emptied at intervals. Bucket augers can be used to drill holes rapidly in firm soils with diameters ranging from 300 mm to 2 m. However; they are not suitable for drilling in cohesionless soils below the water table. 8.4.6 CORE DRILLING The power-operated core-drills are used in hard soils and rocks. The core drill consist of small- diameter hollow tube fitted with a coring bit at the lower end. The core barrel is rotated at speeds ranging between 600 and 1200 rpm, a controlled pressure applied and water circulated through the bit. The more usual standard sizes of core barrel used in site investigation range between 30 and 100 mm (hole diameter) although a larger-diameter equipment is available for special uses. 8.5 Methods of sampling There are two main categories of soil samples: (a) Undisturbed samples: in which the structure and moisture content is preserved as far as possible to truly represent site condition. Undisturbed samples are required for tests of shear strength, consolidation and permeability and are usually obtained by a suitable percussion tube or coring method. (b) Disturbed samples: these should be collected as drilling or digging proceeds, where possible attempting to preserve the in-situ moisture content. Disturbed samples are mainly required for soil identification and for classification and quality tests; as samples are collected they are placed and sealed into glass or plastic containers , or tins, or plastic bags 8.5.1 OPEN-DRIVE SAMPLER The simplest and most common form of sample tube is called an open-drive sampler and it consist of a steel tube with a screw thread at each end. A cutting shoe, and sometimes an extension piece is screwed on to the lower end; a sampler head, which incorporates a non-return valve, is screwed-on to the upper end. As the sample is driven into the valve allows air and water to escape but it remains closed as the sampler is raised to the surface thus holding the sample within the tube. British Standard (BS 5930 : 1981) recommends that the diameter of the cutting shoe (Dc) be typically 1 % less that the internal diameter of the tube (Ds), so as to provide clearance and to reduce the frictional resistance between the sample and the tube. Also the external diameter of the cutting shoe (Dw) should be slightly greater than that of the tube, so as to reduce the force required to withdraw the tube from the hole. The volume of the soil displaced by the sampler in proportion to the volume of the sample is given by the area ratio (Ar) i.e. Ar = Dw2 Dc2 *100% Dc2 To reduce sample disturbance, the area ratio must be kept as low as possible consistent with maintaining sufficient strength in the wall of the tube. Open-drive sampler is suitable for all types of cohesive soils, but a flap-valved extension piece may be needed for loose or wet sands. 8.5.2 THIN-WALLED SAMPLER Using a thin-walled tube (Shelby Tube) with a sharp integral cutting edge rolled slightly inwards gives an area ratio of about 10 % and an inside clearance; so these tubes are suited to soft silts and clays which are sensitive to sample disturbance. Thin-walled samplers are available in diameters ranging from 35 to 100 mm and in a variety of lengths. 8.5.3 SPLIT-BARREL SPT SAMPLER This sampler is used in conjunction with the Standard Penetration Test. Two half-cylinders are jointed together by a threaded open steel cutting shoe at the bottom and threaded onto drilling rods at the top. A sample is obtained following removal from the borehole, by undoing the two semi-cylindrical halves of the sampler. Due to inside friction, a full-length sample is seldom obtained and, because the cutting shoe has thick wall with an outside diameter of 50 mm and an inside diameter of 35 mm, giving an area ratio of more than 100 %, the samples are highly disturbed. 8.5.4 PISTON SAMPLER For very soft alluvial silts and clays it will be necessary to use a piston-sampler consisting of a thin- walled tube fitted with a piston. The sampler is available in diameters between 20 and 120 mm with lengths between 0.15 and 1.5 m but the 100 mm diameter 600 mm long sampler is the most common. The sampler produces good quality, undisturbed specimens because it is pushed beneath any disturbed zone, and it has a low area ratio. 8.6 In-situ testing In certain soils, such as soft sensitive cohesionless soils, it is difficult (almost impossible occasionally) to obtain good undisturbed samples. It is difficult also accurately to model in laboratory truly representative conditions of structure and/or pore pressure under certain site conditions e.g. in very soft alluvium. A number of relatively simple in-situ testing procedures have therefore been devised which will enable good estimates of soil properties to be made under actual site conditions. Although in in-situ testing the degree of accuracy and control possible as lower than would be expected in the laboratory, this is often compensated for by a large number of tests being carried out. 8.6.1 CORE CUTTER TEST 8.6.2 SAND REPLACEMENT TEST 8.6.3 STANDARD PENETRATION TEST 8.6.4 CONE PENETRATION TEST 8.6.5 SHEAR VANE TEST 8.6.6 PLATE BEARING TEST 8.7 Site investigation reports Site investigation report covers the site investigation, site exploration and testing programme. This report will normally include the following: (a) INTRODUCTION: a brief summary of the proposed works, the investigations carried out, the location of the site and significant names and dates. (b) DESCRIPTION OF SITE: a general description of the site: its topography and main surface features; details of access; detail of previous development or relevant history; details of existing works, underground opening, drainage, etc.; a map showing site location, adjoining land and borehole locations (c) SOIL CONDITION: a detailed account of the soil conditions encountered, related to the design and construction of the proposed works; description of all relevant layers, together with results of laboratory and in-situ tests; details of groundwater and drainage conditions. (d) CONSTRUCTION MATERIALS: a detailed account of the nature, quantity, availability and significant properties of materials considered for construction purposes. (e) GEOLOGY OF THE SITE: description of main soil and rock formations and structures; comments on the influences of geology on design and construction. (f) COMMENTS AND RECOMMENDATIONS: comments on the validity and reliability of the information being presented. (g) APPENDICES: it is convenient to assemble most of the collected data into a series of appendices: boreholes log; laboratory test details and results; results of in situ tests; references.