Topic Understand foundation design and construction Today’s Objective 1. Identify different types of site surveys 2. Understand the purpose of site investigation Unit 6: Building Technology in Construction Site Investigation Three stages:1)A desk study. 2)A walk-over survey 3)A physical exploration and inspection, of the ground by means of boreholes or trial pits. This third stage is sometimes called the ground investigation. Unit 6: Building Technology in Construction Site Investigation: Desk Study Stage 1 • The desk study is the first stage in the site investigation. • Essentially, it comprises the collection and analysis of existing information (records) about the site. • The information will come from a variety of sources and, and, once analysed, will form the basis for the second stage, the walk-over survey. Unit 6: Building Technology in Construction Desk Study The desk study has two main objectives: 1) to determine the nature, past use, and condition of the site. 2) to determine whether this has any implications for the proposed building and its foundations It quickly ascertains whether the proposed structure can be built on the site In groups of 2/3 consider what sort of records would you be looking for? Unit 6: Building Technology in Construction Desk Study Sources of Information •British Geological Survey •Ordnance Survey •Landmark Information Group •The Environment Agency •The Local Authority •County Records •Utility Companies and the Coal Authority •Local History Library •The site vendor •Aerial Photographs Unit 6: Building Technology in Construction Desk Study Outcome 1. A check list for closer investigation during the walk-over survey 2. Identification of potential hazards that may require specialist input Unit 6: Building Technology in Construction Site Investigation: Walk-over survey This is the second stage in the site investigation. It's a detailed site inspection which: 1. Enables much of the material discovered in the desk study to be confirmed or further investigated 2. Identifies other potential hazards 3. Enables the surveyor to collect photographic records 4. Gives the surveyor/engineer the opportunity to create a site survey; a detailed drawings of all those items (trees, existing buildings, watercourses, topography, etc) which will have implications for the building design 5. Obtains any additional information required to ensure that the building can be constructed safely and within budget. Unit 6: Building Technology in Construction Walk-over survey Unit 6: Building Technology in Construction Site Investigation: Walk-over survey The NHBC recommend that this stage is divided into 6 sections: 1. 2. 3. 4. 5. 6. Topography Soils and rocks Surface water and ground water Vegetation Structural information Local information Unit 6: Building Technology in Construction Site Investigation: Walk-over survey Topography Unit 6: Building Technology in Construction Site Investigation: Walk-over survey Soils and Rocks Unit 6: Building Technology in Construction Site Investigation: Walk-over survey Surface Water and Ground Water Site Investigation - Stage 2 Walk-over Survey- Water Unit 6: Building Technology in Construction Site Investigation: Walk-over survey Vegetation Unit 6: Building Technology in Construction Site Investigation: Walk-over survey Structural and local information Unit 6: Building Technology in Construction Topic Understand foundation design and construction Today’s Objective 1. Understand the purpose of ground investigation Unit 6: Building Technology in Construction Site Investigation: Ground Investigation A direct ground investigation is the third stage in the site investigation. It provides detailed information on:• The nature and thickness of made up ground/top soil above the subsoil. • The nature, thickness and stratum depth of subsoil • An assessment of allowable bearing pressure. • Groundwater levels, chemicals in the ground etc. • Existing structures or hazards in the ground. Key term: Geotechnical properties How soil is likely to perform when imposing loads on it or what will happen when water is removed to allow work to take place. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Ground investigations should provide the following:• Classification of soils • Soil profile • Soil parameters Key term: Soil Parameters How the soil will react to the building works and imposing loads. The soil can be expected to carry a certain weight depending on its parameters and its characteristics. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Classification of soils Soil is classified by: 1. Geological origin 2. Physical properties 3. Chemical composition 4. Particle size The simplest classification used by geologists is:• Rock • Granular soils ( sands and gravel) • Cohesive soils (clays) • Organic soils ( peat and top soil) • Fill or made up ground. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Soil profile:• Identification of the layers of soil below ground level, depth and type. • Boundaries between the various layers/types are not always distinct, which means it is sometimes difficult to tell what condition will actually be found when digging into the earth Unit 6: Building Technology in Construction Site Investigation: Subsoil survey Soil profile:• Complete the worksheet Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Soil Parameters:The load imposed by a building and the ability of the ground to support this needs to be quantified. Eg. Approved Documents; Part A, Table 10 Analysis of soil type and loadings, giving recommended foundation widths. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation • Plan the direct methods required for site investigation. Identify the depths of investigations required at different locations around the site. • Boreholes should penetrate completely through made up ground or in fill. • Identify suitable in-situ and lab testing methods. • Decide on the number and location of bore holes, frequency of testing. • Plan and Make allowances for any unforeseen ground and groundwater conditions on site. Key term: Borehole Holes sunk into the ground to extract soil samples at differing levels. The information is recorded as the holes are drilled so that the design engineer discovers at what depth each soil is found. Key term: Made Ground Any ground that has been artificially made from previous works, e.g. layers of stone compacted and laid to form a level surface ready for construction work. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Stage 4: Keep Records Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Techniques: Trial pits (Max 5m deep) Trial pits are particularly useful in the investigation of sites intended for low – rise construction Records need to be kept as a ‘log’ with the profile recorded as the hole is dug. Trial pits are relatively cheap but care must be taken with exposed excavations, and some may need shoring with timber or hydraulic earth supports. These also need to be well identified as open excavation can also be dangerous to any person on site. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Trial pits clip Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Trial holes List the advantages and disadvantages Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Techniques: Boreholes (105-200mm) Light percussion equipment is used to drive a hollow tube in to the soil In clay soils a ‘clay cutter’ is used In granular soils a ‘flap valve’ is used The main advantage of light percussion drilling is its ability to make deeper holes in a wide range of ground conditions. This method is considerably more expensive than shallow trial pits and auger holes. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Bore hole clip Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Boreholes List the advantages and disadvantages Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Exploratory holes: How deep? Depth depends on ‘stress distribution’ under the foundation. Boreholes should penetrate all deposits unsuitable for foundation purposes such as unconsolidated fill or materials that need compacting e.g. peat, organic silts, and very soft compressible clays. Depth requirements should be reconsidered when the results of the first tests have become available, as it may be possible to reduce the depth of the subsequent borings. Key term: Stress Distribution How the foundation distributes the load of a building. A very wide, flat foundation will support more load than a narrow thin strip. Unit 6: Building Technology in Construction Site Investigation: Ground Investigation Exploratory holes: How many? The maximum number of boreholes will depend on the complexity of the local geology and construction project. The number can change as more information becomes available from earlier investigations. Exploratory holes: Where? The location of boreholes is dependent on the nature of the site and proposed construction. Additional boreholes are drilled at problem areas and near the site of the proposed structure. Where practicable boreholes should be located along ‘grid lines’ at regular intervals to enable section drawing to be produced across indicated planes. Unit 6: Building Technology in Construction Site Investigation: Radon Gas • Radon is a naturally radioactive gas and is found in many areas within the UK. • It has no taste, colour or smell, it can gather in concentrated levels below construction projects. • Building regulations require that buildings and extensions constructed after 2000 in RADON affected areas have protective measurers installed during construction. • To prevent the build up of radon gas, barriers are built into the structure. This barrier must cover the total area of the foundation footprint of the structure. Unit 6: Building Technology in Construction Site Investigation: Groundwater Conditions Ground water is found below the ground in the spaces and cracks between soil, sand and rock. The level of water in the ground is known as the water table. Unit 6: Building Technology in Construction Site Investigation: Groundwater Conditions Why is it important to know the water table level? • Any design and operations will be affected by the presence of water. • Digging into the ground can cause ground water to fill some trenches and excavations with water. • Construction companies have to plan how to remove or deal with any water during construction; ground water conditions are significant because they can affect construction in a number of ways. Unit 6: Building Technology in Construction Site Investigation: Groundwater Conditions What is the impact on construction of a high water table? • Extra costs – from dewatering techniques, increased trench support and ground stabilisation requirements all making construction more difficult, longer and increasing costs. • The presence of chemicals in any water such as acids can lead to damage of foundations and other materials used below ground level. • A high ground water table implies that pore water pressure in the soil is high (gaps between soils), which usually means that the soil is weaker. As well as influencing foundations, high pore water pressure will and can affect the stability of slopes (landslides?) and the pressure on retaining structures. Unit 6: Building Technology in Construction Site Investigation: Groundwater Conditions What is the significance of this information on the building contractor? • • • • • • • Viability of site Cost of ground works Type of foundations Amount of labour required Skill of labour required Requirement of engineered foundations Any contamination – remediation? Unit 6: Building Technology in Construction Site Investigation Remember! The cost of a typical site investigation is around 0.5 – 1% of the capital cost of a project. It is considered good practice to investigate adequately at this stage. Key term: Capital Cost The total cost of all the equipment and necessary expense to complete the works. Unit 6: Building Technology in Construction