Groundwork and Substructure Overview
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Groundwork and Substructure Explained
Groundwork and subsurface works form an essential part of any build, whether it be a private dwelling, railway line or new road. Most groundwork and substructure works are undertaken to prepare the site for the proposed structure and to create foundations necessary for its support.
Before the structure can be built it is considered good practice and a legal requirement of most local authorities in the UK to conduct a ground investigation at the site, once planning permission has been granted. Ground investigations, also referred to as site investigations, enable geotechnical data to be gathered for foundation design purposes to allow cost effective and efficient development of the site. They can also identify the presence of any voids or abandoned mine workings beneath the site. Various intrusive drilling techniques, such as shell and auger and rotary drilling, are used to recover and test the underlying soil and rock at the site. Once recovered soil and rock samples are subjected to laboratory based geotechnical tests to further aid foundation design parameters.
Planning permission is also likely to stipulate that contamination testing be undertaken, prior to development, to ensure the health of future residents is not put at risk from historical contaminants at the site. Contamination tests can be conducted on the soil samples obtained during the ground investigation. If contamination testing is specified a desk study of the site and its surrounding area should be undertaken, prior to the ground investigation, to help identify the past land usages of the site. This will help to ascertain the likelihood and location of any potential contamination at the site, which should be followed up during the site investigation. A geoenvironmental site investigation is designed to assess both contamination and geotechnical properties present at the site.
Based on the information contained within the ground investigation report a structural engineer will finalise the foundation requirements for the proposed structure. Foundation solutions chosen are dependant on the ground conditions identified at the site, which should be detailed within the site investigation report. Shallow concrete foundations, such as strip footings and pad foundations are likely to be used where underlying soils are relatively stable and are able to offer limited settlement. Where soft or unstable soils are present a reinforced concrete raft foundation may be considered for lightly loaded buildings. Ground improvement solutions, such as vibrocompaction, can be used to compact underlying loose sands and gravels and increase their bearing capacity. For large buildings with high loadings a deep foundation solution will be necessary to support the weight of the building and ensure limited settlement. Deep foundation solutions require piles to be sunk in to more stable ground beneath the structure. Other deep foundation types, such as caissons are used for the construction of bridge foundations. A number of different pile types and pilling techniques can be used to create deep foundations, depending on the type of structure being built. A pilling contractor will recommend pile types and can be employed to sink the required number of piles at the site.
The pilling specialist may use continuous flight augured piles, also known as CFA piles, which can be installed within most soil types and weak rock. First a hole is formed by the helical auger of the drilling rig. On completion the hole is filled with high slump concrete and a steel reinforcement cage is added to increase the cured strength of the concrete. Rotary bored piles, or rotary piles, may be utilised in stable soils and rock. The soil or rock is removed section by section, instead of continuously in the CFA piling process. Once the hole is complete a reinforcement cage is inserted and the hole is filled with concrete to create the pile.
Alternatively the driven piling technique may be used, commonly where soft soils overlie more stable soils or rock. The process involves driving pre cast concrete piles or steel tubular piles in to the ground with a hydraulic hammer or drop weight till they offer the required resistance. Driven piles are a form of displacement pilling as the soil is displaced by the side of the pile as it is driven in to the ground. No soil is removed from the hole during the installation process.
A pilling contractor may need to use a mini piling rig, some times called a micro piling rig, where there is limited access to a site. These piling rigs are small and are suited to house extension projects, at the rear of properties. Mini piles are also used for underpinning subsiding house foundations.
Following installation pile testing should be undertaken to ensure piles are capable of supporting the intended structure.
Specialist Groundwork and Substructures
In order for a site to be developed a number of other groundwork activities may have to take place and other substructures may have to be constructed.
On sloping sites retaining walls may have to be constructed to make a level development platform to enable the best usage of the site area to be achieved. Retaining wall structures prevent slopes from failing, and collapsing or moving down slope, once sections of material have been excavated from the original topography of the site. Sheet piling is often used as a relatively cheap method of retaining wall construction, and is regularly used for temporary applications. The process involves driving steel sheet piles in to the ground with a large scale hydraulic jack hammer. The sheets can be interlocked to form a continuous wall. Retaining walls can also be constructed from reinforced concrete and timbers. Piled retaining walls are commonly used to support basement excavations. They use piled foundation drilling methods such as CFA drilling or rotary drilling. Piled retaining wall types used may be secant piled retaining walls or continuous piled retaining walls. Secant pile walls are constructed of overlapping concrete piles and can stop groundwater flow into the excavations, if designed and installed properly. Continuous pile walls are composed of closely spaced concrete piles and are generally used where groundwater is not present. Steel wire cages filled with rocks, known as gabions, can offer slope retention solutions and are generally used to give support to embankments. They are also used to prevent river bank erosion and in the construction of flood defences. Other methods of slope stabilisation include soil nails and ground anchors. Soil nailing is undertaken by drilling holes and inserting steel bars into the slope face (typically a cliff face), then grouting them in place. A finishing plate or mesh is attached to the bar ends to hold the slope face in place. Ground anchors are generally steel wires with metal wedges on their ends. The metal wedge end is driven pneumatically into the slope face, until the desired depth is reached, and the wire is pulled taught then attached to a finishing plate to maintain tension. Both soil nailing and ground anchoring can be used as part of a retaining wall installation. Other retaining wall types include cantilever retaining walls and coffer damns.
In built up areas the development may require specialist tunnelling or shaft sinking solutions to enable the installation of service ducts and cabling to the property. The trenching of services, such as water and electricity cables may also be required.
A wide array of geosynthetic materials have been designed and manufactured for construction use. They can be incorporated in to features such as retaining walls, flood protection measures and embankments to provide reinforcement. Geosynthetics fit in to two main categories, geomembranes and geotextiles. Geotextiles are woven and allow the passage of ground water through them. Geotextiles are commonly used for filtration and separation purposes within drainage systems, but can also be used for ground stabilisation and reinforcement purposes. Geogrids are a form of geotextile with an open mesh construction. Geomembranes are impermeable and do not allow groundwater to flow though their membrane. Typical applications for geomembranes are as liner materials, within ponds and at landfill sites. Geosynthetics, known as geocomposites, are also widely available, and combine both geomembrane and geotextile materials.