Substructure – Groundwater Control
When executing substructure construction, water may leak into the excavation if the exaction work passes through the water table or if groundwater is present close to the site. To allow for the exaction to remain dry and safe, groundwater control methods are used (most of which are temporary) allowing engineering excavations to be constructed below the groundwater level. The two main groundwater control methods used by engineers are Groundwater Control by Exclusion and Groundwater Control by Pumping.
Groundwater control by pumping involves pumping from a range of wells or slumps to lower the groundwater levels in the area of the construction project. While groundwater control by exclusion uses low permeability cut-off walls to encase the excavation site to prevent the water flowing into the excavated area.
Groundwater control by pumping technique - “well-pointing”:
Well-pointing is used to lower the groundwater below the excavation. This is accomplished by sinking wells close to the excavation area to allow the removal of sufficient water to effectively drop the water table. The water is pumped out of each well until the water table has been lowered to an acceptable level. To ensure that the water level remains low, pumping can be resumed if necessary. The well is constructed using a slotted or perforated pipe connector attached to a riser pipe. Using a jetting shoe fitted to the end of the strainer, high pressure water jets are fed down the riser pipe, effectively sinking the well by jetting it into the ground.
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The soil is pushed to the surface as it is displaced by the water. The cavity left by the well is backfilled using sand or gravel. Ring well-pointing and progressive well-pointing are the two main systems used in this type of groundwater control. Ring well-pointing is where a large area is ringed with wells. Progressive well-pointing is where a line of wells is used for long excavations.
Groundwater control by exclusion technique - “Diaphragm walls”:
The diaphragm walling technique allows the creation of a concrete wall within the ground establishing a barrier between the excavated area and the water table before further excavation takes place. A trench is dug at a required width and depth and filled with concrete to produce a waterproof wall that connects impermeable strata.
A mixture of bentonite and water is used to ensure that the sides of the excavation do not collapse. Bentonite is a thixotropic material formed from montmorillonite clay – when agitated the material takes a liquid form, if allowed to settle it forms a gel. As the mixture settles against the surface of the excavated area it turns to gel which forms a protective skin, preventing loosening and collapse from taking place.
On reaching the required depth, prefabricated reinforcement mats are lowered into the bentonite, which does not stick to the reinforcement. A tremie tube is then used to pour in concrete which displaces the bentonite. The bentonite is runoff for de-sanding and reused further, with the concrete and bentonite being used simultaneously at different sections of the diaphragm wall. Once the concrete at the base of the excavation reaches a depth of three metres, the tremie tube is raised slowly allowing more concrete to be poured on top of the existing three metre. This exercise continues until the required depth of concrete is attained, all the while maintaining the tremie tube within a three metre depth of concrete at all times. By doing so, this allows the best consistency of concrete within the excavation.
Case Study: Diaphram cut-off wall – The Sizewell B Power Station
The groundwater conditions of the site on which the Sizewell B Power Station was to be built consisted of 50 metres of silt and dense sand overlying clay which formed a natural aquifer known at the Norwich Crag Formation. Due to its close proximity to the North Sea, the construction of the Power Station could not commence until the site was isolated – resulting in the deepest ever diaphragm wall in the United Kingdom. Excavations almost eighteen metres below the water table were required for the foundations. Standard dewatering techniques could not be use in this construction for several reasons:- initial calculations established that even with 52 wells, as opposed to only 6 required for the Sizewell A Power Station construction, the water level would only be lowered by 16 metres. Additionally, the use of multiple wells was deemed unacceptable due to settlement beneath Sizewell A, heavy encrustation on the pipework caused by the iron content in the groundwater, excessive draw down below the adjacent bird reserves and a cost of at least £16 million.
The second option was the construction of a diaphragm wall which would extend into London clay and link with a cofferdam to form a 1,260 metre long, all encompassing, cut off wall around the perimeter of the site. Previously diaphragm walling had only ever been used to depth of 30 metres however the new trenching technique using reverse circulation rigs allowed for a very accurate depth of more than 100 metres. The use of this option meant that only 9 dewatering wells would be required and that the construction timescale would be reduced by 50% to just 6 months resulting in a saving of £2,000,000. As a result, this method was selected for use in July 1985. The diaphragm wall required a controlled maximum permeability as well as virtually leak-proof construction joints. A network of piezometers and observation wells was used to monitor the construction performance. By the time the pumps were switched off in spring 1992, more that 4 million metres3 of water had been pumped away, keeping the water table at least 2 metres below the deepest part of the excavation.
Alternatively, a cofferdam could have used in place of the diaphragm wall to connect to the existing cofferdam to control the groundwater on the site. Cofferdams are predominantly used in marine environments making it perfect for this site due to the close proximity to its North Sea. The cofferdam can extend from the ground surface to an impermeable stratum thereby making it almost completely impervious to ground water, only allowing for a slight volume of water to pass through, this can be dealt with by pumping. The most common type of cofferdam construction makes use of a steel sheet piles. Prior to excavation, steel sheet piles are forced into the soil. To maintain stability, struts and walings are fixed inside the sheet piling as the material inside the cofferdam is removed.
Superstructure-Structural Steelwork frames:
Structural steelwork frames consist of vertical columns and horizontal beam that are connected via welding, bolting or riveting. Steel beams resist loads applied horizontally to their axis and columns are perpendicular members that transfer compressive loads to the foundations below. A rectangular grid is used to construct the steel frame and allows for the walls, floors, roof and cladding. Industrial, warehouses, high rise buildings and residential buildings are the most common use for steel frame in construction.
Compared to its weight, steel has a very high strength. It can also be fabricated into multiple span sizes. If fabricated properly, the steel components are easy to install. Steel can be fabricated off site which ensures that the environment in which it is created in is specific to this purpose and the tools used ensure the precision of its components thereby improving the quality of the construction, reduction in errors in measurements and improves the likelihood of timescales and budgets being successfully met. Members that have repeating units can be mass produced. There is also the added bonus of the availability of a variety of readymade structural sections. The strength of steel ensures its ability to resist the forces of storms and earthquakes. Steel can be manipulated into a wide range of shapes and is suitable for a wide range of joining methods. It also has the added benefit of being able to be clad with a large range of materials which can improve its visual appeal.
One of the main disadvantages of steel is its extreme weakness to fire. Therefore, in order to improve the fire resistance of a steel frame structure, many costly methods may have to be used. As a result, this can have a determinantal effect on the financial budget of a project when using this material.
Steel frame structures normally have to have supporting elements built with them, these may include drywalls, insulation or wooden components. The increase in man hours and costs associated with these additional considerations may also make the use of steel within a construction less appealing.
Steel can corrode easy as it is an alloy of iron. Painting steel with anti-corrosion paint will improve the corrosion resistance of the steel, however this means that the steel is high maintenance. Paint will have to reapplied at regular intervals to prevent corrosion. The costs associated with this will continue throughout the life of the streel structure and add to its overall cost.
With larger steel structures, buckling may become an area for concern. The larger the column, the higher the likelihood that buckling may occur and therefore the chance of building collapse also increases.
Types of steel construction:
Conventional Steel Fabrication – This method can be carried out entirely on-site increasing the labour work needed or it can be partly done in a workshop. This method involves the cutting of steel members to required lengths. The members are then lifted into position and welded together to form a frame.
Light Gauge Steel Construction – This method is similar to timber frame construction. The steel forms thin sheets which are then bent into C-sections or Z-sections. Light gauge steel takes the place of timber two-by-fours and is mostly used in the construction of residential and small buildings.
Plant used for erection:
To erect the steel frame there are two methods; mobile and non-mobile. Mobile Elevating Work Platforms (MEWP’s) are used throughout the structural steel frame erection allowing the steel members to be bolted together while the steel members are being lifted and held in place by the crane. MEWP’s can be used both on the ground and on partially erected steelwork (the steel frame should be checked to ensure it can hold the weight of the MEWP).
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Non-mobile methods consist of using tower cranes to lift the beams into position while the MEWPs fix the members together. Due to the scale of a tower crane it must be constructed on site, this normally involves using a truck mounted crane. This increases the overall cost of the steel construction as the crane must be built on site before it can commence work and dismantled once construction is complete. This method of steel frame construction is only used if all other methods are ruled out, its lift speed is slow thus it takes longer for the steel member to be fix together increasing the time taken for the building to be completed. Tower cranes are largely affected by wind loading therefore extra safety considerations must be taken when using this method.
Mobile methods normally use all-terrain cranes, crawler cranes or truck mounted cranes to lift the steel members into position. Truck mounted cranes are usually used in one day commissions as they need very little set up time to get started and there is no need for a secondary crane on site to help with assembly. The big disadvantage for this method is that there is a limit to the height which the truck mounted crane can achieve. This height limit can be increased however, this comes at the cost of a larger footprint. The footprint can be increase by using outriggers, but to obtain good stability adequate ground conditions are required.
Case Study – Moorgate Exchange:
The Moorgate Exchange was the construction of a 12-storey office building. The building was built atop the site of London’s old Telephone Exchange. This project was the responsibility of by HKR Architects and Skanska, with Severfield (UK) Ltd acting as the steelwork contractors. Steel was chosen as the frame material as the design of the structure relied on long “column free spans”, helping to reduce the size of the CHS concrete columns, allowing for the structure to utilise the entire lettable space, while providing the maximum number of storeys. Due to the height of the structure, steel was considered to be a far quicker frame option to use in construction as opposed to a concrete frame. Also, the steel frame saved the project valuable time and money because it was lightweight. This allowed for a raft foundation to be used whereas if a concrete frame were chosen, pile foundations would have to have been used.
The design of the structure incorporated a “complex terrace transfer system” which was only possible due to the steel frame. Furthermore, the plate thicknesses were standardised making it extremely easy for the fabrication of the steel, allowing for the fabrication of diverse beam sizes quickly and efficiently. Slip form cores were used to provide structural support to the steelwork as it was being erected. A tower crane was used to lift the steel member into position with the steel beams connected to the columns using a “splice connection”.
Cellular beams were utilised for the project, this allowed for services to pass through the beam instead of below it, allowing for an increased floor to ceiling height. To protect the steel members from corrosion they were coated in a zinc alloy via hot-dip galvanizing allowing for even the most exposed steel have a life span of 50 years. The project was a huge success and the build achieved the BREEAM excellent rating as well as the Structural Steel Design Award 2015, while satisfying the clients brief.
Other methods that could have been used in the project could have been bolted steel construction. During the construction of the Moorgate Exchange, splice connections were used to connect the beams and columns however, bolted steel could have been easily used instead. Bolted steel construction is extremely fast because most of the fabrication is done off-site meaning the only work required on site is the lifting of the steel members and bolting them to the other structural members on site. This method is the most preferred method of steel construction and can come in large spans of between 6 metres to 12 metres depending on the truck transporting them. The splice connection was the preferred connection of the construction company Severfield (UK) Ltd. The concrete columns could have been replaced by steel columns this would have had the beneficial effect of all structural components being produced by the same fabricator. By using one fabricator, the likelihood of errors during production of the connections would have been minimised as opposed to the columns and beams being produced by different manufacturers.
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