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BUILDING TECHNOLOGY 4Lecture 2Diaphragm Walls and Basement Construction
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DIAPHRAGM WALLDiaphragm walls refers to the construction of in-situ retaining vertical walls by deep trench excavation method.Stability of the sides of the excavation is ensured by bentonite slurry. Hence, these types of walls are also termed at times as slurry walls.The wall is constructed in panels and the length of typical panel is between 2500 to 7000mm, and are usually 600 to 1200mm thick.Standard methods of constructing retaining walls require temporary form work and supports. These become uneconomical after a certain depth. The construction of the work above ground cannot proceed till the basement work is complete. This involves more time for construction.Use of diaphragm wall construction eliminates the need for formwork and temporary support and also allows above ground construction to proceed along with basement construction.
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Purpose-built grabs or milling machines called as hydrofraise mills are used to do the excavation.Since the wall is cast-in-place panels, special care has to be taken to make the joints between the panels watertight.Water bars are constructed within the construction joints to prevent water leakage through them.Standard widths of the walls range from 600,800, 1000 and 1200mm.
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Bentonite slurry is a clay mixed with water which possesses thixotropic property. That means that when it is left undisturbed it acts as a gel and when it is moved it acts like a liquid. The bentonite slurry is poured into the excavation and it seeps into the adjoining soil forming a cake which prevents the sides from caving in. The bentonite is removed while concreting by displacement. The depth of diaphragm walls can reach 50 to 80m. The tolerance for verticality is normally 1:200.BENTONITE SLURRY
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Deep basementsUnderground tanksAccess shaftsRoad and rail under passesTunnelsMultilevel under ground parking Typical Applications
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TOP DOWN CONSTRUCTIONConventionally, buildings with basements are built by bottom-up method where substructure and superstructure floors are constructed sequentially from the lowest level of the basement to the top of the superstructure.Though this method is simple in design and construction, it is not feasible for projects with limited construction time and/or with site constraints.Top-down construction method is a method which builds the permanent structure members of the basement, along with the excavation from the top to the bottom. In this case, the basement floors are constructed as the excavation progresses.The sequence begins with the construction and installation of retaining walls, and then the load-bearing elements that will carry the future superstructure.The basement columns, typically steel sections, are constructed before any excavation takes place and rest on the load-bearing elements. These load bearing elements are typically concrete barrettes or piles constructed under slurry (or caissons).
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Design and construction principles for top-down method primarily call for two major structural elements:Columns with sufficient capacity must be pre-founded in bored piles or barrettes to sustain the construction load and to utilize as part of bracing system.Excavation for basement must be carried out with the support ofpermanent retaining wallso that basement floor slabs can be utilized as lateral bracing.STRUCTURAL MEMBERS REQUIRED FOR TOP-DOWN CONSTRUCTIONDiaphragm wall of 0.8m to 1.2m in thickness with sufficient embedment in firm soil layers is commonly used as aretaining wall,whereas prefabricated steel columns known as Stanchions embedded in either large diameter deep-seated bored piles or barrettes are utilized as structural columns.
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The underground retaining wall which is usually a diaphragm wall, is installed before excavation commences.The soil is excavated just below roof slab level of the undergroundstructure. Struts are installed to support the retaining walls, which in turn support the soil at the sidesThe roof slab is constructed, with access openings provided on the slab for works to proceed downwards. The roof slabs not only provides a massive support across the floor.
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The next level of slab is constructed, and this process progresses downwards till the base slab is completedThe side walls are constructed upwards, followed by removal of the intermediate struts. The access openings on the roof slab are then sealed.After the undergroundstructureis completed, the soil is backfilled to the top strut level before the strut is removed. This is followed by completely backfilling the top of the underground structureand finally reinstatingthe surfaceareas.
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The shortened construction period due to simultaneous construction of the basement and the superstructure.More operational space gained from the advanced construction of floor slabs.The higher stiffness of floor slab compare to steel struts improves the safety of excavation.It is highly suitable for construction for tall buildings with deep basements to be constructed in urban areas.ADVANTAGES
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Higher cost (due to the construction of pile foundation)Since the construction period of the basement is lengthened, the lateral displacement of retaining wallor ground settlement may possible increase due to the influence of creep if the soil layers are encounteredThe construction quality may influence because of worsened ventilation and illumination under floor slabIt requires highly skilled supervision and labor force.DISADVANTAGES
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