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A slurry wall is a civil engineering technique used to build reinforced concrete walls in areas of soft earth close to open water, or with a high groundwater table. [1] This technique is typically used to build diaphragm (water-blocking) walls surrounding tunnels and open cuts, and to lay foundations.
While a trench is being excavated to create a form for a wall, it is simultaneously filled with slurry (usually a mixture of bentonite and water). The dense but liquid slurry prevents the trench from collapsing by providing outward pressure, which balances the inward hydraulic forces and also retards water flow into the trench. The density of the slurry mix must be carefully monitored and adjusted to produce the correct outward pressure to prevent the trench walls from collapsing.
Slurry walls are typically constructed by starting with a set of concrete guide walls, typically 1 metre (3 ft 3 in) deep and 0.5 metres (1 ft 8 in) thick. The guide walls are constructed near the ground surface to outline the desired slurry trench, and to guide the excavation machinery. Excavation is done using a special clamshell-shaped digger or a hydromill trench cutter, suspended from a crane. The excavator digs down to design depth (or bedrock) for the first wall segment. The excavator is then lifted and moved along the trench guide walls to continue the trench with successive cuts as needed. The trench is at all times kept filled with slurry to prevent its collapse, but the fluid filling allows the excavation machinery and excavation spoil to be moved without hindrance.
Once a particular depth of trench is reached, a reinforcing cage is lowered into the slurry-filled pit and the pit is filled with concrete from the bottom up using tremie pipes. The heavier concrete displaces the bentonite slurry, which is pumped out, filtered, and stored in tanks for use in the next wall segment, or it is recycled.
Slurry walls are successively extended to enclose an area, blocking water and softened earth from flowing into it. Once the concrete has hardened, excavation within the now concrete-wall-enclosed area can proceed. To prevent the concrete wall from collapsing into the newly excavated area, temporary supports such as tiebacks or internal crossbeams are installed. When completed, the structure built within the walled-off area usually supports the wall, so that tiebacks or other temporary bracing may be optionally removed.
The slurry wall technique was first introduced in the 1950s during the excavations of the Red Line of the Milan Metro in Italy by the company ICOS (Impresa Costruzioni Opere Specializzate). This new technology became an important component of the top-down tunnelling method also known as Metodo Milano ("Milan method"). [2]
Slurry wall construction was used in 1967–1968 to construct the "bathtub" that surrounded the foundations of most of the World Trade Center site in New York City. [3] In the 1980s, the Red Line Northwest Extension project in Boston was one of the first projects in the US to use the modern form of the technology, with hydromill trench cutters and the "Milan method". Slurry walls were also used extensively in Boston's 1990s Big Dig tunnel project.
The design of a slurry wall (diaphragm wall) includes the design of wall thickness and reinforcements. Thickness of a slurry wall in preliminary design is generally set to about 4-8% of the excavation depth. Slurry wall design is undertaken based on bending moment and shear envelope obtained from the stress analysis. In the design of such underground walls, width of the unit is considered as one meter, and the wall is analyzed under plane strain condition. Since the length-to-width ratio of excavations is generally large, plane strain conditions can be assumed. [4]
In a single-phase diaphragm wall, also known as a "cut-off wall", a binder (usually cement) is added to the supporting fluid so that the supporting fluid hardens without exchange. [5] One application for this type of construction is a landfill seal that is to be retrofitted later.
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.
A trench is a type of excavation or depression in the ground that is generally deeper than it is wide, and narrow compared with its length.
Bentonite is an absorbent swelling clay consisting mostly of montmorillonite which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.
Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to. They are used to bound soils between two different elevations often in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. A retaining wall that retains soil on the backside and water on the frontside is called a seawall or a bulkhead.
A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels. Tunnels are excavated through hard rock, wet or dry soil, or sand. TBMs have been customized to work with each material. TBM-bored tunnel cross-sections range from one metre (3.3 ft) (micro-TBMs) to 17.6 metres (58 ft) to date. Narrower tunnels are typically bored using trenchless construction methods or horizontal directional drilling rather than TBMs. TBM tunnels are typically circular in cross-section although they can be customized for u-shaped, horseshoe, square or rectangular tunnels.
Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing dams, dikes, and other controls for streams and shorelines; and recovering valuable mineral deposits or marine life having commercial value. In all but a few situations the excavation is undertaken by a specialist floating plant, known as a dredger.
Earthworks are engineering works created through the processing of parts of the earth's surface involving quantities of soil or unformed rock.
The New Austrian tunneling method (NATM), also known as the sequential excavation method (SEM) or sprayed concrete lining method (SCL), is a method of modern tunnel design and construction employing sophisticated monitoring to optimize various wall reinforcement techniques based on the type of rock encountered as tunneling progresses. This technique first gained attention in the 1960s based on the work of Ladislaus von Rabcewicz, Leopold Müller, and Franz Pacher between 1957 and 1965 in Austria. The name NATM was intended to distinguish it from earlier methods, with its economic advantage of employing inherent geological strength available in the surrounding rock mass to stabilize the tunnel wherever possible rather than reinforcing the entire tunnel.
In geotechnical engineering, a caisson is a watertight retaining structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships.
Shoring is the process of temporarily supporting a building, vessel, structure, or trench with shores (props) when in danger of collapse or during repairs or alterations. Shoring comes from shore, a timber or metal prop. Shoring may be vertical, angled, or horizontal.
Directional boring, also referred to as horizontal directional drilling (HDD), is a minimal impact trenchless method of installing underground utilities such as pipe, conduit, or cables in a relatively shallow arc or radius along a prescribed underground path using a surface-launched drilling rig. Directional boring offers significant environmental advantages over traditional cut and cover pipeline/utility installations. The technique is routinely used when conventional trenching or excavating is not practical or when minimal surface disturbance is required.
A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.
Landslide mitigation refers to several human-made activities on slopes with the goal of lessening the effect of landslides. Landslides can be triggered by many, sometimes concomitant causes. In addition to shallow erosion or reduction of shear strength caused by seasonal rainfall, landslides may be triggered by anthropic activities, such as adding excessive weight above the slope, digging at mid-slope or at the foot of the slope. Often, individual phenomena join together to generate instability over time, which often does not allow a reconstruction of the evolution of a particular landslide. Therefore, landslide hazard mitigation measures are not generally classified according to the phenomenon that might cause a landslide. Instead, they are classified by the sort of slope stabilization method used:
Penmanshiel Tunnel is a now-disused railway tunnel near Grantshouse, Berwickshire, in the Scottish Borders region of Scotland. It was formerly part of the East Coast Main Line between Berwick-upon-Tweed and Dunbar.
A bucket is a specialized container attached to a machine, as compared to a bucket adapted for manual use by a human being. It is a bulk material handling component.
The hydromill trench cutter is a specialized type of construction equipment designed to dig the narrow but deep trenches used in the casting of slurry walls. Typically, it is a cutter attachment mounted on a crawler crane base machine, with different types of hose handling systems. The machine excavates by cutting the soil using two cutting wheels, while a powerful pump extracts the loose material mixed with some of the slurry, typically bentonite.
John Vernon Bartlett was a British civil engineer, particularly associated with developments in tunnelling technologies. He was President of the Institution of Civil Engineers from November 1982 to November 1983, and received various industry honours including the Telford and Sir Frank Whittle Medals.
The Eurasia Tunnel is a road tunnel in Istanbul, Turkey, crossing underneath the Bosphorus strait. The tunnel was officially opened on 20 December 2016 and opened to traffic on 22 December 2016.
Digging, also referred to as excavation, is the process of using some implement such as claws, hands, manual tools or heavy equipment, to remove material from a solid surface, usually soil, sand or rock on the surface of Earth. Digging is actually the combination of two processes, the first being the breaking or cutting of the surface, and the second being the removal and relocation of the material found there. In a simple digging situation, this may be accomplished in a single motion, with the digging implement being used to break the surface and immediately fling the material away from the hole or other structure being dug.
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management. Tunnel construction is a subset of underground construction.
A portion of the slurry wall was deliberately left exposed in the Foundation Hall of the National September 11 Memorial Museum, set to open next year.
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