RETAINING STRUCTURES

A diaphragm wall is a reinforced concrete wall that is cast in sections or panels excavated in the ground. The trench held open during excavation, and installation of reinforcement and concrete by the use of a supporting slurry. The slurry forms an impervious deposit (cake) on the walls of the trench, isolating the hydraulic pressure of the slurry from the surrounding soil and ground water, such that this pressure exerts sufficient outward force to keep the trench open. The slurry mix can be based on the use of bentonite, or polymers or a mixture of the two.

The temporary guide walls are constructed in advance and consist of two reinforced-concrete sections depending on the thickness of diaphragm wall to be implemented.

The guide walls can be either cast-in-situ or precast. The guide-walls have several functions

  • to provide physical confirmation of the location of the wall,
  • to guide the excavation tool,
  • to provide a reservoir for drilling mud,
  • to provide a fixed support for suspension of the reinforcement cages.

Individual panel lengths are determined by a number of factors including trench stability and the sensitivity of the surroundings to movement, or intensity of the reinforcement. The wall can be constructed very close to existing structures though a minimum clearance is required for the thickness of the guide wall. When excavation of a panel is complete the slurry is treated to reduce the quantity of solids in suspension to a predetermined acceptable level.

Upon completion of excavation of panel, the reinforcement cage is installed and concrete poured using a tremie pipe from bottom to top.

The joint between adjacent panels can be achieved in one of two ways:

  • By use of a temporary steel stop end allowing the placement of a waterstop across the joint and providing at the same time a guide for the excavating tool.
  • By cutting back into the concrete of the?previously constructed panel when excavating with a hydrofraise.

The standard thicknesses of diaphragm walls are in the range of 60 cm to 150 cm typically being 60 cm, 80 cm, 100 cm, 120 cm and 150 cm. Panel excavation is done by diaphragm wall machines like mechanical or hydraulic grab, and hydrofreeze.

Diaphragm walls are ideal solution for deep excavation shoring structures under high ground water level. Diaphragm walls can be built as a temporary shoring structure, as well as permanent structures. Diaphragm walls give great advantages to limit deformations during shoring excavation because of their of rigidity. In addition, they have other functions, such as vertical load-bearing elements and hydraulic cut-off.

The vertical members are composed of bored piles in anchored bored pile structures supported by variable levels and dimensions of horizontal supporting members composed of pre-stressed anchors and waling beams depending on factors such as the soil and groundwater conditions, environmental loads, deformation criterias, etc.

Initially vertical members of bored pile shoring system is constructed followed by step wise excavation and construction of anchors and beams according to the deisgn levels. Guide walls may be used to establish the correct positions of the bore piles, initially.

Bored piles can be constructed by using various capacity and sizes machinery and equipments, depending on factors such as; soil and groundwater conditions, diameter, capacity and the length of the pile. Bored piles are drilled by bentonite mud or casing depending on the ground conditions during the drilling, and length of the piles and the capacity of the equipment.

Bored piles are constructed in the following steps respectively:

  • drilling,
  • placing of reinforcement
  • concreting

After placing of the reinforcement concrete is placed using tremie pipes starting from the bottom of the pile to the surface.

Typically, the bored pile diameters are drilled by conventional drilling equipments to sizes between 65 cm to 150 cm.

Upon completion of the bored pile construction, bored piles are connected to each other with a head beam at the top level.

Head beam sizes may be variable depending on the design requirements and the diameter of the bored piles used. Prior to the construction of the head beam, the heads of the bored piles are cut and cleaned to the elevation specified in the design, and the reinforcement is arranged in order to connect the head beam reinforcement to pile reinforcement.

After completion of the head beam, excavation is performed in steps and the shoring system is formed parallel to the excavation, by constructing pre-stressed anchors and the waler beams. In cases where it is not possible to implement achors due to surrounding conditions, vertical bored pile members can be supported by supporting strut beams, depending on the size of excavation support system.

The only difference between anchored mini piled shoring system and anchored bored pile shoring system is the size of vertical members which are composed of mini piles with smaller diameters. It is a structure supported by prestressed anchors and beams depending on factors such as the soil and groundwater conditions, environmental loads, deformation criterias, etc.

Mini-piles are vertical members that are typically 25 cm to 40 cm in diameter. Initially vertical members of mini pile shoring system is constructed followed by step wise excavation and construction of anchors and beams according to the deisgn levels. Guide walls may be used to establish the correct positions of the bore piles, initially.

Mini piles can be constructed using machinery and equipment in different capacity and size. Thus, they are usually built by standard drilling rigs, under appropriate soil and underground water conditions.

Mini piles are constructed in the following steps respectively:

  • drilling,
  • placing of reinforcement
  • concreting

After placing of the reinforcement concrete is placed using tremie pipes starting from the bottom of the pile to the surface.

Upon completion of the construction of the mini-piles, they are connected to each other with a head beam at the top level. Head beam sizes may vary according to the design requirements and the diameter of the mini-piles. Prior to the construction of the head beam, the heads of the mini piles are cut and cleaned to the design elevation, and the reinforcement is arranged in order to connect the head beam reinforcement to mini pile reinforcement.

After completion of the head beam, excavation is performed in steps and the shoring system is formed parallel to the excavation, by constructing pre-stressed anchors and the waler beams.

In cases where it is not possible to implement achors due to surrounding conditions, vertical bored pile members can be supported by supporting strut beams, depending on the size of excavation support system.

In this shoring system, pre-stressed anchorages are construced with reinforced cast in place concrete panels parallel to the excavation in stages from top to bottom.

It can be implemented in various thicknesses; typically ranging from 20 cm to 50 cm, depending on factors such as excavation depth, environmental loads, soil conditions, etc.

It is usually implemented when the thickness of shoring system is located in limited, and when there is no ground water in the form of free ground aquifer.

It has relatively slow construction rate compared to other shoring systems, because of the fact that it needs time at every stage for the concrete wall to set and reach to design strenth in each level.

Reinforced concrete elements can also be constructed as vertical manual caissons under appropriate conditions.

Shoring system with soil nailing or passive anchors is a flexible shoring system and is constructed from top to the bottom in steps parallel to excavation.

The main elements of the system are soil nails, reinforced shotcrete surface cover and subhorizontal drains.

The length and spacing of the nails are variable depending on the depth of the excavation, environmental loads and soil conditions, etc.

In this system, depending on the ground conditions after each excavation stage pre-shotconcrete is applied, followed by passive anchor drilling. Appropriate reinforcement is placed into the drill holes according to the design and the drill hole around the reinforcement is grouted with cement grout.

In case the system is designed as permanent, anti-corrosion PVC coat or epoxy-coated reinforcement, etc. are applied. Typically excavation surface is covered with wire mesh reinforcement in excavation steps of 1.5 m to 2.0 m in height, and the nails are connected to reinforcement wire mesh by means of a plate and nut. Afterwards final surface is covered with shotcrete and advanced to next excavation step.

Subhorizontal drains with perforated drain pipes are installed on the wall surface at vertical and horizontal spacing and lengths based on the design at excavation levels.

Since the system can be constructed parallel to excavation shoring systems with a very flexible geometry can be achieved through construction.

Cut-off wall systems are shoring systems applied to ensure the impermeability of the shoring system that will be constructed under high ground water table. The main function of this shoring system is to prevent ground water intrusion into the excavation, which can be implemented with diaphragm walls and intersecting bored piles.

Shoring systems with diaphgarm walls is usually constructed serving this purpose. Also intersecting bored piles or piles and jetgrouting serving impermeability as vertical members might be another option.

Reinforced walls implemented on the retaining walls in embankment, are shoring systems constructed from bottom to the top with steel or plastic reinforcement elements.

Reinforcement elements are placed in the embankment in variable length and distance based on the design and the embankment is compacted in layers. The surface of the wall is formed parallel to the fill construction composed of members usually precast conctrete, steel or various materials. Reinforcement within the embankment are connected to surface members. Various areas of application are possible such as roads, bridges, railways, quay walls. The walls can be formed with very flexible geometry and has a high performance under dynamic loads.

 
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Zetas Zemin Teknolojisi A.S.

  Alemdag Merkez Mah.
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