Diaphragm Wall: Construction and Design By Umerfarooq GNDEC, Ludhiana
Diaphragm walls are concrete or reinforced concrete walls constructed in slurry-supported, open trenches below existing ground. Concrete is placed using the Tremie installation method or by installing pre-cast concrete panels (known as a pre-cast diaphragm wall). Diaphragm walls can be constructed to depths of 150 meters and to widths of 0.5 to 1.50 meters. Diaphragm wall construction methods are relatively quiet and cause little or no vibration. Therefore, they are especially suitable for civil engineering projects in densely-populated inner city areas. Due to their ability to keep deformation low and provide low water permeability, diaphragm walls are also used to retain excavation pits in the direct vicinity of existing structures. Diaphragm Wall
Which code to use? IS 14344 : 1996 DESIGN AND CONSTRUCTI-ON OF DIAPHRAGMS FOR UNDER-SEEPAGE CONTROL - CODE OF PRACTICE
TYPES OF DIAPHRAGM WALL Depending on the use of construction materials there are the following types of diaphragm walls: a) Rigid type 1) Reinforced cement concrete. b) Flexible type 1) Plastic concrete, 2) Cement bentonite slurry trench, and 3) Earth backfilled slurry trench.
Depending on the function the following kinds of diaphragm walls are used: Structural walls Load Bearing Elements Cutoff walls Structural Diaphragm walls: they are used as retaining walls for the perimeter walls of deep basements and underground parking facilities, subways, underpasses, etc Load bearing walls: they are used in place of drilled piers in foundation of tall buildings, bridge piers, etc Cutoff walls: in hydraulic structures diaphragm walls are used as impermeable cutoffs to prevent seepage below earth dams, weirs, and leeves
SELECTION OF TYPE OF DIAPHRAGM WALL Selection of type of diaphragm depends upon a number of factors such as: Site conditions Heterogeneity/perviousness of subsurface data Geological features Depth of overburden features Anticipated stress and deformations due to embankment construction and reservoir loading conditions Availability of construction materials Techno-economic considerations
Materials used for the construction of Diaphragm wall Ordinary Portland Cement Aggregate: Course aggregate of size 20mm Sand: Well graded sand consisting of 50% coarse sand Water: Clean water free from impurities Admixtures: if required chemical admixtures shall be used as per IS 456:1978 Reinforcement: Mild Steel bars Bentonite: bentonite used shall conform to IS 12584:1989 Clay: Clay shall conform to IS1498:1970 Concrete Mix: For plastic concrete diaphragm wall the water cement ratio shall not be greater than 0.5.
General Procedure of Construction 1. The excavation is carried out using a heavy self guided mechanical grab suspended from a large crawler crane. 2. The diaphragm walls were excavated and constructed in discrete panels of between 2.8m and 7.0m lengths, with a depth reaching 30m. 3. As the excavation proceeds, support fluid was added into the excavation to maintain the stability of the surrounding ground and to prevent a collapse. This fluid is called “Bentonite”, which is a poser made of a special type of soluble clay and is mixed at the mixing plant with potable water. 4. A heavy chisel may be used if an obstruction of hard strata is encountered, to break up the obstruction for removal by the grab. 5. When the excavation is completed, a submersible pump connected to tremie pipes will be lowered into the panel excavation down to the toe level. This pumped the fluid down to the toe level and then from the bottom of the excavation back to a descending unit, in order to separate the bentonite from the suspended particles contained in it. At the same time, fresh fluid will be added to the top of the excavation to maintain the stability of the ground.
Design Considerations Utmost consideration shall be given, while designing the diaphragm wall, so as to achieve: perfect embedment at both the ends, to avoid/minimize possibilities of cracking both within and surrounding the diaphragm wall, and imperviousness or water tightness. The most important consideration in the design of a diaphragm wall is to form an impervious wall having flexibility to avoid cracking. The best location for a diaphragm wall within a structure is where the loads are reasonably balanced on both sides of the wall. Location of diaphragm wall is often influenced by site conditions. Location shall, therefore, be decided after careful study of site requirements and localised features.
GUIDELINES FOR STRUCTURAL DESIGN Structural Analysis: Rigid type of diaphragm wall is to be analyzed either by the method of beam on elastic foundation or by finite element method (FEM). Method of beam on elastic foundation: The diaphragm wall shall be considered in plane strain state and as such, unit length of diaphragm wall with entire depth as the span is to be considered as a beam resting on elastic soil media on the downstream face. Loads are to be considered acting upon the upstream face of the diaphragm wall. Analysis is to be carried out using appropriate equations for bending moments and shear stresses for a beam resting on elastic foundation with assumed end conditions.
Finite element method (FEM): Finite element analysis takes into account soil structure interaction. Finite element analysis shall be carried out as sequential construction analysis. Sequential construction analysis is, however, preferred as it takes into account the elastic modulus of soil changing with different stress Levels during construction. The finite element analysis shall incorporate interface elements along the contact boundary of the diaphragm wall and surrounding soil mass. Elimination of interface elements results in faulty stress and displacement computations, due to stress transfer through common nodes.
Flexible types of Diaphragm wall Structural Analysis: Plastic concrete diaphragm wall or cement bentonite slurry trench diaphragm wall or earth backfilled slurry trench diaphragm wall are relatively flexible and capable to deform under stresses in surrounding soil. Hence development is not a design problem. These diaphragm wall shall, therefore be designed to undergo deformations compatible with those in the surrounding soil without development of cracks.
Requirements of Slurry • Bentonite slurry is made by passing dry powder through water jet. A conical hopper is used with bottom nozzle through which water is pumped under pressure. The bentonite powder is poured directly from top and when it falls down the hopper, it gets agitated in the water. After getting circulated, the mixed bentonite thus falls in the tank.