4.2.4 Diaphragm walls
Diaphragm walls are also called slurry walls.Since first adopted in Italy in the 1950s，they have been widely used around the world.With technological advances，more and more new methods and construction equipment have been developed.The basement wall（BW）method and Impresa Construzioni Opere Specializzate（ICOS）method，designed separately by a Japanese company and an Italian company are commonly used in some Asian countries.The Masago Hydraulic Long bucket（MHL）method，taking advantage of a bailing bucket to excavate the trenches of the diaphragm wall，are also used in many countries.The teeth of the steel bailing bucket can clutch soils and rocks and store them inside the bucket.Then，the full bucket is lifted out of the trench，and soil and rocks inside are bailed out.Thus，stabilizing fluid need not be pumped out and mud separation equipment is saved.The method is easy in operation.The span of the bailing bucket is about 2.5⁃3.3 m.
The first stage of the constructing of diaphragm walls is to divide the whole length into several panels according to the construction conditions.The construction procedure of each panel is as follows：the construction of guided walls，the excavation of trenches，placing steel cages，and concrete casting，as shown in Figure 4.10.After excavating the trench，mud in the trench must be cleared from the trench.Concrete casting，the last stage of diaphragm wall panel construction，adopts the Tremie pipe to pour concrete into the trench and form a diaphragm wall panel.The construction procedure of a diaphragm wall panel，including excavating trenches，placing connection pipes（depending on the method），placing steel cages，and concrete casting.
Figure 4.10 Construction procedure of a diaphragm wall panel
The joints between panels of a diaphragm wall have to be carefully treated so that they are watertight and are able to transmit bending moments and shear forces.There are many types of joints of diaphragm walls with no standard form.Some patent methods are available.
The connection pipe method and the end⁃plate method are the commonly used joint methods.The former is highly watertight；on the other hand，its capability of transmitting bending moments and shear forces is not good enough.If the diaphragm walls are confined to the tempo⁃rary use of soil retaining，the connection pipe method can serve the purpose well.If they are meant for permanent structures，the end⁃plate method is recommended for its better capability to transmit bending mom⁃ents and shear forces.As shown in Figure 4.11，when the trench excavation of the primary panel is finished，put a connection pipe into the trench，then place the steel cage with a crane and pour concrete into the trench.Once the concrete has been poured，dismantle the connection pipe in two or three hours and proceed to the construction of the secondary panel.
Figure 4.11 Joint of diaphragm walls：the connection pipe method
To ensure the quality of diaphragm walls，many other construction details need to be taken care of.For the construction details of diaphragm walls，readers are advised to read Xanthakos（1994）.
The merits of diaphragm wall methods include：
①Low vibration，low noise，high rigidity，and relatively small wall deformation.
②Adjustable thickness and depth of the wall.
③Good sealing capability.
④May be used as a permanent structure.
⑤The diaphragm wall and foundation slabs form a unity，so that the former can serve as pile foundations.
The drawbacks of the diaphragm wall method are as follows：
①Massive equipment is required，long construction period，and great cost.
②The peripheral equipment（e.g.the sediment pool）occupies a large space.
③This method is not applicable to cobble⁃gravelly grounds.
④It is difficult to construct when encountering quick sand.
Though there are several merits in this method，and it involves matured construction technology，diaphragm walls as retaining walls have engendered quite a few excavation accidents.The reason is either geological uncertainty or bad quality control，or both.Engineers should keep studying how to improve the construction technology under different geological conditions by consulting case histories.