Technology for underwriters
6 Diaphragm walls
1 Application
2 Method
Deep excavations are required for the construction of high-rise buildings, underground garages and underground mass transpor transportt systems operating at several levels. Frequently it is not possible to use conventional methods when excavating the building pit, e.g. driving sheet piles to form a continuous sheet pile wall, either because there is not enough space or because such work would cause excessive noise or vibration.
A diaphragm wall consists of reinforced-concr reinforced-concrete ete components constructed in a mec mechanic hanically ally excavat excavated ed trenc trench.The h.The tren trenc ch is fill filled ed with with bentonit bentonite e mud, a suspension of the clay mineral bentonite in water, to support it against collapse during excavation. Usually, diaphragm walls are 0.80 m thick thic k and may be as much as 30 m deep. The walls are concreted in individual sections, their length varying between approximately 3 m and 10 m.
In such cases the construction of so-called diaphragm walls provides a good solution: – They can be used used in the immediat immediate e vicinity vicinity of existing existing buildings buildings.. – They can be be used not only to secure secure a buildi building ng pit but but also as a loadloadbearing member for the structure to be erected, thus rendering construction more economical.
3 Execution Basically, diaphragm walls are constructed in the following way: First, concrete guide walls are constructed in a shallow trench so that the deep vertical trench for the diaphragm wall may be excavated. After the formwork for the guide walls has been erected and the concrete placed, the cavities outside the guide walls are filled with earth and timber shores are wedged between the walls.
Excavation of the deep trench (cross section).
Next, the trench between the guide walls is filled almost completely with bentonite mud produced in an on-site mixing plant. The bentonite mud is fed through pipes from storage tanks into the trench. The deep trench for the diaphragm wall is dug by a special excavator fitted with a clamshell grab. The earth excavated from the trench is continually replaced by bentonite mud from the tanks. The bentonite mud, which at the same time has the effect of reducing friction between the earth and the grab, also penetrates into the surrounding soil, thus stabilising it and making it impervious. Earth that cannot be excavated with a grab must first be broken up by means of a drop chisel (weighing about 8 tons) and may then be removed with the grab.
When the trench has reached its full length and depth, a steel pipe or a prefabricated concrete element is placed at each end of the trench for the first wall section. For the next sections, such a pipe or element is only placed at the end opposite to the section already concreted. Subsequently,, a reinforcement cage is lowered into the trench by means Subsequently of a crane, the trench being filled with bentonite mud to hold up its sides.
Excavation of the deep trench (longitudinal section).
Lowering of the reinforcement cage into the excavated trench.
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concrete
earth
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Trough Rei einf nfor orce ceme ment nt cag cage e Wall sectio ion n Steel pipe Hydr Hy drau auli lic c extra extract ctio ion n devic device e Guide wa walls
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Next, the diaphragm wall is concreted by means of a tremie, i.e. a sheetmetal hopper with a pipe made up of several sections connected to one another.. The foot of the pipe is held about 1 m above the bottom of the another trench and the concrete is poured in through the hopper. To prevent the concrete from being mixed with the bentonite mud, the pipe must be arranged in such a way that it is always 0.5 m below the level of the fresh concrete. It is not necessary to vibrate the concrete as a sufficient degree of compaction is achieved by the weight of the bentonite mud above the concrete. As the concrete is filled into the trench, the bentonite mud is displaced upwards and is either forced between the guide walls to the adjacent trench or returned to the storage tanks.
Concreting of a wall section. 5
If steel pipes have been placed at the ends of the wall section, they are pulled out hydraulically after completion of concreting. If prefabricated concrete elements have been used, it is not necessary to extract them afterwards as they can be left in the ground as a part of the diaphragm wall. After the concrete has been given the usual time to set and harden, the adjacent trench may be excavated in the same manner.
Excavated trench (top view).
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4 Adv Advantages antages and entrepreneur entrepreneurial ial risks risks Diaphragm walls offer the following advantages:
The construction of diaphragm walls, however, however, may entail considerable entrepreneurial risks:
– Const Construct ruction ion work work entails entails a minimum minimum of noise and and vibration. vibration. – The walls walls can be be construct constructed ed in practica practically lly all types types of soil. soil. – Groundw Groundwater ater lowering lowering is normall normally y not necessary necessary as diaphragm diaphragm walls walls are practically impervious.
– Wedgin edging g or breaking breaking of the grab during during excavation excavation may require require largelargescale and expensive fishing work. – Additional excavation which which is necessary if the trenc trench h wall wall is broken throug thr ough h incr increas eases es the vo volum lume e of con concre crete te req requir uired ed and lea leads ds to ir irreg reguularities in the wall surface (reworking). – More time time and additiona additionall costs may be necessar necessary y if the reinforc reinforcemen ementt cage wedges during installation. – It may not not be possible possible to recov recover er spacer spacer tubes, tubes, so that that they have have to be abandoned. – Leaks may may result if if the excavatio excavation n is not accurate accurate to dimension dimension or from faulty concreting.
5 Insurance cover coverage age The construction of a diaphragm wall usually constitutes part of the contract works covered by a CAR policy policy.. When the policy is being drafted, care should be taken to exclude indemnifiability of additional costs and difficulties of the contractor by means of special conditions. Construction work for a diaphragm wall can lead to enormous liability claims under Section II of the CAR policy if unexpected cavities are cut into and there is a sudden loss of bentonite from the trench. The least damage that may result is the soiling of basements and sewers. Much graver damage may result from reduced trench stability:
– Crac Crack k formation formation in neighbou neighbouring ring build buildings ings – Set Settlem tlement ent of the the surroun surrounding ding ground ground surface surface When such risks are being writt written, en, special consideration should therefore be given to hazards that may involve third party liability.
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