Excavation of the trench wall

Excavation of the floor from the slot wall trench is carried out with slot wall grippers or slot wall milling. The width of the excavation tool is standardized in DIN 4126 (section 7.2) with 40, 50, 60, 80, 100, 120, 150 and 200 cm. The following excavation tools are common:

  1. In Germany, the heavy-duty slit wall grab is combined with a heavy excavator suitable for this purpose. This slot wall gripper usually has a mechanical closing device, if necessary also a hydraulic drive (fig. 1.5). The width of the excavation tool is generally between 60 cm and about 1.20 m. However, also slit walls with a width of 1.50 m to 2.00 m are carried out in individual cases (usually as berets).

  2. Slot wall grippers have lateral stabilizing devices and often a very high weight (70 to 240 kN) in order to produce a vertical wall. The verticality of a slot wall grabber can be checked with a built-in inclinometer (inclination measuring device). An experienced excavator can recognize a slanting of the gripper on the course and the twisting of the rope. With the grippers on the rope, it is possible to carry out slit walls directly next to existing buildings. The performance of a slot wall grabber is approximately 150 m² of slab wall per layer. However, this value depends on the geometry of the lamella and the soil conditions.

Fig. 1.5 Conducted slot wall gripper

  1. Above all in France, a gripper which is rigidly guided on a Kelly rod is often used, which usually has a hydraulic closing device. This gripper has also proved its worth when using large bore holes.
  2. In relatively homogenous, soft and cohesive soils, the suction drills coming from Japan have proved their worth. In this case, rotating tools rotate about a vertical axis, rotating chisels, while the loosened drill material is extracted by means of a bentonite mud and separated from the bentonite suspension in special separation plants (Fig. 1.6.)

Fig. 1.6 Slotted wall milling machine (Source: Brochure of the company Tone Boring Co., Ltd) a) View of the slit wall milling machine

B) Vertical control of the milling machine with inclination sensor

Recent developments in slotted-wall milling work with two chisel heads rotatable about horizontal axes, which can also carry through tougher rock fragments (Fig. 1.7).

  1. In the case of sands and cohesive soils, slot wall milling work quickly, but the machinery for separating the excavation floor from the supporting liquid is very complex. The facilities can have a space requirement of 500 m²

Fig. 1.8 Predrilling of slots

  1. In the case of gripper operation, obstacles such as larger rock blocks or contiguous solid rock fragments with heavy chisels (weight 40 kN to 200 kN), which are attached to the excavator instead of the gripper, must be punctured or laterally displaced (Fig. Concurrent rock fragments may also be perforated with rotating drills.

Fig. 1.9 Slotted wall chisel

The soil material lifted with the gripper is emptied directly into special vehicles or in troughs and removed.

During excavation, the earth walls of the trench wall trench are supported by a supporting liquid which fills the trench wall trench into the guide wall area. This supporting fluid is treated with water by means of intensive agitation or pumping on the construction site of dry bentonite, which is delivered in sacks or in silo containers (Fig. 1.10) and swells until the optimum properties are achieved before use in the slot in special storage containers 4 to 8 hours.

Fig. 1.10 Mixing system and storage tank

During the excavation work and until the end of the concreting process, the suspension level must not fall below the statically required height and not below the guide wall. Insofar as the incision of subterranean cavities (old sewers, basements, etc.) must be expected, a sufficient quantity of replacement suspension must be provided. Cut cavities can be closed by cement and clay in sacks, sand, gravel, etc. If a raised slit is suspended under suspensions over night or over the weekend, the necessary suspension level must be ensured by a standby service.