8.4 Excavating

8.4.1 Supporting the sides of the excavation

A supporting fluid is usually used during excavation. In some cases, it may be possible to excavate using water as a supporting fluid or in dry conditions.

Dry excavation may be used in some soils with cohesive properties or in rock, provided their strength is sufficient to ensure stability of the sides of the trench. In soils where no comparable experience is available, a trial excavation should be made.

During excavation, the level of the supporting fluid will fluctuate, but it shall not be allowed to fall below the level required for excavation stability. In addition, the level of the supporting fluid shall remain above the base of the guide-walls, unless there is no risk of caving of the soil below the guide-walls.

8.4.2 Excavating sequence

The excavation may be carried out continuously or in panels. The sequence of excavation, panel lengths and distances between panels being excavated depend on the ground conditions, the type of wall, and the type of excavating tools.

The excavation of a panel shall not be started before the concrete or the self-hardening slurry in the adjacent panel or panels has gained sufficient strength.

The use of chisels, other tools, or in some cases blasting, which may affect the nearby panels already filled with concrete or self-hardening slurry shall not be made before the material in these panels has sufficient strength to resist the stresses developed during these operations.

8.4.3 Loss of supporting fluid

When a sudden and significant loss of the supporting fluid occurs during excavation, the excavation shall be refilled immediately with an additional volume of supporting fluid, possibly containing sealing materials. If this procedure is impossible or insufficient, the excavation shall be backfilled as quickly as possible with lean concrete or other material which can be excavated later.

In situations where significant loss of supporting fluid may occur (e.g. highly permeable soils, cavities) , an additional volume of supporting fluid, and possibly sealing materials, shall be kept available.

8.5 Cleaning the excavation

Cleaning is necessary when a supporting fluid has to be replaced by concrete or another material. The bottom of the excavation and the surface of the joints shall be cleaned and, if necessary, the supporting fluid shall be desanded or replaced. In the case of a bentonite suspension, the properties specified in Table 1, for the phase "before concreting", shall be obtained. Where elements such as stop ends or reinforcement cages are to be inserted, cleaning shall be carried out before insertion. The cleaning procedure, as well as the time between operations, shall be established on the first few panels.

8.6 Forming the joints

The joints are normally formed either by using steel or concrete stop ends or by cutting into the concrete or hardened material of the previously cast adjacent panel. In some cases, waterstops can be incorporated into the joints.

Stop ends shall be of adequate strength and properly aligned throughout their length.

In the case of stop ends which are extracted vertically, the extraction shall be made gradually during the setting of the concrete or other material. The detailed procedure for forming the joints shall be established during construction of the first panels of each type. In the case of stop ends which are extracted laterally, the extraction shall be made upon completion of the excavation of the adjacent panel.

8.7 Placing the reinforcement or other elements

Reinforcement cages, precast concrete panels or other elements (such as sheetpiles, membranes) shall not rest on the bottom of the excavation, but shall be suspended from the guide-walls.

8.8 Concreting and trimming

In dry excavations, as permitted in 8.4.1, concreting shall be carried out in such a way as to avoid segregation. Direct pumping is permitted in dry excavations. Vibration of the concrete is not permitted when the slump value exceeds 100 mm (class S3, S4, or F3, F4, in ENV206).

In excavations using supporting fluids, concrete shall be placed beneath the supporting fluid through one or more concreting pipes. Concreting pipes are usually tremie pipes, which are pipes equipped with a hopper at the top, but may also be pipes connected directly to concrete pumps.

The concreting pipe shall be clean and watertight. Its inner diameter shall be at least 0,15 m and 6 times the maximum aggregate size. Its outer diameter shall be such that it passes freely through the reinforcement cage.

The number of concreting pipes to be used in a panel shall be adjusted so as to limit the horizontal distance which the concrete has to travel from the concreting pipe.

In normal circumstances, it is recommended that the horizontal distance the concrete has to travel be less than 2,5  m.

It is recommended that at least one concreting pipe is used per reinforcement cage, where there is more than one cage per panel.

When several concreting pipes are used, they shall be arranged and supplied with concrete in such a way that a reasonably uniform upward flow of the concrete is assured.

When starting concreting, the supporting fluid and the concrete in the concreting pipe shall be kept separate by a plug of material or by other suitable means.

To start concreting, the concreting pipe shall be lowered to the bottom of the trench and then raised approximately 0,1 m.

After concreting has started, the concreting pipe shall always remain immersed in the fresh concrete. It is recommended that the minimum immersion should be 3 m, but it can be reduced to 2 m when the level of concrete is accurately known. The immersion depth may have to be reduced when the concrete approaches ground level to facilitate concrete flow.

The average rate of concrete rising over the full height of the panel should not be less than 3 m/h.

When delays are anticipated which may adversely affect the quality of the concrete, e.g. due to traffic conditions, an appropriate percentage of retarder may be added to the concrete during the mixing process.

Since the top of the cast concrete may not be of the required quality, sufficient concrete shall be placed in the panel to ensure that the concrete below the required cut-off level has the specified properties.

The required quality of the concrete at the cut-off level can be achieved by providing an additional height of concrete above the cut-off level, its value depending on the depth to the cut-off level, the wall dimensions and the number of concreting pipes. In cases where the cut-off level is close to the top of the guide-walls, this can be achieved by allowing the concrete to overflow.

After concreting, empty excavations above the concrete level shall be backfilled with lean concrete or other suitable material. It may also be necessary in some circumstances to insert props between the guide-walls.

Trimming of the concrete to cut-off level shall be carried out using equipment which will not damage the concrete, reinforcement or any instrumentation installed in the panels. Final trimming to cut-off level shall only be carried out after the concrete has gained sufficient strength to avoid damage. Where possible, some preliminary trimming above cut-off level may be carried out before the concrete has set.

EN 1538:2000 Execution of special geotechnical works — Diaphragm walls