Double-walled cofferdams are those in which two parallel rows of steel sheet piles
are connected with means of a track or a plurality of steel-enclosed purlin brackets. The gap between the two rows of steel sheet piles
is generally filled with granular material, such As gravel or gravel.
When the external steel sheet pile as anchor pile, when the inner steel sheet pile anchorage retaining wall design. U-shaped or 2-type steel sheet pile is more suitable for this type of structure.
Such cofferdams shall be analyzed as a whole for gravitational structures. In order to avoid capsizing and sliding failure, the width of the cofferdams shall not be less than 0.8 times the height of retaining or retaining walls. It is suggested to use Jehn's small logarithmic helix Method to check the overall stability of the structure.
The ends of the cofferdams and the middle position of the double cofferdams shall be provided with transverse compartments to form sturdy joints to facilitate construction and to prevent possible damage. The nodes may consist of a square-shaped rectangular cell connected in both directions.
It is recommended that the bottom of the inner surface of the steel sheet piles be provided with with holes at the bottom of the steel sheet pile so that the packing can be freely drained and the water pressure acting on the inner piles can be reduced to prevent The shear strength with the increase of time. Drain holes are only effective for small cofferdams because the fillings are not always drained and if necessary drainage can be achieved with well point dewatering. Even if these measures are taken, the water pressure acting on the sheet Pile is still designed to have a margin. Clay, silt and all materials of this type can not be used as backfill material. If the soil is contained in the upper part of the foundation soil in the sheet pile cofferdam, it must be removed before Backfilling.
Steel sheet piles must be driven to a certain depth below the excavation or surface in order to have sufficient depth to generate passive resistance. Under normal circumstances, the steel sheet pile will move to the excavation side, acting on the retaining side of the earth When the excavation surface or its lower part is non-cohesive soil, the sheet pile must have enough depth of embedment to control the seepage. In addition, the bearing capacity of foundations need to be calculated to meet the capacity Of the cofferdam and the upper part of the overload capacity requirements.
If the excavation surface has a rock layer, it is not appropriate to use this cofferdam except for the following circumstances:
1) A steel sheet pile can penetrate into the rock layer and reach a sufficient depth (see Chapter 10).
2) can be installed in the cofferdam at the bottom of the rod (most likely installed underwater)
3) A groove may be pre-set on the rock, into which the steel sheet pile can be inserted and filled with grout.
4) The end of the pile can be fixed to the rock through the positioning pin.
If the steel sheet pile hit the bedrock or a depth below the excavation surface can only rely on the friction and the weight of the base to resist the overturning and sliding cold-formed Z-beam sheet pile prices. In this case, the earth pressure Acting on the retaining side is between the resting earth pressure and the active earth pressure, and its magnitude depends on the magnitude of the displacement.
As the cofferdam internal vertical stress distribution is not uniform (affected by bending moment) on the outer row of earth pressure on the pile may be greater than the active earth pressure, so the design of the active earth pressure to increase by 25%.