Bored pile wall
A bored pile wall is a component used to secure a (mostly artificial) jump in terrain, for example an excavation pit . A bored pile wall consists of many individual reinforced and unreinforced bored piles made of in-situ concrete .
Manufacturing
( 1 ) First, a suitable device (rotary drill, grab, etc.) is used to make a hole with the diameter and depth of the pile.
( 2 ) To prevent the borehole from collapsing, the borehole is cased (standard today) or a support fluid made of bentonite is used (shown here). The support fluid is continuously refilled, compensates for the earth pressure and seals the borehole. If there is groundwater, it can happen that the soil drifts into the borehole from below despite supporting piping (hydraulic ground failure). In order to equalize the water pressure of the groundwater, it is possible to drill with water load. The borehole is filled with water. The water level must be at least as high as the groundwater level in the surrounding soil. In the case of stable ground (rock), the borehole can also be made uncased and unsupported.
( 3 ) Once the final depth has been reached, the prefabricated reinforcement cage is normally lifted in one piece. In the case of very long piles, it may also be necessary to assemble the pile reinforcement from several strands up to the required length.
( 4 ) The concrete is then poured in using the contractor method : A concreting pipe (pouring pipe) is inserted into the borehole up to the pile base. The concrete emerges at the lower edge and pushes water, mud and the supporting liquid upwards. The concreting process therefore takes place from bottom to top. This means that there is no contamination, segregation or watering down (change in the w / c value ) of the concrete. During concreting, the displaced suspension is continuously sucked off and recycled, the water and mud in the borehole run out over the top of the drill pipes.
execution
Depending on the arrangement of the piles, there are three types of bored pile walls.
Overcut bored pile wall
The overlapping bored pile wall is produced using the so-called pilgrim step method.
In this design, the center distance of the piles is smaller than the pile diameter.
First of all, the so-called primary piles (every second pile) are drilled and concreted without reinforcement. In order to be able to maintain the even spacing of the bored piles, a drilling template is first made, through which the piles are then drilled. It also serves as a guide for the secondary piles. The use of a drilling template is mandatory according to VOB / C DIN 18301.
After the concrete has partially set, the secondary piles in between are drilled, with the secondary piles cutting into the primary piles. The secondary piles will be reinforced. The concrete of the primary piles should not yet have reached full concrete strength so that it can be drilled over more easily.
The overlapping bored pile wall should be impermeable to groundwater and is used for deep construction pits in the groundwater. In the case of completely enclosed construction pits, the outside groundwater does not have to be lowered if the pile wall is tight at the bottom, i.e. it is integrated into the groundwater reservoir or supplemented by a sealing base at the bottom.
Tangent bored pile wall
In this design, the center distance of the piles corresponds exactly to the pile diameter, so that they are tangent (touching). One pile after the other is drilled and all piles are reinforced.
Tangent bored pile walls are not used as often as detached bored pile walls. Their center distance is equal to their diameter, so there are no gaps between the piles. This type of wall has a higher load-bearing capacity than the detached bored pile wall and the positioning of the anchors is easier because there is no need for transoms. A disadvantage of this construction is the high consumption of concrete. The tangential bored pile wall is usually not impermeable to water, as the subsequently drawn drill pipes do not guarantee an absolute form fit.
Dissolved bored pile wall
In this design, the center distance of the piles is larger than the pile diameter. Between the piles so a distance which usually remains with shotcrete or injection bodies is infilled.
Statics
The piles are clamped in the ground and therefore act as cantilever arms . At great depths it may be necessary to anchor the bored pile wall back. This is done with grouted anchors , with temporary anchors or permanent anchors , depending on the purpose. Opposite pile walls can also be braced against each other with belts and stiffeners made of steel or reinforced concrete.
use
Bored pile walls are used where other (consistently cheaper) construction pit structures are no longer possible due to the loads or the depth. Another reason for its use - especially in inner-city areas - is the almost vibration-free production and the low flexibility of the shoring. The bored pile wall is one of the rigid types of construction and also enables loads from neighboring buildings, traffic systems and other loads to be absorbed without settling next to deep construction pits. With the usual drilling methods and their correct use, bored pile walls can be used in all existing soils ( soil class 1 - 7) and at all groundwater levels.
The bored pile wall usually remains permanently in the ground and, in contrast to other types of shoring such as B. girder pile wall , sheet pile wall not reusable.
The bored pile wall becomes economically interesting if it is not only used as a temporary excavation support, but is also included permanently in the foundation of the building. It is also suitable for permanent security measures such as slope protection or the outer walls of underground garages.