Design of injection well
In order to prevent serious water and soil loss in the process of excavation, a 200 mm diameter reinjection well is set around the foundation pit.
The injection well is set between the foundation pit and the building, and the north side is set within the enclosure. The distance between the reinjection wells is 15-20 M.
Construction technology and technical requirements of injection well
- Construction technology
Construction technology: setting out and positioning → reinjection well drilling → cleaning and mud changing → Johnson well screen pipe installation → filling → well flushing → reinjection → regular reinjection。
- Construction method
(1) Construction preparation
“Three connections and one leveling” shall be done well on the construction site to meet the requirements of equipment entry and exit, mud preparation, complete with filter materials, pipes and water injection equipment.
(2) Reinjection well drilling
The casing of geological drilling machine is used for vertical drilling, the verticality is controlled within 1%, and the borehole diameter is 200mm. Before drilling, it shall be connected with the site management personnel to check the condition of nearby pipelines to prevent damage to the pipelines
(3) Hole cleaning and slurry exchange
When drilling to the design hole depth, in order to prevent mud sedimentation and hole collapse, hole cleaning and mud change shall be carried out in time. The sand content of the replaced mud is not more than 5%, and the sediment thickness at the bottom of the hole is not more than 20cm.
(4) Well pipe installation
1) Fabrication of Johnson well screen p: lower Ф 70mm PVC pipe, arrange quincunx filter holes every 20cm on the pipe wall, including one layer of wire mesh and two layers of nylon mesh filter layer.
2) Lower the Johnson well screen pipe: the bottom of the pipe shall be filled with 250mm thick 2-5mm stone meter filter material. The upper end of the well pipe shall be at least 50cm higher than the ground. It is strictly prohibited to force the well pipe into the well hole.
5) Backfill filter material
When filling, the well pipe must be in the middle and the circulating water filling method shall be adopted. The filter material shall be 2-5mm stone meter filter material, which shall be evenly and symmetrically scattered around the well pipe with spade. The filler shall be within 1.0m from the ground, and the upper 1.0m shall be backfilled with cement slurry.
(6) Well flushing
After the completion of filling, the well shall be washed with circulating water, and the thickness of sediment in the well shall not exceed 200mm.
(7) Reinjection
After the well pipe is cleaned and qualified, the reinjection well shall be sealed. The top end of the well pipe shall be welded with steel plate that can reach the bearing thickness. The opening of the circulating water pipe shall be welded on the steel plate to ensure the reinjection volume of the reinjection well.
(8) Regular back lifting
In order to prevent and deal with the plugging of tube well, the method of back lifting is mainly adopted. The number and duration of reinjection are mainly determined by the size and permeability of aquifer particles. In karst fissure aquifer, pipe well reinjection can maintain the reinjection capacity without reinjection for a long time; for fine-grained aquifer, reinjection is particularly important, and the reinjection time should be once a week.