Consider CCTV survey of well to establish extent of steel casing fitted and status of casing in well. In addition CCTV may give more insight to rock formation, water strike levels etc. Can the PVC casing(if fitted) be retrieved fully and can steel casing be withdrawn as the grout returns to near surface.
Emplacement of a clean rounded gravel from bottom of borehole to xx mtr as a filter medium for groundwater passing through well shaft. In addition the gravel pack will be topped up with 1-2mtr sand layer or use of bentonite pellets to support the grout column in the top section of the well. Anticipate gravel pack & sand layer to be brought back up borehole shaft to provisionally 15-20mtrs of surface… unless otherwise instructed and/or subject to information from CCTV survey ensuring a sufficient head of grout is injected to deal with the artesian conditions.
On completion of backfill of clean pea gravel/sand to install tremie pipes to correct depth and using a grout plant to proceed to backfill cement and bentonite mix to surface. To achieve a seal in an artesian well *effectively, it is advised to grout with cement bentonite mix say 15-20m below bottom of outer casing back up to land surface.
*Effective borehole grouting should compromise 7.5kg bentonite with 75ltrs of approved water and 150kg cement. Neither additives nor borehole cuttings should be mixed with grout. Grouting should be carried out in one continuous operation, filling the annular space of the drilled openhole and the inner uPVC casing string from below the bottom of the outer steel casing to land surface. The grout should be installed by an Approved Method which emplaces the grout from the bottom upwards e.g. injection of grout by a specialist grout pump and tremie pipe method. The grout returning to the surface shall be wasted until it is deemed that the grout has fully returned to surface.
Headworks to compromise excavating 1.5mtr square and up to 1mtr deep around steel liner in centre of excavation. Where artesian conditions arise,a surface pump may be required to discharge water collecting into excavated area away from excavation. Readymix concrete to be poured to backfill excavation around and over liner to marry-in with surface finish.Or complete concrete plug 1 mtr below surface and proceed to reinstated with native clay & soils and reseed.
Others to consider excavating on outside of concrete plinth shuttering/forme to facilitate external drainage stone and rigid slotted drainage pipe on the perimeter of concrete plinth below surface up to 1mtr deep and lay pipeline to storm drain at lower level away from well head.
2 man crew to emplace gravel and complete excavation around protruding steel liner.
1st man responsible to operate plant and mix to correct cement weights, adjust pumping ratio and take samples.
2nd man to deliver cement bags to mixer pot and remove waste bags to waste skip. When cement is back to surface or all cement has been pumped to wash out tremies, the valves are to be closed and pipe works disconnected. Tremies are withdrawn and Grout Plant cleaned down.
All relevant borehole information is to be on site and reviewed before any operations take place. Full PPE to be worn at all times.
Health & Safety
All relevant COSHH sheets for cementing procedures should be available on request.