Borehole grouting by; Tremie pipe method including use of Well Grouting Band
To cement either steel casing or uPVC casing to prevent surface pollution and support any weak geological structures at annulus to cemented casing.
On completion of running permanent casing or installing uPVC screen/casing string, tremie pipes to be installed. When tremies are at correct depth, the well grouting plant will be set up and all pressure lines checked and whip-checked. A fresh water slug will be pumped to check for circulation that no back pressures are encountered.
Well Grouting Options
To achieve proper sanitary seal protection *effectively, it is advised to grout with cement bentonite mix emplaced at annulus to a uPVC casing (or steel casing) say 10m below bottom of outer temporary casing into competent rock socket back up to land surface.
Emplacement of a clean rounded gravel pack at annulus to the uPVC screen is below the grouted section of the well may be necessary as a filter medium for groundwater entering the uPVC screen but in addition the gravel pack with 1-2mtr sand layer on top, is essential to support the grout column in the top section of the well. Either backfill circa 1 linear mtr with sand layer (or bentonite pellets) overlaying the gravel pack. Note groundwater sourced at depth does not come into contact with the outer steel liner (this is relevant where the uPVC cemented in).
ORWell Grout by Grout band fitted onto the circumference of uPVC casing which in effect is a steel ring plate with an O.D. slightly less than the openhole drilling diameter and the centre cut out to allow a tight fit with outer circumference of uPVC casing. The grout band is generally secured to uPVC casing by placing pop-rivets through the "L" brackets (90° steel brackets) into the uPVC casing on the underside of the grout band. The grout band should generally be positioned for example10mtr below the butt of the steel casing ie where bedrock/overburden is encountered.
*Effective borehole grouting should comprise 9.5 parts cement to one half part (5%) bentonite with 45ltrs approved water per 50kg bag of cement. Neither additives nor borehole cuttings should be mixed with grout. Grouting should be carried out in one continuous operation filling the annular space between the drilled open hole and the inner uPVC casing string from below the bottom of the outer steel casing to the land surface. The grout should be installed by and Approved Method which emplaces the grout from the bottom upwards by injection of grout by specialist grout pump and tremie pipe method. The grout returning to the surface shall be wasted until it is deemed that the concentrated grout has full returns oozing out over the liner at surface. No further work should be done on the well until the grout has firmly set.
1st man responsible to operate plant and mix to correct cement weights, adjust pumping ratio and take samples.
2nd & 3rd 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, the valves should be closed and pipe works disconnected. Tremies are withdrawn and the Grout plant and tremie pipes washed down thoroughly.
All relevant 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.