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Proper Borehole Grouting NOT "grouting" for the aesthetics
Proper borehole grouting NOT "grouting" for the aesthetics
Unless a water well is properly "grouted" or sealed into bedrock, surface water with potential ecoli & faecal coli and other harmful bacteria will leak down the outside of the permanently installed steel or uPVC casing and enter the groundwater under the bottom of the casing in the rock or through perforations in the welded steel casing joints.. When a borehole is drilled a wider diameter socket is drilled into the rock to accommodate the smaller diameter permanently fitted steel casing or uPVC casing.....so there is little or no resistance to prevent contaminated surface water "destroying" your groundwater source.
We are regularly asked.... can you effectively grout the annulus of a permanent uPVC or steel casing installed in a borehole without a temporary conductor casing being fitted to faciliate injection of the cement grout ?...
This is an important question to consider when it comes to drilling a properly sealed well to adhere to IGI Guideline (Water well guidelines,March 2007) and EPA Advice Note 14 "Borehole Construction and Wellhead Protection",Sept-2013.
Too often, lately, Drilling Contractors will suggest that they can place grout/cement into an open hole (with no conductor casing) and push down the permanent steel casing or uPVC casing through the cement and ........"magic" the well will be sealed. There is no magic in drilling or well sealing. That blob of cement tipped into the openhole prior to installing the permanent steel or uPVC casing is not down and tightly packed around the outside of the permanent casing achieving an even annular column of cement/grout at this critical interface where the overburden changes to competent rock.The blob of cement is perched at the first point of collapse of the sides of the well,near surface, before installing the casing and pushing a liner through it clearly doesn't push on the the cement down the hole following the casing.
From an Irish drilling experience....it's highly unlikely that you can confidently expect an unprotected openhole to remain stable while you install an inner uPVC or steel casing and proceed to effectively grout the annulus of the inner casing. (.......Leave aside the exceptions,for example; mud drilled boreholes & the occassional borehole with particularly stiff firm boulder clay overburden from surface to rock head).
An openhole in overburden is typically unstable,liable to lateral movement as drill spoil is removed from the well during the overburden drilling process,susceptible to shallow water strike ingress,drilling strata often comprising loose sand & gravel layers/weathered rock encountered and by its nature,this drilling strata collapses as soon as you drill through these layers.
The "open" borehole in the unlined overburden generally collapses as soon as the drill tools are removed from the borehole and prior to casings being installed.
It's essential to use a outer temporary conductor casing to ensure that the cement slurry has a clean open annulus around the inner casing in order to grout from deep into the competent rock socket into which the inner casing is fitted right back up the borehole to surface to ensure an effective grout seal.The grout is generally injected at the annulus of the inner uPVC casing/steel casing by a tremie pipe method ( nominal 1" hollow pipe).This grouting operation is undertaken by use of a suitable Grouting Plant which displaces cement slurry to a specific gravity under pumping pressure through the tremie pipe and maintaining the pressure until displaced deep into fissures into the rock. As grout returns to surface,sections of the threaded tremie pipe are pulled back,unscrewed and removed from the tremie pipe string.In addition, the outer temporary conductor casing needs to be retrieved in sections ensuring that it remains just above the level in the borehole where the grout is being displaced. Pumping of grout should continue in one continuous operation until the grout slurry clearly returns to surface having displaced any standing water in the annulus of the hole as the grout returns to surface.
Please refer to a related article on Domestic Borehole Grouting Review which puts the facts and figures around how a domestic well can be sealed effectively. Yes,it does cost more money to achieve an effective domestic well seal vis-a-vis an aesthetic attempt at sealing a standard 6" diameter well where cement is placed into an openhole prior to hammering in 6" steel casing. (Extra costs arise due to wider drilling diameters,grout band & bentonite pellet seal etc).The article attempts to dispell the myth that a 6" diameter domestic well in which no temporary conductor casing is fitted and cement is emplaced into the openhole prior to installing the 6" steel casing in the overburden.... that while it may aesthetically tick a box that the well is cement/sealed......in reality........save the €100-€200 that the Driller may charge for this element of works. Borehole grouting needs to be done right or its ineffective.
Borehole Grouting is advised in all instances for water wells per IGI Guideline (Water well guidelines,March 2007) and EPA Advice Note 14 "Borehole Construction and Wellhead Protection",Sept-2013