A sanitary apron is a solid concrete pad, 3 m across and 10 cm thick, poured around the wellhead and sloping away from it so that spilled water and rain run off to a drain instead of soaking in beside the well. Pour it in one piece with the wellhead collar, slope it between 2% and 5% to its edge, drain it at least 5 m away, and keep it sound, because a crack in the apron is a channel straight into the seal.
Part of Deep Wells
The sanitary apron is the paved area around a wellhead. It keeps the ground next to the well dry and sealed, so that water spilled while drawing, and rain, cannot soak down along the lining.
The Ground Beside The Well Is The Easiest Way In
Every bucket, pump spout and rainfall puts water on the ground at the wellhead. Without an apron it pools and soaks in beside the lining, and the more it is used, the wetter and muddier the ground. A paved, sloping apron sheds that water to a drain instead. It works together with the sealed upper 3 m (Surface Seal), which stops water that does get into the ground from running down to the aquifer.
The Apron Needs Only A Few Centimetres Of Fall And Under A Cubic Metre Of Concrete
The Deep Wells guide specifies an apron 3 m in diameter and 10 cm thick, sloping away from the wellhead. Other guides in this wiki give smaller aprons (a radius of 1.5 m as the recommended size and 1 m as the minimum in Well Lining, a 1.2 m slab in Well Development, 1 m in Hand Pumps); where space allows, build the largest.
Fall. The radius is 1.5 m. The collar stands at the high point and the edge is lower. The drop to the edge depends on the slope:
- At 2%, the fall of the pad round a standpipe in Standpipe Construction: 0.02 × 1.5 m = 0.03 m, 3 cm
- At 1 in 20, 5%, the apron slope in Well Development: 0.05 × 1.5 m = 0.075 m, 7.5 cm
Over a 10 cm slab a fall of 3-7.5 cm leaves the edge only 2.5 cm to 7 cm thick unless the slab is formed with a constant thickness, so cast the slab with parallel faces, the bottom sloped as well, or thicken the edge.
Volume. The area of a disc 3 m across is π × 1.5² = 7.07 m². At 0.10 m thick that is 0.71 m³ of concrete before subtracting the well opening. Taking the opening as a ring of outside diameter 1.3 m (π × 0.65² = 1.33 m²), the net area is 7.07 − 1.33 = 5.74 m² and the volume is 0.57 m³. Order 0.6-0.7 m³.
What You Need
- Concrete — cement or lime concrete (Concrete Making), enough for 0.6-0.7 m³ in one pour
- Forms — a circular edge of boards or earth
- A gravel base — firm and compacted
- Light reinforcement — wire mesh or bars, to control shrinkage cracks
- A drain channel — stone-lined or concrete, leading at least 5 m from the well
- A soakaway — at the end of the drain (Soakaway)
- Damp curing materials — wet burlap or sand to keep the slab damp
Method: Collar, Base, Pour, Cure, Drain
Step 1: Prepare the base. Remove soft soil round the well, compact a gravel base, and form the circle. The base must be firm; a slab on soft ground cracks.
Step 2: Set the collar. The lining or casing stands at least 50 cm above the finished apron surface (the wellhead height in the Phase 0 well guides), so the apron slopes away from it.
Step 3: Pour in one piece. Place the concrete in one continuous pour, bonded to the collar, working it into the gap at the lining. A cold joint between collar and apron is the weak point.
Step 4: Slope and edge. Strike the surface to fall evenly from the collar to the edge, and thicken or turn down the edge so rain cannot undercut it.
Step 5: Cure. Keep the slab damp for days; thin slabs dry out and crack. Use the Curing guide as the model (Curing).
Step 6: Drain it. Cut a channel from the edge, falling away from the well, to a soakaway at least 5 m from the wellhead (the distance used in the Phase 0 well guides and Well Development).
Step 7: Inspect. Check regularly for cracks and for water standing on the slab; repair cracks before the next rain.
Common Mistakes
- A gap between collar and slab. Water goes straight down the lining.
- A flat slab. Water pools on it and soaks through the joint.
- A slab on soft ground. It cracks and tilts.
- A drain that stops near the well. Spilled water soaks in beside it.
- Livestock troughs on the apron. Animals foul it; keep stock away.
Safety
Wet concrete and heavy forms
Wet cement and lime burn skin and eyes: wear gloves and eye protection and rinse splashes at once. The opening of the well stays covered while you work around it, with nothing dropped in, and no one works over it without a cover. Do not let concrete or wash water fall into the well.
What’s Next
- Learn next: Well Protection — the whole set of barriers
- Combine with: Surface Seal — the seal that meets the apron