A brick lining holds a well wall up by acting as a ring in compression: earth pressure squeezes it evenly from all sides, so a round, tightly bedded ring of well-fired brick set in lime mortar carries the load with an average stress of about 0.3 MPa at 10 m depth in dry ground. What fails such a lining is not crushing but a ring that is out of round or has soft ground behind it, so keep it circular, plumb and backfilled behind, mortar the upper 3 m watertight, and leave the lower water-bearing part open.
Part of Deep Wells
Brick lining is a cylinder of masonry built inside a well shaft to support the soil and, in its upper part, to seal out surface water. It is laid by hand course by course, and is the main alternative to pre-cast concrete rings.
A Round Ring Turns Sideways Pressure Into Compression
Soil pushes sideways on the wall of a shaft with a pressure that rises with depth. In a circular shaft that pressure acts evenly all round, and a circular ring of masonry answers it by compressing along its circumference, which is the way masonry is strongest. This is why wells are round. A square shaft would bend its walls, and masonry is weak in bending.
The soil pushes on the outer face of the ring, so for an even external pressure p the compressive force in the ring is N = p × b per metre of height, where b is the outer radius. Divided by the wall thickness t it gives the average compressive stress.
Dry ground at 10 m. Assume soil weighing 18 kN/m³ and a sideways pressure of half the vertical pressure (typical textbook values for sandy or silty soil; the real values vary):
- Vertical pressure = 18 × 10 = 180 kPa
- Sideways pressure p = 0.5 × 180 = 90 kPa
- A 1.0 m bore (inner radius 0.5 m) with a wall one brick length thick, 0.22 m (assumed; measure your bricks), has outer radius b = 0.5 + 0.22 = 0.72 m
- Ring force N = 90 × 0.72 = 64.8 kN per metre of height
- Average stress = 64.8 ÷ 0.22 = 295 kPa, about 0.3 MPa. The inner face of a thick ring carries more than the average, about 1.2 times it from the thick-cylinder formula, so about 0.35 MPa
Saturated ground at 10 m. Below the water table the water presses at 9.81 kPa per metre of depth and the soil’s effective weight falls to 18 − 9.81 = 8.2 kN/m³:
- Soil part: 0.5 × 8.2 × 10 = 41 kPa
- Water part: 9.81 × 10 = 98 kPa
- Total p = 139 kPa; N = 139 × 0.72 = 100 kN/m; average stress = 100 ÷ 0.22 = 455 kPa, about 0.45 MPa
These stresses are small beside the crushing strength of well-fired bricks, which is in the order of several MPa or more (a recalled order of magnitude; test yours with the method in Fired Bricks). The water part of the load is relieved when the well is full, because the water inside balances it; it acts in full when the well is bailed dry during digging or cleaning, and the lining must be built for that case.
Set a working stress of 0.5 MPa (an assumed conservative value, not a standard) and solve p × (0.5 + t) ÷ t = 500 kPa for the thickness t. Dry ground: 90 × (0.5 + t) = 500 t gives t = 45 ÷ 410 = 0.11 m. Wet ground: 139 × (0.5 + t) = 500 t gives t = 69.5 ÷ 361 = 0.19 m. One brick width (about 0.1 m) is borderline even in dry ground at this depth, and a double ring (about 0.21 m) clears the wet case. Thicker linings are not only about crushing: they give the ring the stiffness to stay round while the ground settles and the backfill is placed.
Lay The Bricks As Stretchers In A Tight Ring
Radial bricks (headers) in a 1 m bore leave wedge-shaped joints. With an inner circumference π × 1.0 = 3.14 m and bricks 0.105 m wide, 30 headers fit on the inner face; on the outer face of a 0.22 m wall the circumference is π × 1.44 = 4.52 m and the gaps open to (4.52 − 30 × 0.105) ÷ 30 = 46 mm, far too wide for mortar. Lay the bricks lengthwise (stretchers) instead: about 14 bricks (π × 1.0 ÷ 0.22 = 14.3) make a ring in a 1 m bore, each course shifted a half brick from the one below, as in Brick Bonds. For a thicker wall lay two such rings, with the joints of the outer ring offset from the inner.
What You Need
- Well-fired bricks — fully fired, not soft or powdery; underfired bricks soften in water (Fired Bricks, Brick Making)
- Lime mortar — a hydraulic lime mortar for the sealed upper zone, which sets in wet conditions (Hydraulic Lime, Lime Mortar)
- A plumb line and a fixed radius rod — a stick pivoted at the centre of the shaft to check every course is round
- Gravel and puddled clay — for backfilling behind the lining
- A curb — a timber or concrete ring with a sharp lower edge, if you will sink the lining
Method: Build It Round, Plumb And Backfilled
Step 1: Soak the bricks. Dry bricks pull water from the mortar. Wet them before laying.
Step 2: Lay on a firm base. In dig-and-line, trim the section, lay the first course on a firm level base, and build up in courses. Check each course against the radius rod and the plumb line.
Step 3: Pack behind. Fill the gap between lining and earth as you go: gravel in the lower, water-bearing zone; clay or grout in the upper 3 m (Surface Seal).
Step 4: Mortar by zone. In the top 3 m, fill every joint and plaster the inside watertight. In the lower aquifer zone, leave joints open or dry-stack so that groundwater can enter.
Step 5 (alternative, sinking): Build on a curb above ground. Build the brick cylinder on a curb, allow the mortar to set so it can bear handling, dig out inside it and let the lining sink, adding courses on top. Keep it plumb: dig more under the high side to bring it back. Do not force a lining down by overloading it.
Common Mistakes
- An out-of-round ring. Bending, not crushing, is what cracks masonry. Check the radius every course.
- Fresh mortar loaded too early. Lime mortar gains strength slowly; do not sink or backfill against fresh work.
- No backfill behind. A void behind the lining lets the ground move onto it unevenly.
- Soft bricks. Bricks that crumble in water make a lining that dissolves, and cannot be fixed without relining.
- Fresh lime in the water. New mortar is strongly alkaline; bail and flush the well before using it (Community Water Testing).
Safety
Bricks and mortar fall, and the shaft is a confined space
Lower bricks in a covered basket, never by throwing; nobody is below while a load is lowered or lifted. Every entry follows the ventilation, harness and two-above rules in Safety Protocol: force air to within half a metre of the bottom for 20-30 minutes, keep it running, and haul a collapsed worker out from above, never going down. Wet lime mortar burns skin and eyes: wear gloves and protect the eyes, and rinse with plenty of water if splashed.
What’s Next
- Learn next: Surface Seal — sealing the top of the lining against surface water
- Combine with: Concrete Rings — the alternative lining