What the Earth Asks of a Well Lining: Lateral Pressure, Ring Load, and a Thickness Rule

A circular lining in a hand-dug well works mainly in hoop compression: the ground presses inward on the ring, and the ring carries that pressure by squeezing itself. This paper computes the inward pressure by depth for four stated soil classes, using at-rest earth pressure above a stated water table at 10 m and adding full hydrostatic pressure below it. The pressure grows faster below the water table than above it, because water pressure is transmitted to the lining without reduction by any earth-pressure coefficient. At a depth of 30 m the computed total lateral pressure is 383.1 kPa in loose sand and gravel, 417.8 kPa in medium clay, 468.5 kPa in stiff clay, and 385.4 kPa in weathered rock and hardpan, producing hoop forces of 229.9 kN/m, 250.7 kN/m, 281.1 kN/m, and 231.3 kN/m respectively in a shaft of 1.2 m diameter. In a reference ring 0.100 m thick those forces correspond to hoop compressive stresses of 2.299 MPa, 2.507 MPa, 2.811 MPa, and 2.313 MPa. Plain concrete, taken at an allowable stress of 5.00 MPa, is nowhere stressed beyond its allowance in the computed range, so its thickness is governed by a stated practical minimum of 75 mm. Masonry, at an allowable stress of 1.67 MPa, governs by stress at depth: it requires 100 mm at 20 m in loose sand and gravel, medium clay, and weathered rock and hardpan and 125 mm in stiff clay, 150 mm at 30 m in loose sand and gravel and in weathered rock and hardpan, and 175 mm at 30 m in medium clay and stiff clay. The originated rule is stated in closed form — thickness is the greatest of the practical minimum and the hoop force divided by the allowable stress, rounded up — and tabulated by depth. On a stated stability criterion for unlined digging, the admissible unlined depth is 0.00 m in loose sand and gravel, 3.70 m in medium clay, 10.53 m in stiff clay, and 6.67 m in weathered rock and hardpan.