Erosion at the Base of an Exposed Mud-Block Wall
A garden wall 2.4 m high is built from 300 x 150 x 150 mm unstabilized mud blocks with a 1:3 cement-sand mortar. It faces south-west and is exposed to wind-driven rain. After six years, the base course has eroded back 40 mm over a 2 m length, with a white efflorescence band at 300 mm above grade and a 3 mm gap between wall and concrete footing. Previous repairs used a stiff 1:3 cement mortar, which has cracked and fallen off. The ground slopes toward the wall and a downpipe discharges 1.5 m away.
1. Diagnose the erosion mechanism and explain why the cement repair failed.
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Erosion is splash-driven: 40 mm loss at the base, white efflorescence at 300 mm indicates salts migrating with water and evaporating, and the 3 mm gap at the footing lets water enter. South-west driving rain wets the wall; ground sloping toward it and a downpipe 1.5 m away concentrate water. Stiff 1:3 cement mortar is stronger and less permeable than mud blocks; it cracks at the interface, traps water, and spalls. The repair failed because it was incompatible, not because cement is inherently bad.
2. Propose a repair and detailing strategy covering materials, profile, drainage, and splash protection.
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Repair: cut back eroded block to sound material, shape a sloping plinth, and repoint with a weak lime-earth mortar (about 1:1:6 lime:earth:sand), not cement. Provide a 150 mm high, 1:20 sloping concrete or stone plinth with a drip edge and a separation layer. Redirect the downpipe to a drain at least 3 m away; regrade ground to fall 2% away. Add a 600 mm roof overhang or tile creasing and cap the wall. Use a breathable silane or siloxane water repellent after repair, not film-forming paint.
3. Outline a maintenance and inspection plan for the repaired wall.
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Maintenance: inspect the base and downpipe after each wet season; probe for soft spots; repoint cracks under 2 mm with lime mortar; keep the splash zone clear; check the plinth drip; reapply repellent every five to seven years. Install erosion pins and record annual loss. This plan monitors performance rather than assuming the repair is permanent.