Bulging Rubble Retaining Wall in the Monsoon
Your gang has built a 1.8 m high random rubble retaining wall in Kandy to hold back a cut terrace. The stones are rounded river boulders, the mortar is 1:6, and the wall has some batter and a 450 mm base. No through stones were placed and there are no weep holes. Clay soil was tipped straight in behind the wall and compacted with a jumper. Two weeks into the monsoon a vertical crack appears near mid-length and the face bulges about 40 mm. The client wants you to re-point the crack with rich mortar and paint the wall before the rains get worse. The terrace above carries a small shed and a footpath runs along the toe of the wall.
1. What is actually happening to this wall, and why is it not a surface defect?
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The wall is being pushed over by water and it cannot resist it.
- Clay backfill with no weep holes and no drainage means monsoon rain soaks the clay; saturated clay also loses shear strength and exerts far more pressure than dry soil, plus direct water pressure.
- No through stones means the wall is really two thin skins of stone around a loose core. The two faces are not tied together, so the wall has very little resistance to overturning.
- Rounded river boulders roll rather than interlock, and 1:6 mortar is weak; thick, poorly filled joints let water sit inside the wall itself.
- The vertical crack plus a 40 mm bulge is the beginning of a structural failure at the section carrying the most pressure, not a cosmetic crack. The shed above and the footpath below both load the risk.
2. What do you do in the next hour, and what do you refuse to do?
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Next hour
- Stop work at the wall and keep people, stock and vehicles off the terrace above and away from the toe of the wall; cordon with tape and a barricade.
- Remove or move back any stacked material or shed contents that are adding surcharge.
- Tell the client and your supervisor or the engineer the same day, in writing, describing the crack, the bulge and the drainage faults. This wall needs an engineer's assessment.
- If the wall could fall on the footpath, keep the path closed and, only under direction, relieve the backfill or shore the face from the front.
Refuse
- Re-pointing the crack with rich mortar and painting the wall. That hides a movement that is still going on and puts the blame on the mason when it collapses. Cosmetic treatment of a structural failure is not a repair.
3. When the wall is rebuilt, what details would you insist on so that the same failure does not repeat?
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- Foundation: on firm ground, wider than the wall base, taken down below the ground level in front of the wall, stepped where the slope falls.
- Stones: angular, flat-bedded stones laid on their natural bed, largest at the bottom; no rounded boulders in the face unless they are tied in with through stones.
- Through or bond stones: full wall thickness, at roughly 1 m spacing both horizontally and vertically, so the two faces act as one wall.
- Drainage: 50 to 75 mm weep holes through the wall at the lowest level, about 1 to 1.5 m apart, with granular or gravel drainage material behind the wall and a geotextile to stop clay washing into it.
- Backfill: granular material placed in layers and compacted, not clay tipped in and jumped.
- Mortar and workmanship: a 1:4 or 1:5 mix with joints fully filled, and curing of the mortar; maintain the batter and check it with a plumb board as the wall rises.
- Record the details and the engineer's instructions so the next gang does not repeat the omission.