Choosing a Borrow Pit for a Two-Storey Rammed Earth Wall
A two-storey rammed earth house is planned with 450 mm walls. Three borrow pits are available. Sieve and Atterberg results: Pit A - 25% gravel, 30% sand, 45% silt/clay, PI 14, linear shrinkage 6%; Pit B - 50% gravel, 30% sand, 20% silt/clay, PI 7, linear shrinkage 3%; Pit C - 5% gravel, 55% sand, 40% silt/clay, PI 26, linear shrinkage 11%. No cement or lime is budgeted; the architect wants an unstabilized wall with a natural finish. The contractor prefers Pit C because it 'sticks together best' in a hand ribbon test. The site is in a warm, semi-arid climate with occasional driving rain. The client asks you to recommend one pit and to state what you would verify before construction.
1. Recommend a pit and justify the choice using grading, plasticity, shrinkage, and the structural role of the wall.
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Pit B is the best starting point, but not automatically acceptable. Its 50% gravel gives an internal skeleton, and PI 7 with 3% linear shrinkage indicates low water sensitivity. Pit C's PI 26 and 11% shrinkage are too high for 450 mm unstabilized walls: shrinkage cracking and rain erosion risk rise sharply. Pit A is intermediate; PI 14 is workable but 6% shrinkage is borderline. The key is not 'most clay' but a graded skeleton with enough active fines to bond. B's 20% fines may be too low for durability; a trial blend of 80% B plus 20% C would raise fines while keeping PI below about 10. Verify with trial walls, not just index tests.
2. State what you would verify before construction and why index tests alone are insufficient.
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Before construction: (1) sample each pit at five depths and repeat grading and PI to quantify variability; (2) run modified Proctor compaction and make 300 mm trial panels at three moisture contents, measuring dry density; (3) test 28-day compressive strength on 300 x 300 x 600 mm prisms or 100 mm cubes, targeting a safe factor above the design stress; (4) run spray erosion and wet-dry tests on panel surfaces; (5) check linear shrinkage of the actual mix. Accept the pit only if the trial panel is dense, crack-free, and erodes less than 10 mm under a standard spray. This verifies performance rather than relying on the contractor's ribbon test.
3. If only Pit C were available, what mix or design changes would you require?
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If only C: blend with 30-40% sharp sand or gravel to bring fines to about 30% and PI near 10-12; or stabilize with 4-6% hydrated lime, since high-plasticity clay responds to lime, and cure seven days damp. Reduce lift thickness to 100 mm, use heavier compaction, and add a reinforced bond beam at roof level, vertical reinforcement at corners and openings, and a 300 mm splash course. Increase roof overhang and plinth height. These changes address shrinkage and strength rather than hiding cracks.