Grouting and Backfilling
What the exam tests
Grouting and backfilling questions focus on sealing the borehole, improving thermal contact, and restoring the trench. Know grout types, mixing, tremie placement, and backfill compaction.
Core rules
- Grout seals the borehole against surface water and aquifers and improves heat transfer by filling the annular space.
- Common grouts include bentonite, cement-bentonite, sand-cement, and thermally enhanced bentonite or graphite blends.
- Place grout from the bottom up with a tremie pipe; do not pour from the top.
- Backfill trenches with suitable material, compact in lifts, and install warning tape and surface restoration.
- Keep grout and backfill compatible with the pipe and the environment.
Worked example
A 240-ft borehole is 5 in in diameter and contains a 1.25-in U-bend. Borehole area = π × 2.5² = 19.6 in². Two pipes displace about 2 × 2.2 = 4.4 in². Annulus = 15.2 in² = 0.106 ft². Volume = 0.106 × 240 = 25.4 ft³. Add 10% for loss = about 28 ft³. Order grout by volume, not by bag count alone.
Traps
Grouting from the top; using neat cement that shrinks; skipping the tremie; backfilling before the test; failing to compact; not accounting for lost circulation.