Borehole and Trench Excavation
What the exam tests
Excavation questions cover borehole drilling and trenching methods, depths, clearances, soil stability, and protection of the loop. You need to know how geology and site conditions choose the method and how to keep the hole or trench open long enough to install and grout.
Core rules
- Bury horizontal loop pipe below frost depth and deep enough to avoid damage; maintain required cover over the pipe.
- Keep boreholes straight and sized for the U-bend, grout pipe, and tremie; typical small-diameter holes are 4 to 6 in.
- Match drilling method to formation: mud rotary in unconsolidated soil, air rotary or hammer in rock, casing in unstable ground.
- Bench, slope, or shore trenches; keep spoil back from the edge; control water.
- Protect pipe from sharp rock, traffic, and kinking; use sand or fine backfill where required.
Worked example
A site has 36-in frost depth and the specification requires 48 in of cover. A 1.25-in IPS pipe has an outside diameter of about 1.66 in. Minimum trench depth is 48 + 1.66 = 49.7 in, so a 54-in trench provides the required cover with margin. The 36-in frost depth is satisfied, but the specification controls because it is deeper.
Traps
Trench too shallow at frost; borehole too small for grout pipe; no casing in gravel; leaving an open trench overnight without barricades; damaging pipe on sharp rock.