A gravel pack is a ring of clean, washed, rounded gravel, 3-6 mm in the Deep Wells design, poured into the space between the screen and the borehole wall so that it holds back the aquifer sand while the slots only need to hold back the gravel. It only works if its grains are coarser than the slots and suited to the sand behind it, if the gap is wide enough to pour it without bridging, and if it is sealed above with clay or cement.
Part of Deep Wells
The gravel pack is the filter layer that surrounds a well screen. It stabilises the hole, keeps formation sand out of the screen, and holds a permeable zone open around the intake so that water flows to it easily.
Two Grain Sizes Have To Fit: The Pack To The Sand, The Slot To The Pack
The pack is a two-stage filter. The screen slots must stop the pack, and the pack must stop the aquifer’s sand.
Slot to pack. With slots of at most 1 mm and gravel of at least 3 mm, the smallest pack grain is three times the slot width, so the slots hold the pack with a margin.
Pack to sand. The pack must be fine enough to hold the sand behind it. The usual design rule in well-drilling practice is that the pack gravel should be about four to six times the size of the formation sand (stated as the ratio of the 30%-retained grain sizes; recalled from Driscoll, Groundwater and Wells, not from the Deep Wells guide). So a 3-6 mm pack suits a coarse sand of about 0.5-1.5 mm: 3 ÷ 6 = 0.5 mm and 6 ÷ 4 = 1.5 mm. If the aquifer is fine sand, the 3-6 mm pack is too coarse: the sand will wash through the pack and the slots into the well, and you need a finer pack with narrower slots (0.5 mm, the narrow end of the Deep Wells range). Judge the sand from the bailings: sand with grains you can distinguish one by one is coarse; sand that feels like flour is fine.
The Gap Must Be Wide Enough To Pour Into
Gravel poured down a narrow gap can wedge, bridge across the gap and leave a void below. The Deep Wells bit of 10-15 cm is too narrow for 110-160 mm casing with a pack. A clearance of at least 25 mm on each side (an assumed figure from general drilling practice, not from the guide) is about four times the largest grain.
Worked example: volume per metre. A hole of 160 mm round 110 mm casing has an annulus area of π ÷ 4 × (0.160² − 0.110²) = π ÷ 4 × (0.0256 − 0.0121) = 0.0106 m², which is 10.6 L per metre of height. For a 1.5 m screen with the pack carried 1 m above the top of it (a minimum from drilling practice, so that settling cannot uncover the screen) the pack is 2.5 m tall: 2.5 × 10.6 = 26.5 L of gravel in place, and order extra for settling and waste, judging the amount from the first fill. For a narrower hole of 150 mm the gap is only (150 − 110) ÷ 2 = 20 mm, 3.3 times the largest grain.
What You Need
- Gravel of 3-6 mm — rounded, hard, preferably quartz or similar; avoid limestone or chalk, which can dissolve and re-cement
- Two sieves — hardware cloth or mesh at 6 mm and 3 mm, to remove oversize and undersize grain
- Clean water — to wash the gravel until the water runs clear
- Centralisers — wooden or plastic spacers fixed round the casing every few metres so that it stays in the middle of the hole
- A tremie or a slow pour — a small pipe to guide gravel down the gap, with water running
- Clay or cement — to seal above the pack (Surface Seal)
Method: Grade, Wash, Centre, Pour, Settle, Seal
Step 1: Grade. Pass the gravel over the 6 mm mesh and discard what stays, then over the 3 mm mesh and keep what stays. Fine material is what clogs a pack.
Step 2: Wash. Rinse in clean water, in batches, until the water runs clear.
Step 3: Centre the casing. Fit spacers round the screen and the casing above it, so the gap is even all round.
Step 4: Pour slowly. Pour a little at a time into the gap, with water flowing, and track the level of the gravel with a weighted line against the volume poured. If the level rises faster than 10.6 L per metre predicts, the gravel has bridged and left a void below: stir with a rod and pour again.
Step 5: Settle and top up. Let it settle, and top up to the planned height above the screen.
Step 6: Seal. Above the pack, backfill with clay or cement to the surface, or at least the top 3 m (Surface Seal), so that surface water cannot run down outside the casing.
Step 7: Develop. Bail or pump hard for hours to clear the fines from the pack and the aquifer face (Well Development describes the completion procedure for dug wells; the same principle applies).
Common Mistakes
- Pack too coarse for the sand. The aquifer sand moves through, the well silts up and the pump wears.
- Pack too fine for the slots. Grains pass through, or the slots clog.
- Bridged gravel. A void below a bridge lets sand pour into the hole.
- Unwashed gravel. The dust clogs the pack from the start.
- No seal above. Without it the pack is a channel for surface water.
Safety
Contamination is the main hazard here
A well is a conduit from the surface to the aquifer. Gravel, wash water and tools that touch the ground before they go in carry bacteria into the pack. Wash and handle gravel clean, keep tools off the ground, and disinfect the finished well (Community Water Testing covers verification). Keep people clear of the open gap while pouring.
What’s Next
- Learn next: Surface Seal — sealing above the pack
- Combine with: Slot Cutting — making slots that hold the pack