Rig Site Setup and Mobilization: Ground Bearing Capacity, Mast Raising Safety, and Move-In Time Control

The most expensive hours on a water well project are the ones before the bit touches rock. A leg that sinks 200 mm into rain-softened ground tilts the mast, a gust above 20 km/h stops a raise halfway up, and a three-day move eats half the drilling time of a 200 m hole. Bearing capacity, layout, and mast safety set the schedule before any footage is made.

Ground Bearing Capacity Before the Rig Unloads

Track pressure versus leg load

A rig that rolls across ground at 40-70 kPa of track pressure does not sit on the ground the same way when it stops. Jack the legs down and one leg can carry 80-150 kN on a pad the size of a small table, two to three times the ground pressure the tracks left behind. That is why rigs that drive comfortably onto a site can sink a leg an hour later. The fix is area: pad area equals leg load divided by allowable bearing capacity. A 100 kN leg on firm clay rated at 80 kPa needs 1.25 square metres, which is a 1.2 by 1.2 m pad. On loose sand at 50 kPa the same leg needs 2 square metres, so a 1.5 m pad at minimum, with hardwood mats under the steel to spread the edge load.

What rain does to the numbers

Rain rewrites those numbers. A clay that reads 80-120 kPa dry can drop 30-50 percent after a week of showers, down to 35-70 kPa, and the pad sized for the dry value goes into the ground anyway. This is the whole argument for a November mobilization window: the African dry season from November to March offers ground that holds its rated capacity, while pads sized during the wet season are guesses. Two more limits close out the site survey. Slope above 5 degrees is not a comfort question, because it unloads the down-slope legs and loads the up-slope ones, so either pick level ground or cut a platform. And a leg that has already sunk 200 mm has tilted the mast before the first metre is drilled, which costs at best a half-day of re-leveling.

Table 1. Pad areas are leg load divided by allowable bearing, worked for a 100 kN leg. Scale the area in proportion for heavier legs.

Laying Out the Site Before the Trucks Arrive

Layout is cheaper to fix on paper than with a wheel loader. For a 200-300 m well drilled by a trailer rig, a worked yard of about 30 by 20 m covers the rig position, the pits, and material staging; a truck-mounted rig needs less, and a full top-head rig on level terrain can work from about 25 by 18 m. What matters more than total area is the flow. Mud pits sit 5-8 m from the centreline and downwind of the crew positions, the casing and material yard sits within 12 m of a forklift path that never crosses the mud pit access, and the camp sits 30-50 m away, upwind, beyond the noise of the compressor.

The mud pit is a calculation, not a tradition. A 215 mm hole to 250 m displaces about 9.1 cubic metres of formation, and a working pit sized at 2.5-3 times the hole volume, 25-30 cubic metres including a settling section, keeps solids away from the pump suction and holds enough volume to fill the hole during trips. Disposal sits 10-15 m further downwind on lined ground away from any watercourse, and cuttings go back into the pit only after the slurry has set or been solidified, per the permit terms of the project. Fuel and the generator sit together on bunded ground with cable ramps across every vehicle path.

Table 2. Distances are working minimums for a trailer rig on a 200-300 m water well; larger rigs and rotary tables with standalone power widen them.

Mast Raising on the Safe Side of the Wind

Wind writes the first rule of mast work: raising and lowering stop above 20 km/h. That is the point where flags stand out and small trees sway, and on a 10-12 m mast the wind load at that speed already exceeds what the raise cylinders and the leg locks were sized to hold. Gusts, not the average, do the damage, so crews that work between gusts are betting the mast on a lull. The second rule is distance from power lines: at least 3 m from lines under 50 kV and 4.5 m from 50-200 kV lines, confirmed from the tower or pole plates before mobilizing rather than judged by eye at the site. The third rule is people: everyone not needed on the raise stands outside a radius of 1.2 times the mast height, which is 14-15 m for a 12 m mast.

Checks before the raise button

The raise itself is a five-minute job made safe by checks that take longer than the raise. Leg lock valves are confirmed mechanically, locks seated, not inferred from a gauge reading. Machine level reads within 0.5 degree in both directions at the deck before the mast leaves its cradle. Wire rope runs its full length, sheaves turn by hand, and lifting hooks carry latches. We recommend the raise be made at low speed with one stop part-way up to watch for drift; our rig manuals ship with the exact sequence for each model, and it is the one procedure crews should not improvise.

Table 3. The wind limit covers raising and lowering; a mast that stays up in service follows the rig manual for the maximum operating wind.

Move-Time Arithmetic Between Wells

Move time is a schedule line item, and it is worth pricing honestly. A re-spot on the same pad takes 2-4 hours: legs up, slew, legs down, level. A same-area move under 5 km with partial breakdown takes 4-8 hours with a crew of three to four. A cross-district move of 20-100 km means full breakdown, a lowbed, and 1-2 days; add a border or a ferry and it is 2-3 days with permits. Set that against drilling time: a 200 m hole in mixed sediment takes 5-7 days of actual drilling, so a three-day move eats half a well. What keeps the short moves short is discipline in breakdown order: electrical disconnected and tagged, hydraulic circuits depressurized and capped, mast lowered and pinned, pins and bolts returned to a counted tray.

Table 4. Times assume a trailer or truck-mounted rig in the 10-20 t class with a local lowbed; floats for wider loads add permit lead time.

The Mobilization Budget and the Pre-Move Checklist

Mobilization money is 5-12 percent of a single-well cost, and most of the spread is chosen, not suffered. Site spacing is the biggest lever: where the next well is under 5 km away, the same yard and camp can be reused and the move drops back to a partial-breakdown day. Where spacing is wide, the lowbed rate, the escort width permit, and an empty return leg all land on that one well. Timing is the second lever. The African dry season runs from November to March with the drilling peak in December to February, so November is the window in which ground is firm, roads are passable, and the move costs a day instead of three.

Table 5. Run the sheet at every new site before the first raise, not after; a failed line costs minutes to fix on arrival and days to fix once drilling has started.

A checklist only works if every line produces a reading. We supply each rig with a pad sizing table, a mobilization checklist, and a mast raise procedure matched to the model, and crews that log the level readings, the lock checks, and the earth resistance against the well file turn the next move-in into a comparison instead of a fresh start. The pad that held the last rig in the same district is the pad size to beat; the checklist signed at this site is the first page of the next one.

Ground bearing, layout, mast safety, and honest move-time arithmetic decide how much of the dry season a rig actually drills. Pads sized from leg load, a wind limit of 20 km/h, and a checklist that produces readings keep the schedule.

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