Getting Plant and Deliveries Onto Soft Ground

Coming off the deck is geometry before it is driving

Ramp length falls out of two measurements and one decision. The rise is the deck height where the ramps actually land, taken on the loaded trailer standing where it will stand — not the empty figure from the haulier's brochure, and not before the driver dumps the suspension if that is part of his routine. The decision is the steepest grade you will run this machine on this surface, which is not a number to be carried over from the last job.

Fifteen per cent is a workable ceiling for wheeled equipment climbing under its own power; anything with low clearance, a long fixed overhang or a load on it wants ten or less, and hydraulic beavertail ramps run steeper than either because a tracked machine will climb what would defeat a wheel. A deck 1.05 m above the landing point held to fifteen per cent needs about seven metres of ramp, and ten and a half at ten per cent. The accessibility figures do not belong here at all: the 1:12 ceiling in the ADA and the equivalent limit in Approved Document M describe a person in a wheelchair, not a twenty-tonne excavator.

Grade is rarely what stops the machine anyway. What stops it is one of three angles — the approach where the track first meets the ramp, the breakover at the crest where the ramp meets the deck, and the departure where a counterweight or rear overhang swings down onto the ground. A ramp long enough to give a comfortable average grade still forms a sharp break at each end, and it is at those breaks that a counterweight touches, a belly plate grounds, or the machine pivots and drops the last half metre onto the deck.

The ramp foot is what everyone forgets. Set it on soft ground and it sinks as weight comes onto it, steepening the ramp while the machine is halfway up — the one moment you least want the geometry moving. A steel plate, a short mat or a bed of stone under each foot fixes that for ten minutes' work. So does turning the machine round: reversing a tracked excavator up with the boom out over the low end keeps the centre of gravity where you want it.

The road journey either side is governed, and worth checking rather than assuming: in Britain the vehicle and its load sit under the Road Vehicles (Construction and Use) Regulations 1986, an abnormal indivisible load moves under the Road Vehicles (Authorisation of Special Types) (General) Order 2003, restraint is covered by the DVSA's load securing guidance, and the securing forces are calculated to EN 12195-1. The site-side duties for the machine itself sit in OSHA 29 CFR 1926.602, Material handling equipment.

1. Agree where the trailer will stand before it arrives, and check the road position leaves the ramps landing on something rather than in a ditch.
2. Measure deck height at the landing point with the machine on and the suspension where it will be, not from a specification sheet.
3. Make up the ground under both ramp feet with plate, mat or stone so the geometry cannot change under load.
4. Set the exclusion zone at the foot of the ramps and put the banksman where the operator can see him without turning his head.
5. Release the chains and transport locks in the handbook's order, then walk the machine off squarely with somebody watching the counterweight rather than the tracks.

What a temporary track is made of

A haul track across soft ground is four purchases: fabric, stone, the top-up stone nobody budgets for, and the plant time to lay it — and the fabric decides whether the other three were worth spending. Under repeated wheel loads a soft subgrade behaves as a pump. Each pass squeezes pore water out under the wheel line and drags subgrade fines up into the voids between the stones, and none of it shows from the surface until the day the track is suddenly a rut, by which point the stone has become a dirty gravel with no interlock left in it.

Fabric holds that boundary, and on a track it does a second job as well: once a rut has begun to form, the sheet is in tension across it and returns a little of the wheel load sideways instead of letting it all go downwards. Which fabric is not a preference. AASHTO M 288, Geosynthetic Specification for Highway Applications, classes separation geotextiles by survivability and states the properties each class must meet against named test methods — grab tensile to ASTM D4632, trapezoid tear to ASTM D4533, static puncture to ASTM D6241 — alongside an apparent opening size to ASTM D4751 and a permittivity to ASTM D4491 so the sheet filters the subgrade without damming the water it is trying to release. On a track built by tipping angular stone straight onto the sheet from a wagon, it is survivability that gets tested first and hardest, long before anybody finds out whether the filtration was right.

That specification also sets how much fabric you buy, because it fixes the overlap: M 288 widens the required lap as the subgrade weakens, and below a stated strength stops accepting an overlap at all and calls for sewn seams. The lap is no rounding error on the order — it comes off the usable width of every roll, so a wide lap on soft ground can add a whole extra pass down the track. Shingle the sheets so a wheel crossing a joint runs onto the top sheet rather than under its leading edge, roll fabric out ahead of the stone rather than across the site, and keep every wheel off the bare sheet: one turning tyre will take it apart.

The stone, and the wagon that has to bring it

There is a circularity at the start of every one of these jobs: the first load of stone has to arrive on the ground the stone exists to protect. The answer is always the same and always resisted, because it is slower. Build out from the hard end — tip on the last built metre, push the material forward with a machine that is itself standing on stone, and never let a loaded wagon put a wheel on raw subgrade. One bogged eight-wheeler costs more of the day than the whole morning's sequencing, and it leaves a hole exactly where the track was going.

Thickness is designed, not chosen. In Britain the document a piling or crane contractor asks for by name is BRE report BR 470, Working platforms for tracked plant, together with the working platform certificate that goes with it — somebody's signature accepting the platform as temporary works, which a verbal assurance at the gate is not. Where the design uses a geogrid, the Giroud–Han method for reinforced unpaved roads is the usual reference. Both start from a strength that has been measured: a CBR from a dynamic cone penetrometer to ASTM D6951, or a shear strength from a hand vane, taken at the wettest condition the track will see. A description of the soil is not an input to either, and neither is a thickness that worked on a different field.

Once a thickness exists the ordering is straightforward and the traps are in the units. Nine hundred square metres at 350 mm compacted, with the quarry's density for the material, comes out around 660 tonnes — thirty-three twenty-tonne wagon loads, a delivery programme rather than an order line, and at 3.5 m wide it is only about 257 m of track. Add the difference between loose delivered volume and compacted in-place volume, then add the stone that disappears into a very soft subgrade on the first pass, which on bad ground is a real tonnage rather than an allowance. Verify what you built against a Proctor density to ASTM D698 or ASTM D1557, by nuclear gauge to ASTM D6938 or by proof rolling with a loaded wagon — on a temporary track the proof roll is often the more honest of the two, because it applies the load the track was built for.

Buy stone for the width the wagons need, not the width they occupy. A track built to the exact width of a tipper leaves the driver no margin, so the outer wheels ride the shoulder — the part with no confinement on one side and the least compaction. It ravels, the edge drops away, and the usable width narrows every week until two vehicles cannot pass where the plan said they could.

The leg that goes through

Most deliveries to a soft site are unloaded by the vehicle's own crane, and the lorry loader is where the ground problem turns from slow into sudden. Wheels spread weight along a track; stabiliser legs concentrate the vehicle and its load onto four small feet, and the reaction on the offside leg with a load out at full reach bears no relation to kerb weight divided by four. That reaction comes from the loader manufacturer's own outrigger load data for the configuration in use — it is not a figure to estimate at the gate — and in Britain the operation sits under BS 7121, Code of practice for safe use of cranes, in the part covering lorry loaders.

The arithmetic afterwards is only a division, and it is brutal. A leg carrying twelve tonnes on a 500 mm square pad is 0.25 m² of contact and about 470 kPa on the ground. Put the same leg on a 1.2 m square mat and the contact area is 1.44 m², which is about 82 kPa — the same load, a shade under a sixth of the pressure, because area goes as the square of the side. Doubling the dimensions of a pad quarters what the ground feels, which is why arguing about pad size is one of the few conversations on a soft site where a small decision produces a large change.

Two cautions on the spreading. A mat only spreads if it is stiff enough to spread: a thin sheet under a concentrated point load dishes and delivers most of the pressure straight down under the foot, so the mat's own capacity matters as much as its footprint. And the figure you compare against is the allowable bearing pressure of whatever is under the leg at that spot, which is not automatically the ground the wheels are on — a track designed against repeated wheel loads was not designed against a static point load pushed into it in one place. That is the argument for building the unloading bay wider and thicker than the track, and for marking where the legs go rather than letting each driver find out.

Where a wagon turns round

Reversing a loaded eight-wheeler four hundred metres down a single-track haul road in the wet is how people get hurt, and it is the default arrangement wherever nobody drew a turning head. HSE guidance HSG144, The safe use of vehicles on construction sites, and the site traffic duties in Part 4 of the Construction (Design and Management) Regulations 2015 push the same way: separate vehicles from people, and design the reversing out rather than manage it with a banksman. In stone that means a one-way loop or a properly sized turning head — the item most often missing from a haul track take-off, because it is the only part that is not a length times a width.

A turning vehicle occupies two different outlines and they fail against two different things. The rear wheels track inside the front ones, so on a made track the inside of every bend is precisely where a tyre leaves the stone and finds the subgrade; the front corner of the body swings outside everything, which is what takes mirrors off gateposts and finds the low branch nobody looked at. Widen the bends on the inside, and check the body envelope against the physical obstructions rather than against the kerb line.

Take the dimensions from the vehicle you are actually receiving. A rigid eight-wheel tipper, an artic with a walking floor and a low-loader with a machine on it are three different templates, and the difference between them is the difference between a turning head that works and one that gets driven over. Where the delivery comes through a farm gate, measure the gate before the wagon is booked.