What to Prepare Before Construction Site Cabins Are Delivered

Ordering construction site cabins settles what arrives. It does not settle whether the unit can be put where the project wants it. That part is site work, and most of it sits on the project team’s side of the contract: the ground the cabin stands on, the route the vehicle takes to reach it, the lift that puts it down, and the power, water and drainage that make it usable.

The reason this matters more than it sounds is that the pieces arrive together. The vehicle, the offload equipment and the crew all turn up for one slot. A base that is out of level, a gateway a few centimetres too narrow or a scaffold over the drop position does not delay one item, it stops the whole movement, and hired plant is paid for whether it lifts or not.

The order the preparation has to happen in

Sequence matters because several of these tasks bury each other. Service ducts that run under a cabin cannot be installed once the base is down. A base cannot be excavated safely until the ground under it has been checked for buried services. And a delivery date should not be fixed until the route and the lifting position are confirmed, because either one can force a different unit size or a different offload method.

1. Confirm the loaded weight, external dimensions and bearing point layout of the specific unit
2. Trace and mark buried services across both the base position and the lifting setup area
3. Excavate, lay any ducting or drainage that has to pass under the unit, and form the base
4. Survey the route from the public highway to the drop position, including overhead obstructions
5. Agree the offload method and have the lift planned by a competent person
6. Bring temporary power, water and drainage to the edge of the base, terminated ready to connect
7. Book the delivery slot and settle who meets the vehicle, where it waits and who directs the reversing
8. Level, connect and commission the unit once it is standing in its final position

Who arranges what

The split between supplier and project is set by the contract rather than by custom, so it is worth reading instead of assuming. Two items in particular are worth pinning down: who provides the offloading equipment, and who carries the risk if the ground turns out not to be fit to receive the unit.

Where a task is shared, name the individual rather than the organisation. The contractor is not a person who will be standing at the gate when the vehicle arrives.

What the base actually has to carry

A site cabin does not spread its weight evenly across its footprint. Most units bear through a small number of points, typically corner castings or adjustable jack legs, so the load reaches the ground concentrated rather than distributed. A yard surface that comfortably carries plant driving over it can still deform under a handful of static points, particularly on made ground, recently backfilled ground or anything that holds water.

The specification that matters is therefore not a slab thickness copied from a general guide. It is whether the base can take the loaded weight of the actual unit at the actual bearing positions, and those figures come from the supplier rather than from a rule of thumb. When requesting a quotation for Module-T UK construction site cabins or any other unit, ask for the loaded weight and the bearing point positions at the same time, because the groundworks cannot be specified without them.

Most base problems come down to a short list of physical causes:

• Standing water, because a base level with or below the surrounding ground collects run-off and turns the area by the door into the wettest part of the compound
• Differential settlement, where one corner dropping is enough to bind a door or crack a window unit, and packing under the sinking leg only treats the symptom
• Soft or made ground, where fill that was not compacted in layers moves under point loads even when the surface looks sound
• Falls in the wrong direction, so that surface water drains towards the unit instead of away from it
• No allowance for what sits underneath, when tanks, pumps and duct entries need to stay reachable after the unit is standing, not only before

Before excavating for a base or driving a pin into the ground, treat the utility drawings as a starting point rather than an answer. HSE guidance on avoiding danger from underground services is explicit that plans alone are not sufficient, and that cable avoidance tools and signal generators, the CAT and genny in site language, should be used to trace routes on the ground and repeated as the dig progresses. A cabin base is a shallow excavation, which is exactly the depth at which service cables and ducts tend to sit.

There is a genuine choice of base rather than one correct answer. A poured concrete base suits a compound that will stand for the duration of a long project or carry linked and stacked units. Compacted stone with spreader slabs or sleepers under each bearing point is often proportionate for a short compound on firm ground, and it is far easier to take out again. Whichever is chosen, check what the site is contractually obliged to reinstate at the end, because breaking out a slab belongs to the same decision that created it.

Proving the route before the vehicle is booked

The constraint is rarely the site entrance. It is the tightest point anywhere on the chain from the public highway to the drop position, and it is often something that was not there when the compound was planned: a skip, a scaffold, a stacked material delivery, a parked van.

Walk the route rather than measuring it on a drawing, and walk it in the direction the vehicle will travel. The obstructions that catch projects out are vertical rather than horizontal. Low branches, cables crossing an access road, signage gantries and the underside of scaffolding all reduce the working height, and an articulated trailer needs that headroom along its whole length, not just at the gate. A loaded vehicle also needs somewhere to stand that will not sink, and a haul route that works in dry weather can strand a delivery after rain.

Reversing is the other half of the problem. CDM 2015 requires a site to be organised so that vehicles and pedestrians can move without putting each other at risk, and for a one-off delivery that usually means a defined waiting position clear of the highway, a banksman briefed before the vehicle arrives rather than found once it has, and a pedestrian route that stays segregated while the trailer is manoeuvring. If the vehicle has to reverse in from a live road, or a parking bay needs suspending, the local highway authority becomes part of the arrangement, and that is a conversation to open early rather than on the morning of the delivery.

Planning the lift and the space above it

Under the Lifting Operations and Lifting Equipment Regulations 1998, every lifting operation has to be properly planned by a competent person, appropriately supervised and carried out safely. For a cabin offload that plan is a document, not an intention. In practice, principal contractors and lifting equipment suppliers expect it to be written or approved by an appointed person, the role that carries responsibility for planning the lift and selecting the equipment and accessories.

The clearance most often underestimated is the one above the lift. HSE guidance on overhead power lines recommends a minimum horizontal distance of 6 m between ground level barriers and the point where the nearest conductor projects vertically onto the ground. Where plant such as a mobile crane is involved, the recommendation becomes 6 m plus the length of the jib or any other overhanging part of the machine, and the owner of the line may advise a greater distance depending on the voltage. That is a far larger exclusion than most compound layouts allow for, and it is measured from the conductor rather than from the pole.

Whatever lifts the unit also has to be supported. A lorry loader or a mobile crane transfers much of its own weight and the load through its outriggers, and the pressure under an outrigger foot is high because the contact area is small. Spreader pads and mats exist to bring that pressure down to something the ground can take, and the calculation belongs in the lift plan rather than in a judgement made on the day. The setup area therefore needs the same ground assessment as the base itself, and it is frequently on the softer side of the compound precisely because it was never intended to carry anything.

Radius decides more of this than weight does. A crane’s capacity falls as the jib reaches further out, so a machine that can lift the unit close in may not be able to place it on the far side of an obstruction. Fixing the exact drop position before the equipment is selected avoids a lift plan written for a position the compound never adopted.

Not every unit has to be craned. Where the ground and the approach allow it, a unit can sometimes be delivered on a trailer that lowers or skids it into place, or handled by a telehandler with a suitable attachment. That changes the readiness requirement rather than removing it, because a skid or roll-off placement needs a firm, level run right up to the final position, which is often harder to provide than a single lifting position with clear airspace over it. Confirm the intended handling method with the supplier instead of assuming either.

Bringing services to the unit

Cabins used as offices, canteens, drying rooms or toilet blocks are treated differently from the construction supply proper. The construction and demolition site requirements in Section 704 of BS 7671 do not apply to the parts of a site provided for administrative, welfare or sanitary purposes, which includes portable cabins standing in a fixed position. That is not an exemption from anything, because those installations still have to meet the general requirements of BS 7671. The practical effect is that the cabin sits closer to an ordinary fixed installation than to the reduced low voltage arrangement feeding tools around it, and the design has to make that boundary deliberate rather than accidental.

Where the cabins take one earthing arrangement while the rest of the site runs on another, the separation between the two needs handling properly rather than being left to chance. Put that to the site’s electrical designer before the supply is laid in, since a cable is easier to route than to re-route.

Water and drainage are decided by geometry. A gravity connection needs a fall to an accessible drain, which the base level either permits or prevents, and that has to be worked out before the base is formed rather than after. Where no reachable drain exists, the alternatives are a holding tank or a pump set lifting the discharge to the nearest connection, and each carries its own access requirement. A tank has to stay reachable by a tanker with its service ports clear, and a pumped line has to be routed where nothing will drive over it. Units raised on jack legs sometimes allow a tank to sit underneath, which works neatly until the tank needs emptying and the space around it has been filled with stored materials.

Data and communications are the connection most often left until last, and the one most likely to need a duct that has to be laid before the base goes down. Decide early whether the cabin will run on a fixed line, a shared site network or mobile connectivity, because a fixed line is the only one of the three that constrains the groundworks.

Where the unit stands in the compound

Position is a fire question before it is a convenience question. The Joint Code of Practice on the protection from fire of construction sites and buildings undergoing renovation, which many construction insurance policies are written against, addresses separation between site accommodation and the building under construction. Its guidance points to a clear space of 10 m, treats 6 m as the minimum break, and looks for around 20 m where a large timber framed structure is involved. Where that separation cannot be achieved, the code turns instead to the fire performance of the temporary building itself. Whether the code applies to a particular project depends on the contract and the insurance conditions rather than on the size of the cabin, so it is worth checking before a compound layout is fixed.

The rest of the position is decided by details that are easy to miss on a plan and obvious on site. Keep the unit clear of the slew radius of any tower crane that will be erected later, and outside the influence zone of excavations that have not been dug yet. Put the door where people will actually approach from, not where it happened to face on the trailer. Leave a lifting position accessible if a second unit may follow, because bringing equipment back into a compound that is by then full is a much harder lift than the first one.

What tends to stop a delivery on the day

The problems that abort a delivery are usually small and physical rather than complicated:

• The base is finished but was never checked against the tolerance figure the supplier gave
• The unit delivered is a different configuration from the one the base was set out for
• An overhead obstruction was noticed only once the jib went up
• The offload equipment was assumed to be on the delivery vehicle and was not
• Nobody with authority to accept the unit was on site when it arrived
• Services reached the compound but terminated too far from the unit to connect to it
• The crane setup area was firm on the day it was surveyed and soft on the day of the lift
• The movement clashed with another delivery at the same access point, or fell outside the site’s permitted working window

Every item on that list is easier to prevent than to resolve on the day, and the prevention is the same in each case: fix the exact unit, the exact position and the exact route early, then check all three again shortly before the vehicle is booked, because compounds change while nobody is looking. Module-T UK has been designing and manufacturing modular buildings and portable cabins for professional and commercial use for more than 15 years, and the Get A Quote route on module-t.co.uk is where unit weights, dimensions and handling details are confirmed for a specific project. Site specific questions can also go to [email protected] or +44 20 4587 1172.