Temporary Works and Excavation Support: The Design You Did Not Know You Owned

A contractor prices a stormwater upgrade. The drawings show pipe sizes, invert levels, bedding, backfill and reinstatement. Every permanent element is designed, detailed and certified by somebody else. Nothing on the drawings says how the trench stays open while the pipe goes in.

That is not an omission. Temporary works are the contractor’s responsibility almost everywhere in Australian civil contracting, and they are the one category of engineering a civil SME is expected to own without anyone saying so. They are also where the industry’s most serious injuries happen: a trench collapse gives no warning, and a cubic metre of soil weighs more than a tonne. The same design obligation applies to concrete work — see our guide to formwork, falsework and steel reinforcement.

The design you did not know you owned

The division is worth stating plainly because it explains everything downstream.

	Permanent works	Temporary works
Who designs	The principal’s designer	You, or an engineer you engage
What you get	Drawings, specification, certification	Usually nothing
Who carries the risk	Principal, subject to the contract	You, almost always
When it is designed	Before tender	Frequently the morning it is needed
What happens if it fails	A defect	An injury, a fatality, or a structure lost

The last row is why this belongs in a compliance guide rather than a methodology one. Temporary works failures are not commercial events. They are the events that end businesses and put directors in front of a regulator.

What counts as temporary works

Broader than most contractors assume. Anything that holds something up, holds something back, or carries a load, and which is removed when the job is finished.

Category	Examples	How often it is engineered
Excavation support	Shores, shields, sheet piles, soldier piles and walers, drag boxes	Sometimes — usually only when hired with a certificate
Batters and benches	Cut slopes left unsupported, benched faces, temporary stockpile faces	Rarely, and this is the biggest gap in the industry
Working platforms	Piling platforms, crane pads, EWP standing areas, pump setups	Rarely, and failures are common
Haul roads and access	Temporary haul roads, ramps into excavations, creek crossings	Almost never
Formwork and falsework	Headwall and structure formwork, props, back-propping	Sometimes, on structures work
Propping and support of existing structures	Underpinning support, propping adjacent buildings, service supports across a trench	Sometimes, and the consequences of not doing it are third-party damage
Water control	Coffer dams, bunds, diversions, dewatering drawdown, temporary levees	Rarely — see coastal, marine and flood mitigation civil works
Temporary traffic and edge protection	Barriers, edge protection at excavations, temporary bridging over trenches	Usually covered by the traffic management plan

Two rows deserve highlighting because they are where competent contractors still come unstuck. Unsupported batters get treated as “no temporary works” when they are in fact a designed slope that happens to be made of soil. Rock faces and permanent cuttings are covered separately in our guide to rock batters and slope stabilisation. And working platforms get treated as ground preparation when they are a load-bearing structure carrying a machine that can weigh more than the excavator that built it.

The trench, and why depth is the wrong question

Ask most civil workers when a trench needs support and the answer is a number. The model work health and safety regulations do set a trigger depth for trenches — commonly cited as one and a half metres — above which excavation work is classified as high risk construction work requiring a safe work method statement, and at which ground support is required unless a competent person has determined that the ground is not at risk of collapse. Jurisdictions adopt and amend the model regulations differently, so confirm the depth and its application with your regulator rather than relying on a number you heard on site.

But the more important point is this: the regulatory trigger is a depth, and the engineering trigger is not. Complying with the depth rule is not the same as being safe.

A trench shallower than the trigger can still collapse and still kill, and does, when:

• The ground is saturated, or water is seeping through the face.
• The ground was previously excavated and backfilled — a service trench, an old structure, uncompacted fill.
• Spoil, plant or a stockpile is loading the edge.
• Vibration from plant, traffic or compaction is acting on the face.
• The trench has been open through a rain event, or through several days of drying and cracking.
• The soil is a cohesionless sand or gravel that will not stand at any depth.

The professional posture is to treat every entry into an excavation as a decision requiring a reason, not a depth check. If the answer to “why is it safe to go in” is “it is under the trigger depth”, that is not an assessment.

Support systems and what each actually does

The distinction in this section is the single most misunderstood thing in civil excavation work.

System	What it does	What it does not do
Trench shield or box	Protects people inside it if the trench collapses around it	It does not support the trench. The ground outside can and does move
Hydraulic or screw shores	Actively supports the faces by pushing against them	Does not work in ground that cannot take the reaction, or where the face is already loose
Sheet piling	Retains ground and, driven deep enough, cuts off water	Not economical for short runs; needs plant and design
Soldier piles and walers	Retains deeper or longer excavations, often with anchors or struts	Always an engineered system, never an off-the-shelf one
Battering or benching	Removes the vertical face so there is nothing to collapse	Needs room, generates spoil, and is still a designed slope

Read the first row again. A trench shield is a protective structure, not a retaining structure. Workers inside a correctly used shield are protected; the trench walls outside it are not held up, which is why services, adjacent pavements and neighbouring structures can still be undermined even when nobody is hurt. Contractors who believe a box makes a trench stable also believe they can leave one in a face overnight, or step outside it to make a connection, which is the moment the incidents happen.

Two further practical points. Shields and shores have manufacturer’s limits — depth, soil type, and the way they must be installed and removed — and using them outside those limits voids the basis on which they are safe. And installation and extraction are themselves the highest-risk moments, because they involve the machine, the face and often a person in proximity.

Battering, benching and the room you do not have

Battering is the cheapest support system when you have room, and the reason it goes wrong is that people treat “no shoring” as “no design”.

A batter is a slope chosen so the soil stands up. The angle that achieves that depends on the soil type, its moisture, whether it is disturbed, what is loading the top, and how long it will be open. Generic angles passed down on site are rules of thumb for the ground someone else was working in.

The commercial consequences are worth understanding at tender stage, because they are large and they are usually missed:

• Battering multiplies excavation volume. A deep trench battered on both sides can involve several times the volume of a supported vertical trench, with the same multiplier on backfill, compaction and cartage. On a rate priced for a vertical trench, that difference is your margin.
• It needs a footprint you may not have. In a road reserve, between services, or beside a boundary, there is frequently no room to batter at all — which means the choice was made for you and shoring should have been priced.
• It changes the reinstatement. A wider disturbed footprint means more pavement restoration, which on a road opening can exceed the trenching cost.
• Benching is not a lesser option. A benched face is still an engineered profile and, in poor ground, benches can concentrate loading rather than relieve it.

The decision between battering and supporting is therefore a pricing decision as well as a safety one, and it should be made when the estimate is built rather than on the day. Our guide to preparing civil works cost estimates covers the build-up; this is one of the items where a wrong assumption is invisible in the rate and fatal to the result.

Surcharge: the load nobody drew

Surcharge is any load near the top of an excavation that increases the force trying to push the face in. It is the most common contributor to collapses and the least often considered, because it arrives after the excavation was assessed.

• Spoil placed beside the trench — the single most common surcharge, and the most avoidable. Spoil belongs away from the edge, not next to it.
• The excavator itself, tracking along the edge while it digs.
• Delivered material — pipe, bedding aggregate, precast — stacked where there is room, which is beside the hole.
• Traffic, including the live road you are working beside and your own trucks.
• Adjacent structures — a fence footing, a pole, a building, a retaining wall whose own stability depends on the ground you just removed.
• Water, which is both a surcharge and a strength reducer, and which arrives without anyone deciding to put it there.

Where an adjacent structure or a service is involved, the exposure is third-party rather than just internal — undermining a neighbour’s footing or dropping a live service is a liability event as much as a safety one, and it interacts directly with the cover discussed in our guide to making an insurance claim on a civil job. Locating and proving services before excavating is the related discipline, covered in our guide to utility and telecommunications civil works.

Who designs it, and who certifies it

Three levels, and knowing which one you are in is most of the competence.

Level	Typical situation	What is required
Standard solution	Proprietary shields or shores used strictly within the manufacturer’s published limits, in ground those limits cover	The manufacturer’s data, a competent person confirming the ground matches, and correct installation
Competent person assessment	Battering in known, uniform ground with no surcharge and no water	A documented assessment by someone with the training and experience to make it, recorded and re-checked as conditions change
Engineered design	Deep excavations, anchored or strutted systems, work beside structures or services, poor or variable ground, water, working platforms for heavy plant, falsework	Design by a suitably qualified and, where the jurisdiction requires it, registered engineer, with calculations, drawings and a certificate

Engineer registration requirements differ by state and territory — Queensland has long required registration to practise professional engineering, and other jurisdictions have introduced or are introducing registration schemes. Where registration applies, a design certificate from an unregistered person may not satisfy the client or the regulator. Check the requirement in the jurisdiction you are working in rather than assuming your usual engineer qualifies everywhere; the same border-by-border caution applies here as in our guide to contractor licensing by state and territory.

Larger clients increasingly require the formal structure that has become standard on major projects: a temporary works register listing every temporary works item on the job with its design status, and a named temporary works coordinator responsible for ensuring each item is designed, checked, installed as designed, inspected and removed in a controlled way. For an SME this need not be elaborate — a one-page register and a named person is a proportionate version, and having one is increasingly a differentiator in a tender response. Where the excavation goes below the water table, the dewatering system belongs on that register too — see our guide to dewatering, water take and discharge.

Ground and water

Every temporary works decision rests on an assumption about the ground, and the ground information you were given was gathered for the permanent works.

What that means in practice:

• Boreholes are points, and your trench is a line. Between two boreholes showing stiff clay there can be a sand lens, an old creek channel or a backfilled trench.
• The water table in the report is the water table on the day it was measured. Seasonal variation, rainfall and tidal influence all move it, and a trench that stood open in October may not in March.
• Disturbed ground behaves nothing like natural ground. Any excavation crossing an existing service alignment is crossing previously disturbed, often poorly compacted material.
• Dewatering changes the ground you are supporting, and drawdown can affect adjacent structures and neighbouring bores. It also requires its own authorisation — see our guide to environmental approvals and permits.

Where the ground turns out to differ materially from what a competent contractor could reasonably have anticipated, there may be an entitlement — but that is a contractual question with notice requirements attached, and it does not help anybody standing in the trench. Our guide to latent conditions in civil contracts covers what a site information clause gives you and what it does not. Treat the claim and the safety response as separate: stop first, notify second.

Platforms: the temporary works most often forgotten

Working platforms deserve their own section because they fail regularly, the failures are expensive, and almost nobody prices them as an engineered item. Designing one also engages the designer duty covered in our guide to safety in design. Where a platform or an excavation edge creates a fall risk, the separate control order in our guide to working at height in civil construction applies alongside the ground stability question.

A piling rig, a large crane or a concrete pump imposes loads on the ground that are concentrated, eccentric and dynamic. A rig that is stable on a properly designed platform can overturn on one that merely looks flat and firm. The failure mode is usually a punch-through on one track or outrigger, and it happens in seconds.

What a competent approach looks like:

• Get the machine’s actual bearing pressures from the supplier, for the operating case rather than the travelling case. Outrigger loads in particular are far higher than people expect.
• Design the platform to carry them — thickness, material, compaction, and where necessary geogrid reinforcement — on the ground that is actually there.
• Check what is underneath. Platforms built over recently backfilled trenches, over services, or beside an excavation are the classic failure setting.
• Certify and hand over. Where a specialist subcontractor brings the rig, they will normally ask you for the platform and for confirmation it is fit for their loads. That request is not paperwork; it is the transfer of a real risk, and answering it casually is how contractors end up liable for someone else’s overturned machine.
• Maintain it. Platforms degrade under trafficking and rain, and a platform certified in week one is not certified in week six.

The same thinking applies to crane pads and outrigger packing on smaller jobs, and to the ground beneath an elevated work platform. It is also relevant to the plant you describe in your plant and equipment schedule: heavier plant is a stronger submission only if the ground can carry it.

Where the contract puts the risk

Australian standard-form construction contracts generally leave the contractor responsible for the means, methods and sequence of construction, which is where temporary works sit. That default is rarely varied, and amended government contracts more often strengthen it than soften it. Our guides to AS 4000 and AS 2124 and contract forms beyond construct-only cover how the forms differ.

Four clauses worth finding before you price:

• Does the specification prescribe a method? If it mandates a particular support system or prohibits an approach, your options are constrained and your price must reflect it. A specification that prescribes the method also shifts some design responsibility back — worth knowing, and worth confirming in writing.
• Does the superintendent approve temporary works? Some contracts require submission of temporary works designs. Approval by a superintendent generally does not relieve the contractor of responsibility, and contracts usually say so explicitly.
• What are the obligations toward adjacent property and services? These usually sit squarely with the contractor and are the practical exposure from surcharge and undermining.
• What does the insurance respond to? Damage to temporary works themselves, and damage caused by their failure, are treated differently across policies — see insurance requirements in government civil tenders.

Pricing temporary works

Cost line	Notes
Engineering design and certification	A real fee, and a lead time. Budget it where the work is anything beyond a standard solution
Shoring or shield hire	Charged for the whole period on site, not the days used — see plant hire agreements on standing time and off-hire
Additional excavation and backfill for battering	Frequently the largest single item, and the one most often omitted from a trench rate
Additional cartage and disposal	The extra spoil from battering has to go somewhere, at a rate per cubic metre
Platform construction and removal	Material, placement, compaction, testing, and reinstatement of the area afterwards
Installation and extraction time	Shoring is not free to install. It is machine time and crew time on a cycle
Dewatering	Pumps, running hours, discharge treatment and authorisation
Inspection and re-inspection	Competent person time daily and after every rain event
Reduced production rate	Working inside a shield is slower than working in an open trench. If your production rate came from an open-cut job, it is wrong here

The last row is the one that quietly destroys margins, because it does not appear as a cost line at all — it appears as a job that takes longer than programmed. Tracking it is exactly the sort of thing the monthly cycle in our guide to job costing and cost control is designed to surface while there is still time to respond.

Inspection and the daily discipline

Temporary works are the one part of a civil job whose condition changes overnight without anyone touching it.

• Inspect before every entry, by a competent person, and again after anything that changes the conditions: rain, vibration, a change in the water, a load placed near the edge, or a period of hot dry weather that has opened cracks.
• Look for the warning signs — tension cracks parallel to the edge, spalling or sloughing from the face, water seepage, a change in soil type appearing in the face, or movement in a support system.
• Record it. The record is what demonstrates the duty was discharged, and it belongs in the same system as your other quality and safety records — see quality management plans and ITPs.
• Have a rule about the ladder. Access and egress within a short distance of any worker is a basic requirement and the first thing to disappear as a trench lengthens.
• Treat a trench as a potential confined space where atmosphere could be an issue — deep excavations, work near sewers, contaminated ground or where engines are operating nearby.

One further overlap worth naming: excavation work frequently happens beneath overhead services, and the excavator boom and the tipper tray are the two pieces of plant most often involved in powerline contact. Our guide to working near overhead powerlines covers those controls, which belong in the same daily check.

What tenders actually ask for

Temporary works appear in tenders in four places, and treating them as a scored opportunity rather than a compliance box is the difference between a generic response and a credible one.

• The safe work method statement. Excavation work is high risk construction work, so a SWMS is mandatory. A generic excavation SWMS that does not name the support system, the soil assumption and the inspection regime is the most common weakness — our guide to WHS management plans and SWMS covers the structure.
• The construction methodology. This is where you can score. Stating which excavations will be supported and which battered, on what basis, with what design status, and how the decision changes if the ground differs, demonstrates competence that competitors assert rather than show. Our guide to writing a construction methodology statement covers the format.
• The programme. Installation and extraction cycles, and the slower production inside a supported trench, belong in the programme rather than being absorbed.
• Prequalification. Larger clients ask whether you operate a temporary works register and who coordinates it. A one-page register and a named person is a legitimate answer for an SME and a better one than silence.

Checklist

• Have you listed every temporary works item on the job, including batters, platforms and haul roads?
• Is each item assigned to a level — standard solution, competent person assessment, or engineered design?
• Where an engineered design is required, is the engineer appropriately qualified and, where required, registered in that jurisdiction?
• Is there a temporary works register and a named person responsible for it?
• Does everyone on site know that a trench shield protects the people in it and does not support the trench?
• Are shields and shores being used within the manufacturer’s stated limits and soil types?
• If you are battering, is the angle based on the actual soil rather than a rule of thumb?
• Has the extra excavation, backfill, cartage and reinstatement from battering been priced?
• Is there physically room to batter, or was shoring always the only option?
• Is spoil placed away from the edge, and are plant, materials and traffic kept clear of it?
• Have adjacent structures, services and their footings been assessed for undermining?
• Does the geotechnical information actually cover your excavation line, and how old is the groundwater reading?
• For working platforms, do you have the machine’s operating bearing pressures and a platform designed for them?
• Has anything been built over recently backfilled ground or beside an open excavation?
• Is dewatering authorised, and has drawdown on neighbours been considered?
• Is a competent person inspecting before every entry and after rain, vibration or loading changes?
• Is safe access and egress within a short distance of every worker in the excavation?
• Does your SWMS name the actual support system, soil assumption and inspection regime?
• Does the programme include installation and extraction cycles and reduced production inside supports?

The short version

• Temporary works are the contractor’s design responsibility almost everywhere, and nobody hands you drawings for them.
• The regulatory trigger for trench support is a depth; the engineering trigger is ground, water, surcharge and time. Meeting the depth rule is not the same as being safe.
• A trench shield protects the people inside it. It does not support the trench, and the ground outside can still move.
• Battering is a designed slope, not the absence of design — and it multiplies excavation, backfill, cartage and reinstatement.
• Surcharge from spoil, plant, materials, traffic and adjacent structures is the most common contributor to collapse and the least considered.
• Know which of the three levels you are in: standard solution, competent person assessment, or engineered design.
• Working platforms for piling rigs, cranes and pumps are load-bearing structures. Get the operating bearing pressures and design for them.
• Boreholes are points and your trench is a line; disturbed ground along old service alignments behaves nothing like natural ground.
• Price design fees, hire for the whole period, extra excavation and cartage, installation cycles — and the slower production inside a supported trench.
• Inspect before every entry and after rain, vibration or a load placed near the edge, and record it.
• A one-page temporary works register with a named coordinator is a proportionate SME answer and increasingly a scored differentiator.

Sources and further reading

This guide is general information for Australian civil construction businesses and is not engineering, geotechnical, safety or legal advice, and nothing in it is a design or a substitute for one. Temporary works design is a professional engineering activity and must be carried out by a suitably qualified person; in some jurisdictions it must be carried out by a registered professional engineer. Work health and safety regulations, including trigger depths for excavation work, the classification of high risk construction work and ground support requirements, are made and amended by each state and territory and must be confirmed with the regulator for the jurisdiction you are working in — no depth, angle, soil classification or support arrangement is stated here as a safe value and none should be inferred. Proprietary shoring and shielding equipment must be used strictly within the manufacturer’s stated limits. Always work from a design prepared for your actual ground conditions, the executed contract, and current professional advice.

• The model Work Health and Safety Regulations and the versions enacted in each Australian state and territory, which classify excavation work at a specified trench depth as high risk construction work requiring a safe work method statement, and which require ground support unless a competent person has determined the ground is not at risk of collapse. Referenced in §03 and §13. Trigger depths, definitions and duties differ between jurisdictions and are amended; the regulator for your jurisdiction is the authority. The general SWMS and management plan obligations these sit within are sourced in full in our guide to WHS management plans and SWMS for civil tenders.
• Regulatory codes of practice and guidance on excavation work published by Australian work health and safety regulators, which address ground support systems, battering and benching, surcharge loading, access and egress, and inspection by a competent person — the basis for §03 to §06 and §12. These are jurisdiction-specific and periodically revised.
• Professional engineer registration requirements, which apply in some Australian jurisdictions and are being introduced in others, referenced in §07 as affecting who may prepare and certify a temporary works design. The related question of contractor licensing across borders is sourced in full in our guide to contractor licensing by state and territory.
• Temporary works management practice as applied on major Australian projects — the temporary works register and temporary works coordinator structure described in §07, together with the design, check, install, inspect and remove control cycle. This is contractual and client-imposed practice rather than a statutory requirement, and the proportionate SME version described here is a recommendation rather than a standard.
• Related TenderBuilt guides carrying the primary-source detail referenced above: latent conditions, AS 4000 and AS 2124, contract forms beyond construct-only, utility and telecommunications civil works (service location and proving), environmental approvals and permits (dewatering discharge), insurance requirements, plant hire agreements, preparing civil works cost estimates and job costing and cost control. See also rock batters and slope stabilisation.