Temporary works design covers the engineering of temporary structures needed to support excavations, retain ground, and provide safe working platforms during construction. Proper design of temporary works is essential for safety, constructability, and cost control.
What Are Temporary Works?
Temporary works are structures or systems required during construction but not forming part of the permanent works. In geotechnical engineering, common temporary works include:
- Shoring systems — support for excavation walls
- Temporary retaining walls — soldier piles, sheet piles, contiguous piles
- Ground anchors and tiebacks — lateral support for shoring walls
- Working platforms — stable surfaces for cranes and piling rigs
- Temporary drainage — dewatering systems for excavation
- Temporary slopes — cut slopes for access ramps or excavations
Excavation Support Systems
Soldier Pile and Lagging
Steel I-beams (soldier piles) are installed at regular spacing (typically 1.5–2.5 m) and timber or shotcrete lagging is placed between them as the excavation proceeds.
- Suitable for: Stiff clays, weathered rock, sands (with dewatering)
- Advantages: Cost-effective, flexible spacing, staged construction
- Limitations: Not watertight, limited for very soft soils
Sheet Pile Walls
Interlocking steel profiles are driven into the ground to form a continuous wall.
| Type | Description | Application |
|---|---|---|
| Straight web | Low section modulus | Light retaining, cut-off walls |
| U-section | High bending resistance | Deep excavations, waterfront |
| Z-section | Very high bending resistance | Deep excavations, heavy loads |
- Advantages: Watertight (if interlocked), reusable, quick installation
- Limitations: Noise and vibration during driving, limited depth in dense ground
Contiguous Pile Walls
Reinforced concrete piles are installed at close spacing (typically 150–300 mm gaps) to form a retaining wall.
- Suitable for: All soil types including rock
- Advantages: High stiffness, can be installed in limited headroom
- Limitations: Not watertight (gaps between piles), more expensive
Secant Pile Walls
Overlapping piles form a continuous, watertight wall. Alternate piles (primary piles) are installed first, then secondary piles cut into them.
- Suitable for: Water-bearing soils, deep basements
- Advantages: Watertight, high stiffness, can take vertical loads
- Limitations: High cost, requires specialised equipment
Diaphragm Walls
Cast-in-situ concrete walls constructed in a trench under bentonite slurry.
- Suitable for: Deep basements (20+ m), cut-and-cover tunnels
- Advantages: Watertight, very high stiffness, part of permanent structure
- Limitations: High cost, specialised contractor
Ground Anchors and Tiebacks
Ground anchors provide lateral restraint to shoring walls by transferring load to stable ground behind the excavation.
Types of Anchors
| Type | Description | Typical Capacity |
|---|---|---|
| Temporary anchors | Unbonded length, bonded length in grout | 200–1,500 kN |
| Permanent anchors | Double corrosion protection, grouted | 300–3,000 kN |
| Macro-piles | Small-diameter grouted pile used as anchor | 100–500 kN |
Anchor Testing
| Test | Purpose |
|---|---|
| Proof test | Verify anchor capacity to 1.25–1.5× working load |
| Performance test | Verify load-deflection behaviour |
| Suitability test | Confirm design assumptions on-site |
Working Platform Design
Design Parameters
A safe working platform must support the maximum loads of construction plant:
| Plant Type | Working Load (kPa) | Typical Platform Thickness |
|---|---|---|
| Small excavator (< 20t) | 50–80 | 300–400 mm |
| Medium excavator (20–40t) | 80–150 | 400–600 mm |
| Piling rig | 200–400 | 500–1,000 mm |
| Crawler crane (50–100t) | 150–300 | 500–900 mm |
| Mobile crane outriggers | 300–600 | 600–1,200 mm |
Working Platform Design Process
- Subgrade investigation (CBR, DCP, plate load test)
- Platform thickness design (limit equilibrium or bearing capacity)
- Material specification (DGB-20, crushed rock, recycled aggregate)
- Construction quality control (lift thickness, compaction tests)
- Certification (safe working platform certificate)
Dewatering
Types
| Method | Suitable For | Application |
|---|---|---|
| Sump pumping | Small excavations, granular soils | Simple, low cost |
| Wellpoints | Shallow excavations, sands | Close spacing, continuous pumping |
| Deep wells | Deep excavations, variable soils | Large volumes |
| Eductor wells | Fine sands, silts | Vacuum-assisted dewatering |
Design Considerations
- Predicted groundwater inflow
- Drawdown radius of influence
- Impact on adjacent structures (settlement from consolidation)
- Water treatment and discharge (EPA requirements, trade waste)
- Duration of operation
Key Design Checks
| Check | Requirement |
|---|---|
| Wall stability | Overturning, sliding, bearing capacity |
| Structural capacity | Bending, shear in wall elements |
| Anchor design | Pullout capacity, corrosion protection |
| Global stability | Deep-seated failure surface through anchor zone |
| Settlement behind wall | Ground loss, consolidation from dewatering |
| Basal heave | Bottom heave in soft clays |
| Groundwater control | Inflow rates, filter design, settlement risk |
Australian Standards and References
| Standard | Title |
|---|---|
| AS 4678-2002 | Earth Retaining Structures |
| AS 4100 | Steel structures (soldier piles, walers) |
| AS 3600 | Concrete structures (piles, walls) |
| AS 1726-2017 | Geotechnical site investigations |
| Safe Work Australia | Excavation and shoring guidance |
| CIRIA C517 | Temporary works design guidance |
| BS 8004 | Foundations (retaining wall design) |