Last reviewed 16 Sept 2026 · 9 min read
Site preparation and setting out
- Site investigation — soil exploration, water table, existing services, access.
- Site clearance — removal of vegetation, debris and topsoil; demolition of old structures.
- Levelling and grading; provision of temporary drainage, access roads, stores, water and power.
- Temporary benchmark (TBM) established near the site.
- Setting out of foundations — baseline from the site plan; centre lines of walls or column grids set out with tape, theodolite or total station; right angles by 3-4-5 method; diagonals checked; profile boards fixed outside the excavation to preserve lines; trench widths marked with lime/chalk powder (see Setting Out of Works).
Excavation
Methods and equipment
- Manual excavation — pickaxes, spades, crowbars, baskets — small works, restricted spaces, near services.
- Mechanical excavation:
| Equipment | Use |
|---|---|
| Hydraulic excavator (backhoe) | Trenches and pits below machine level, loading trucks |
| Face shovel | Excavating above track level (cuts, quarries) |
| Dragline | Large, loose or wet excavations below level, long reach |
| Clamshell | Deep, confined vertical excavations (wells, shafts) |
| Bulldozer | Pushing and spreading earth, clearing, short-haul |
| Scraper | Excavating, hauling and spreading over medium distances |
| Loader (front-end/wheel loader) | Loading loose material |
| Grader | Fine levelling and shaping |
| Trenchers | Narrow continuous trenches for pipes and cables |
- Excavation in rock — rock breakers (hydraulic hammers), controlled blasting, chemical expansive agents where blasting is not permitted.
Stability of excavation sides
Trench sides may collapse because of soil weight, water, vibration, surcharge from spoil and equipment. Protection by:
- Sloping the sides to a safe angle (depending on soil) or benching (steps).
- Timbering (shoring of trenches) or proprietary trench boxes and hydraulic shoring.
- Keeping spoil and equipment away from the edge.
- Dewatering where the water table is high.
Excavations deeper than about 1.5 m generally require protective measures and safe access (ladders), barricading and lighting.
Timbering of trenches
| System | Soil condition | Arrangement |
|---|---|---|
| Stay bracing | Firm soil, moderate depth | Vertical sheeting planks (poling boards) placed opposite each other at intervals and held apart by horizontal struts |
| Box sheeting | Loose soil, moderate depth | Vertical sheeting boards placed close together, held by horizontal walings and struts — forming a box |
| Vertical sheeting | Deep trenches in soft soil | Box sheeting installed in stages (with offsets/steps as depth increases) |
| Runner system | Very loose soils (running sand) | Long vertical runners (thick planks with pointed ends) driven ahead of excavation, supported by walings and struts |
| Sheet piling | Very soft/waterlogged soils, deep and wide excavations | Steel (or timber/concrete) sheet piles driven before excavation, interlocking to resist earth and water pressure, supported by walings and struts or anchors |
Components: sheeting/poling boards (vertical planks against soil), walings/wales (horizontal members), struts (horizontal compression members across the trench), wedges, puncheons (short vertical props between walings).
Dewatering (control of groundwater)
| Method | Description / suitability |
|---|---|
| Open sump pumping | Water collected in sumps and pumped out — shallow excavations, cohesive or coarse soils with modest inflow |
| Well point system | Closely spaced small-diameter wells (well points) connected to a header pipe and vacuum pump — lowers water table in sands and gravels, a few metres per stage (multi-stage for deeper lowering) |
| Deep well system | Large-diameter wells with submersible pumps around the excavation — deep lowering in permeable soils |
| Electro-osmosis | Electric current drives water towards cathode wells — silts and fine soils of low permeability |
| Grouting | Cement, clay or chemical grouts injected to reduce permeability (and strengthen) |
| Freezing | Ground frozen by circulating refrigerants — very difficult waterlogged ground (temporary) |
| Cut-off walls | Sheet piles, diaphragm walls, secant piles to exclude water |
Shoring
Shoring is the construction of temporary supports to structures (walls, buildings) that are unsafe or require support during repairs, alterations, adjacent excavation or underpinning.
| Type | Arrangement | Use |
|---|---|---|
| Raking (inclined) shores | Inclined members (rakers) with their upper ends against a wall plate fixed to the wall (with needles and cleats), and lower ends on a sole plate bedded on the ground; bracing between rakers | Support walls that tend to bulge or lean outward; common for walls of damaged or adjacent buildings. Rakers are commonly inclined at about 60–70° to the horizontal (not flatter than about 45° or steeper than about 75°) |
| Flying (horizontal) shores | Horizontal struts spanning between two parallel walls, with wall plates, needles and inclined struts (straining pieces) | When an intermediate building is demolished and rebuilt between two existing buildings |
| Dead (vertical) shores | Horizontal needles passed through holes in the wall and supported on vertical props on both sides, with sole plates | Support the upper part of a wall while its lower portion or foundation is removed/rebuilt, or openings are made |
Underpinning
Underpinning is the process of strengthening, deepening or extending the foundations of an existing structure.
Purposes
- Foundations have settled or are inadequate (weak soil, deterioration).
- Deeper excavation adjacent to the building (new basement or foundations nearby).
- Increased loads (additional storeys, change of use).
- Creating a basement below an existing building.
- Repairing damage from subsidence, tree roots or leaking drains.
Methods
| Method | Description |
|---|---|
| Pit (mass concrete) method | The wall is supported (by needles and dead shores if needed); short sections (pits) of foundation are excavated below the existing footing and filled with new foundation concrete, working in alternate bays so that the wall is never unsupported over a long length |
| Pile method | Piles installed along both sides of the wall; needles or pile caps (beams) pass through the wall to transfer loads to the piles |
| Cantilever needle and pile method | Piles on one side only (where access is restricted), with cantilevered needle beams |
| Mini-piles / micro-piles | Small-diameter bored and grouted piles drilled through or alongside existing foundations — minimal disturbance, restricted access |
| Jack piling | Precast pile segments jacked down using the weight of the structure as reaction |
| Grouting and ground improvement | Cement, chemical or jet grouting to strengthen soil below foundations |
| Beam and base / raft underpinning | New reinforced concrete beams or rafts transferring loads to new bases |
Precautions: survey and monitor the building (cracks, levels); provide shoring; work in short sections; avoid undermining; ensure new foundations are well bonded (dry pack/grout between old and new).
Demolition
- Planning: structural survey, disconnection of services, safety and environmental measures (dust, noise), permits.
- Methods: manual (top-down, element by element), mechanical (excavators with breakers, crushers, wrecking balls, high-reach machines), controlled implosion (explosives — tall structures in open surroundings), diamond wire and wall sawing (precise cutting).
- Sequence: generally reverse of construction — non-structural elements first, then roof and floors from top downward.
- Waste management: segregation and recycling of concrete, steel, bricks, timber (construction and demolition waste rules).
Construction equipment
| Category | Equipment |
|---|---|
| Earthmoving | Excavators, shovels, draglines, clamshells, bulldozers, scrapers, loaders, graders, dumpers/tippers |
| Compaction | Smooth-wheeled, sheep-foot, pneumatic-tyred and vibratory rollers, plate compactors, rammers |
| Hoisting | Tower cranes, mobile cranes, crawler cranes, derricks, hoists, winches, gantry cranes |
| Concrete equipment | Batching and mixing plants, drum mixers, transit mixers, concrete pumps (boom and line), vibrators, power trowels |
| Piling equipment | Pile drivers (drop hammers, diesel and hydraulic hammers, vibratory hammers), rotary piling rigs |
| Pavement equipment | Hot mix plants, pavers, slip-form pavers, bitumen distributors |
| Tunnelling | Tunnel boring machines, drill jumbos, muck loaders |
FormulaOutput of an excavator (approximate)
= bucket capacity (m³); = bucket fill factor; = job efficiency factor (e.g. working minutes per hour ÷ 60); = cycle time (seconds).