Earthwork and excavation takeoff

Soil exists in three conditions, and the reported number changes by about 10 to 70 percent between them. Bank is the natural, undisturbed volume you read off the drawings: the cut prism between existing ground and the design surface, or the fill prism between original ground and finished grade. Loose is the excavated, swelled volume that fills a truck, equal to bank multiplied by one plus the swell percentage. Compacted is the placed and rolled volume a finished embankment occupies, equal to bank multiplied by the shrinkage factor.

Two factors connect the states, both referenced to bank. Swell expands bank into loose, and its inverse, the load factor, converts loose back. Shrinkage reduces bank into compacted, so a finished fill always needs more bank cut or borrow than its own geometric volume: the required borrow in bank terms equals the compacted fill volume divided by the shrinkage factor. Netting raw cut against raw fill without applying shrinkage is the classic earthwork balance error.

The factors vary strongly by material. As approximate planning values, granular sand and gravel swell about 12 to 18 percent and shrink about 5 to 14 percent; common earth swells about 25 percent and shrinks about 10 to 20 percent; clay swells about 30 to 40 percent and shrinks about 10 to 20 percent; and blasted rock swells about 50 to 70 percent and has negative shrinkage of roughly 30 percent, because broken rock occupies more space than the bank it came from. These are published averages for planning; the real values come from a soils test, in place density by ASTM D1556 or D6938 and maximum dry density by Proctor testing under ASTM D698 or D1557.

Which state to report follows the purpose. For a bid you start from bank cut and compacted fill, then add the bank borrow needed for any shortfall; for haul and disposal you convert to loose; for an embankment paid in place you report compacted. A bare cubic yard or cubic metre is ambiguous, so the unit should always be tagged with its state. Under most United States highway specifications, roadway excavation is measured in bank position and embankment in compacted position, with the contractor absorbing swell and shrinkage at no separate payment.

Two geometric methods dominate, and the right one depends on the work type. For linear and roadway earthwork, the average end area method takes the cut or fill cross sectional area at each station, averages two adjacent areas, and multiplies by the distance between them. In United States units the cubic yards equal the average of the two end areas multiplied by the length, divided by 27. The method slightly overstates volume where sections change rapidly, and a prismoidal correction refines it where precision matters. Accuracy depends on spacing: straight ground is sectioned at roughly 50 to 100 feet, commonly 100 feet rural and 50 feet urban, reduced to about 25 feet or less on ramps, sharp curves, and rapidly changing ground.

For sites, building pads, and ponds, where there is no single alignment, a grid or spot elevation method is used instead: overlay a grid, compute the cut or fill depth at each node from the existing minus the proposed elevation, and sum the prisms. Both methods produce a bank volume for cut and a compacted volume for fill; the state conversions are applied afterward, never built into the geometry.

The pay and design quantity is the neat line: existing ground down to the theoretical cut surface or finished grade, at the design side slopes. The contractor almost always excavates more than this, because soil cannot stand vertical, but that extra dirt is means and methods, not the measured quantity. Reporting the actual battered prism instead of the neat line overstates the pay quantity by the slope volume.

When a takeoff models the true excavated prism for cost estimating, the side slope sets the over cut. OSHA Subpart P fixes maximum allowable slopes for excavations up to 20 feet deep, with a protective system required at 5 feet or more unless the face is stable rock, and an engineered design beyond 20 feet. The maximum slopes are vertical for stable rock, three quarters horizontal to one vertical (about 53 degrees) for Type A soil, one to one (45 degrees) for Type B, and one and one half to one (about 34 degrees) for Type C. These are safety limits, not the pay line.

Trench excavation is measured to a specified pay width, typically the pipe outside diameter plus a working clearance each side, or a width stated in the contract or standard detail, regardless of how wide the contractor digs. Per side clearances of roughly 150 to 300 millimetres (6 to 12 inches) are common practice rather than a fixed figure, so confirm the pay width against the project trench detail. Over width beyond the pay line is the contractor's cost.

Earthwork volume is measured net, with no allowance for bulking, shrinkage, or waste built into the geometric quantity. This is a stated principle in CESMM4 and is shared by NRM2, the Australia and New Zealand method, and DIN 18300. Padding the geometry with swell and then also applying a state factor double counts, which is why the geometry stays net and the conversions stay explicit.

There is no earthwork specific codified void threshold, and minor isolated obstructions such as single piles or small services are ignored and absorbed. The dominant mechanism for existing structures and services in the dig is extra over, which adds the cost of excavating around or across them rather than deducting their volume; NRM2 measures extra over for excavating alongside or across existing services and for breaking up rock, reinforced concrete, or brickwork. Only substantial voids are deducted, and where a size threshold is wanted, the roughly 1 cubic metre figure from the building works void convention is used by analogy.

Excavation is split by material because the cost differs by an order of magnitude with how hard the ground is to dig. United States and AASHTO practice separates common excavation, rock excavation (material needing ripping or blasting, with boulders above a stated size counted as rock), and unsuitable or subsoil excavation, which is soft or organic muck removed below grade and replaced as its own pay item. A single unclassified excavation item is also common, where the contractor takes all the material risk. The boulder size that triggers rock varies by agency; some use a volume such as about 1 cubic yard and others a rippability test. Under NRM2 and CESMM4, excavation is split into topsoil, material other than topsoil or rock, and rock. Germany's VOB with DIN 18300 replaced the old fixed soil classes with project specific homogeneous areas.

How rock is measured follows the same split. In the quantity surveying tradition, rock is measured extra over the base excavation: the rock volume is still counted in the base dig, with an additional rate for the difficulty, irrespective of depth. United States highway practice instead measures rock as its own separate pay item that replaces the base quantity. Getting this wrong either double counts the rock or omits the base dig beneath it.

Topsoil is stripped and stockpiled separately from bulk excavation, because it is reused for landscaping. It is measured by area with an average strip depth stated, typically about 100 to 150 millimetres (4 to 6 inches), and can also be reported as a stockpile volume of area multiplied by depth. NRM2 measures it this way, for example as removing topsoil 150 millimetres thick by area.

Disposal of surplus is itemised by destination, conventionally priced for haul on the loose truck volume, while quantity surveying bills often measure it on the bank volume of the excavation it came from; imported fill is billed on the compacted volume it forms in place. Haul distance is governed by the mass haul diagram, which plots cumulative cut minus fill on a common bank basis along the alignment. Up to a contract free haul distance the move is included in the base excavation price; beyond it, overhaul is paid separately as a volume distance quantity, such as cubic yard stations or cubic metre kilometres, rather than a pure volume.

The United Kingdom is the most codified. NRM2 and CESMM4 measure excavation net in cubic metres, with the commencing surface and reduced level stated. NRM2 bands bulk and foundation excavation in 2 metre depth stages (not exceeding 2 metres, 2 to 4 metres, 4 to 6 metres, and so on), while CESMM4 classifies by total maximum depth. Working space is left to the contractor's discretion under NRM2, and its second edition reintroduced measuring earthwork support to all excavation faces over 250 millimetres deep, whether or not it is thought necessary.

United States highway practice has no legal method of measurement: roadway excavation is in bank position by cubic yard, embankment is compacted, depth is not banded into stages, and the contractor absorbs swell and shrinkage. In Australia and New Zealand, civil cut and fill is measured under AS 1181, while the Australia and New Zealand Standard Method of Measurement covers building substructure, where excavation depth is classified in 1 metre increments (0 to 1, 1 to 2, 2 to 3, 3 to 4 metres, so a total depth of 3.5 metres falls in the 3 to 4 metre band) and working space is girth along the footing multiplied by depth. Across Europe, VOB with DIN 18300 bills to actual dimensions with homogeneous area material classification.

For progress billing, the contractor is paid either the plan quantity or a field measured quantity from final cross sections. Highway departments commonly pay the plan quantity when no design change occurs, re measuring only when a defined trigger is hit, such as consecutive end areas varying beyond a threshold (a 5 percent variance is common but agency specific), undercut, a slide, or settlement. This pay basis is distinct from both the bid quantity and the order quantity, and the three should never be reported as one another.