CIVL1810 Chap.3 Preparing the Site and Choosing Excavation Plant
Preparing the Site and Choosing Excavation Plant
Week 2 begins with an empty parcel of land. Three commencing tasks turn it into a buildable site: clearing, setting out the building, and establishing a datum level from which every later height is read. Site clearance is specified in depths and distances rather than adjectives, and excavation is defined to include both digging the material and getting it out.
The plant catalogue then splits into machines chosen by reach, by the material they can handle, and by whether the spoil still needs hauling.
What this chapter covers
- 01
Three tasks that open the job: clearing, pegging the building out, and fixing a datum level
- 02
The land surveyor's role in fixing original ground level and marking survey pegs for services
- 03
What clearing involves: demolition, grubbing out vegetation, and removing soil to reduce levels
- 04
Clearance specifications: topsoil sterilised to 300 mm, saplings to 300 mm girth measured 1 m above ground, rubbish to 50 m beyond the periphery
- 05
How deep roots go: the deepest of 600 mm under the existing surface, 300 mm under formation and 150 mm under subgrade, then earth fill, ram and level
- 06
Anything already in the ground rerouted or taken out on the site engineer's instruction
- 07
Excavation as a two sided operation: loosening the ground and carrying the spoil away
- 08
The four planning inputs: soil type, nature of the excavation, haul distance and working space
- 09
Excavating equipment classified into excavators and tractor based equipment
- 10
Power shovel, backhoe and trench digger, and what each one suits
- 11
Dragline and clamshell: the cable suspended machines for reach and for depth in a small area
- 12
Loader, tractor shovel, scraper, bulldozer and angle dozer
- 13
Arranging temporary buildings and site facilities once the structure is marked out
Choosing plant for a described excavation
- +1Strip and clear first. A dozer, described as the most common equipment for clearing and levelling, takes off vegetation and topsoil; a tractor shovel loads the stripped material. Justification: vegetation and roots must be removed before earthworks begin.
- +1Bulk excavation of a large open area. A dragline works from the side of a large open excavation and reaches well beyond a backhoe, which suits a wide basin in open ground; a backhoe would need to stand within the dig and would be limited by reach.
- +1Moving the spoil 400 m. A scraper excavates, loads, hauls and dumps in one cycle, which removes the need for a separate excavating machine and a haulage fleet on this movement. The planning inputs that justify it are haul distance and available working space, both of which are generous here.
- +1Say what would change the answer. A clamshell would be right for depth in a confined footprint and is specified for all soils except rock; it is wrong here because the excavation is wide and shallow. If the material were rock rather than clay, the scraper could not cut it and the operation would separate again into breaking, loading and hauling.
Key terms
- Setting out
- Marking the position of the works on the ground before construction begins. It is one of the three commencing tasks, alongside clearing the site and establishing a datum level.
- Sterilising the topsoil
- Removing the top 300 mm of soil to contain plant life and decaying vegetation. Organic matter left in the ground rots, loses volume and leaves voids beneath whatever is built above.
- Power shovel
- Its dipper is an open topped bucket whose floor swings open to discharge, carried on a stick that both slides and rotates where it meets the jib. It cuts into a face or bank and will take rock and stone, but it cannot work a flat surface.
- Dragline
- A bucket hung on cable alone from a slim boom, with the machine standing on natural ground alongside the cut rather than inside it. It handles wide open digs a backhoe cannot reach across, and it is standard plant in open cut mining.
- Clamshell
- A pair of hinged jaws on a frame, hung on cable from a long jib. It goes deep in a small plan area, in any ground other than rock.
- Scraper
- A machine that excavates, loads, hauls and dumps material in one cycle. Options include self loading elevators, twin engines and push pull capability, and it earns its keep where the haul is long and the ground is open.
Preparing the Site and Choosing Excavation Plant FAQ
Why is establishing a datum level treated as a separate task?
Because every height obtained later on the job is read back to it. Fixing the untouched ground level is the surveyor's work, and it produces the pegs that locate drainage, buried pipework, cabling and the sewage treatment plant, all before any bulk earth moves. Once earthworks have altered the ground surface there is nothing left to measure from, so the reference point has to exist first.
What does the phrase whichever is lower mean in the root removal rule?
Roots go to whichever is deepest of three limits: 600 mm under the existing surface, 300 mm under formation, or 150 mm under subgrade. The three depths are measured from three different reference surfaces, and which surface governs changes as the earthworks proceed, so quoting a single depth in isolation misses the rule.
Is a plant answer complete if it only names diggers?
No. The definition is two sided: digging counts, and so does lifting the loosened material off the face and taking it away, so haulage belongs to the operation rather than following it. Roughly half the plant on a real dig is there for the second of those, and an answer that lists excavators without trucks, loaders or a scraper has described only part of the job.
Which planning input do students most often forget?
Working space, closely followed by haul distance. Soil type and the nature of the excavation are obvious, but a machine that cannot turn or cannot be reached by a truck is the wrong machine however well it digs. On constrained sites those two inputs usually decide the answer, and the unit lists them alongside the other two for that reason.
Exam move
Learn the clearance figures as a short table rather than as prose, because the numbers are the answer to the specification question. Then build a one line profile for each machine covering how it works, what it suits and what it cannot do, and drill the three selection questions in order: can it reach, can it handle the material, and does the spoil still need hauling.
Practise on described sites rather than on the catalogue: take an open paddock, a narrow city block and a rock cutting, and write the plant sequence for each with a justification tied to a stated condition. Finish by sketching a section with the machine at each level and an arrow for the spoil route, because that sketch is what the paper asks for.
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