Earthwork Production Planning and Excavator Selection

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06
LECTURE 6
Earthwork
Production
Planning excavations, calculating soil volumes, and choosing the right
excavator for foundation construction
COURSE
TECHNOLOGY OF CONSTRUCTION
PRODUCTION
TECHNOLOGY OF CONSTRUCTION PRODUCTION · LECTURE 6
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WHAT WE'LL COVER
From site survey to excavator selection
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02
03
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Initial Data
Volume Calculations
Machine Parameters
Excavator Selection
Reading a site plan and geological
cross-section before any digging
starts
Working out how much soil to
remove, move, and bring back
The vocabulary engineers use to
describe earthmoving equipment
Matching a machine and bucket size
to the job
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SECTION ONE
Initial Data
What an engineer needs to know before sizing an excavation
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3.1 INITIAL DATA
Every excavation starts on paper
Earthwork design happens at the technology-planning stage, before any
machine reaches the site.
Two things every design needs
The size of every pit, trench, or embankment depends on the size, material, and
construction of the building's underground structure.
Where each foundation sits relative to the
building's grid of column axes
It also depends on the site's own ground and water conditions — no two
building sites are ever quite the same.
A geological cross-section of the soil layers
actually present at the site
Getting these numbers right matters: an excavation that is too small forces
rework, while one that is too large wastes machine-hours and money.
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3.1 INITIAL DATA
Locating foundations on the grid
Figure 1. Foundation positions relative to the building's grid axes
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Buildings are set out on a grid of reference lines, lettered (Я, Б, А…) one
way and numbered (1, 2… n) the other.
“Пролет / span” is the distance between two lettered lines; “шаг / step”
is the distance between two numbered lines.
Every foundation is described by which pair of grid lines it sits on — this
fixes its exact position before any digging is planned.
The overall building footprint is simply the sum of all the spans, or all the
steps, in each direction.
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3.1 INITIAL DATA
A worked example: a strip-foundation plan
Reading this drawing
Each labelled block is a precast foundation panel; the count in
brackets is how many identical panels are needed.
The numbers along the edges give the overall building
dimensions in millimetres.
This plan is exactly what an excavation plan is drawn on top of.
Figure 2. Layout plan of a precast strip (ribbon) foundation, with panel lengths and grid-line spacing
marked
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3.1 INITIAL DATA
Reading the ground: a geological cross-section
Layer 1 — topsoil / vegetation layer: organic, unsuitable for bearing loads and always stripped off
first.
Layer 2 — the first main soil layer beneath the topsoil.
Layer 3 — a second main soil layer, often below the groundwater level (GWL).
GWL — the depth at which groundwater is found; when an excavation reaches this level, pumping
or drainage may be needed.
Figure 3. Soil layers at the site, with the groundwater level
(GWL) marked
From this cross-section, the design records each layer's group (dig-difficulty), density, safe slope angle, and how
much it swells once dug — the full soil profile the excavation plan is built on.
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SECTION TWO
Calculating Earthwork Volumes
Turning a cross-section into cubic metres of soil to move
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3.2.1 CHOOSING THE EXCAVATION TYPE
Leave room to work — and round up
Earthwork design must keep machines, workers, and formwork installers safe,
and leave room for site machinery to move between trenches, pits, and
foundations.
Every excavation dimension is rounded up to the nearest 0.1 m, and every
volume up to the nearest 0.1 m³ — never down.
≥ 0.7 m
≥ 0.3 m
≥ 0.6 m
0.1 m / 0.1 m³
Clearance from a vertical-walled
excavation face to the structure,
allowing for waterproofing and
formwork
Minimum clearance when the
excavation has sloped (battered) sides
instead of vertical walls
Clearance at the base when workers
must stand and work from two or three
sides
Rounding precision for every excavation
dimension and every calculated volume
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3.2.1 CHOOSING THE EXCAVATION TYPE
Cross-section 1–1: reading a transverse cut
To decide what shape of excavation is needed,
engineers draw two cuts: section 1–1 across the
building and section 2–2 along it.
c₁ is the width of undisturbed ground (“soil island”)
left standing between two neighbouring
foundations in this cross-section.
H₂ is the total dig depth; Hv is the depth of the
excavation itself above the working level.
The slope ratio “1:m” fixes how far the battered side
leans out for every metre of depth.
Figure 4. Transverse cross-section through two neighbouring foundations, one with vertical walls (left) and one with sloped
sides (right)
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3.2.1 CHOOSING THE EXCAVATION TYPE
One pit, a trench, or one big excavation?
The rule of thumb
Figure 5. Cross-section along the outer grid axes, showing the soil islands c₁ and c₂ left between
foundations
TECHNOLOGY OF CONSTRUCTION PRODUCTION · LECTURE 6
Both c₁ and c₂ wider than 4.5 m → dig a separate pit for each
foundation.
One of them narrower than 4.5 m → dig a continuous trench
along that row of foundations.
Both narrower than 4.5 m → excavate one common pit for the
whole area.
4.5 m is simply the minimum safe width for a truck, crane, or
bulldozer to pass without risking a soil-island collapse — below
that, it's more efficient to remove the island entirely.
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3.2.2 CALCULATING VOLUMES
Step 1 — strip the topsoil
Before digging begins, the organic topsoil layer is stripped off across the
whole working area (not just where foundations sit).
How far back to strip
Volume of stripped topsoil:
≥ 20 m
V = K×l×h
p.c
m³
p.c
K and l — the overall width and length of the whole building site, found once the
site layout is finalised.
• hp.c — the measured thickness of the topsoil layer at the site.
TECHNOLOGY OF CONSTRUCTION PRODUCTION · LECTURE 6
back from the edge of every excavation, if all excess soil is stockpiled
directly by excavator
20 – 40 m
back, if stockpiles are instead placed outside the building's own
footprint
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3.2.2 CALCULATING VOLUMES
Step 2 — the volume of the excavation itself
Once the type of excavation is fixed (pit, trench, or common excavation), its volume can be estimated from its average plan
dimensions and its depth.
V = a + a₁ ÷ 2 × b + b₁ ÷ 2 × H × n m³
e
v
v
a, a₁ and b, b₁ — the excavation's width and length at its top and bottom edges (they differ whenever the sides are sloped).
Hv — the depth of the excavation; nv — the number of separate excavations of this kind.
If the first and second soil layers differ in dig-difficulty group, their volumes are calculated separately.
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3.2.2 CALCULATING VOLUMES
Finishing the base by hand
A sand cushion (bedding layer) of at least 0.1 m goes under every
foundation, resting on undisturbed soil.
On sandy ground, the excavator deliberately leaves a thin under-dig δ
(delta), finished off by hand or by a laser-guided screed rather than by
the machine.
Hand-worked volume, clayey soils: Vd = Fd × Δ, where Fd = Ad × Bd × nf
(nf = number of foundations).
Hand-worked volume, sandy soils: Vd = Fd × δ, where Fd = a × b × nf.
Figure 6. Hand-finished excavation base: (a) clayey soils, (b) sandy soils
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3.2.2 CALCULATING VOLUMES
From total dig to backfill and haulage
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2
3
Total volume
Backfill volume
Haulage volume
Add the excavation volume to the topsoil-stripping volume:
V-total = Ve + Vp.s
Subtract the volume the foundations themselves occupy,
then apply the soil's residual bulking factor ko: V-fill = (Vtotal − ΣV-fdn) × (1 − ko)
The difference between what was dug and what is needed
for backfill, adjusted for initial bulking kp: V-haul = (Ve − Vfill) × (1 + kp)
A positive haulage volume (+) means there is surplus soil to truck away to spoil; a negative value (−) means extra soil must be brought in from a borrow pit
to make up the shortfall.
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3.2.2 CALCULATING VOLUMES
Where does the surplus soil go?
Surplus soil is temporarily stockpiled on site — spread and shaped by
bulldozer into low ridges called stockpiles (“cavaliers”) — ready for
backfilling later.
In cross-section a stockpile is either triangular or trapezoidal.
3 m, up to 5 m
≤ 60 m
1m
4 m or 7 m
typical stockpile height — taller piles
are harder to shape and to take back
down
maximum stockpile length
minimum passage width required
between adjacent stockpiles
vehicle-access roadway width left
between stockpiles, sized to the
machinery used
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3.2.2 CALCULATING VOLUMES · BACKFILL & COMPACTION
Not just any soil can go back in
Backfill soil must be free of large stones, construction debris, and frozen
clumps, and its fines content is capped at about 15%.
A sand-and-gravel backfill mix is typically proportioned around 1 : 1.2 (sand
to gravel).
Compaction is checked with a coefficient k: about 0.90–1.0 for ordinary
backfill, rising to 0.95–1.0 close to structures and under floor slabs.
Table 1. Settlement coefficient k₁ by soil type (illustrative values)
Soil type
k₁, settlement coefficient
Sand, sandy loam
0.01 – 0.03
Loam
0.03 – 0.06
Clay
0.06 – 0.09
Bulked / disturbed soil
0.10 – 0.15
k₁ tells the crew how much extra soil to add on top of a fill so that, once it settles
naturally over time, the surface still ends up flush at the design level.
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3.2.2 CALCULATING VOLUMES · BACKFILL & COMPACTION
Matching compaction machine to layer thickness
Table 2. Typical compaction-machine parameters (illustrative reference values)
Compaction machine
Layer thickness, m
Passes per point
Best suited to
Hand tamper / plate
0.1 – 0.3 (up to 0.4)
3–4
Narrow gaps beside foundations and pipe trenches
Vibratory plate compactor
0.2 – 0.4
2–4
Granular backfill near structures
Towed / self-propelled roller
0.3 – 0.6
4–8
Large open backfill areas
Vibratory roller
0.5 – 0.8
3–6
Cohesionless (sandy/gravelly) soils
Heavy pneumatic-tyre roller
0.6 – 1.0
4–6
Large embankments and site pads
Figures illustrate the kind of reference table used to pick equipment and layer thickness together — actual values always come from the manufacturer's technical data sheet.
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3.2.2 CALCULATING VOLUMES · BACKFILL & COMPACTION
How a roller actually moves across a backfill
The roller runs back and forth in overlapping strips rather than
in one straight line, so no strip of soil is compacted only once.
At each end it must turn — the turning radius R sets how much
extra room the machine needs beyond the fill itself.
Neighbouring passes overlap by roughly 0.2–0.3 m so that no
strip is left under-compacted at its edge.
Figure 7. Roller movement pattern for compacting loose (cohesionless) soils: passes 1–8 in sequence, R =
the machine's turning radius
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3.2.2 CALCULATING VOLUMES · BACKFILL & COMPACTION
Where hands take over from machines
Right against a foundation, machines can't safely compact — that narrow zone (δ₁) is always tamped by hand
or with small powered tools.
Hand-tamped layers stay thin: 0.1–0.3 m by hand, up to 0.4 m with powered hand tools.
Once far enough from the foundation — about 0.8–1 m for unconnected elements, or 0.4–0.6 m for a
monolithic one that hasn't yet reached full strength — heavier machine compaction (δ₂) can take over.
Any buried element (pipe, duct) must keep at least 0.2–0.4 m of soil cover above it before machine
compaction passes over it.
Figure 8. Manual vs. machine-compacted zones around a
foundation: (a) neighbouring foundations, (b) a continuous strip
foundation
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SECTION THREE
Earthmoving Machine Parameters
The vocabulary behind every excavator spec sheet
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3.3 MACHINE PARAMETERS
Three families of numbers on every spec sheet
Main
Basic
Auxiliary
The parameters that define the machine's
core working capability
Parameters needed to check a machine will
actually fit and operate on site
Supporting figures used for logistics,
transport, and running costs
e.g. bucket capacity, digging depth/height, digging
radius
e.g. overall dimensions, ground clearance, turning
radius
e.g. transport weight, engine power, travel speed
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3.3 MACHINE PARAMETERS
Four letters worth memorising
m
N
q
Q
Number of working cycles the
machine completes per hour — how
quickly it can dig, swing, and dump,
over and over
Installed engine power — the energy
available to drive the digging,
swinging, and travel systems
Bucket capacity — the volume of soil
scooped up in a single working cycle
Machine output — the total volume
of soil the machine can move per
hour or per shift, combining q and m
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SECTION FOUR
Excavator Selection
Choosing a machine, an attachment, and a bucket to match the job
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3.4 EXCAVATOR SELECTION
One machine, three ways to dig
1
2
3
Backhoe
Dragline
Face (forward) shovel
Digs below its own standing level, pulling the
bucket toward itself
Casts its bucket out on a cable, then drags it back
Digs above its own standing level, pushing the
bucket forward and up
The default choice for most pits and trenches —
the site's excavator sits at ground level and digs
downward
TECHNOLOGY OF CONSTRUCTION PRODUCTION · LECTURE 6
Reaches wide or deep excavations, and can work
from a distance when the ground near the edge is
too weak to carry the machine
Best where the machine can stand at the bottom
and the soil face rises above it — e.g. cutting into a
slope or quarry face
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3.4 EXCAVATOR SELECTION
Sizing a trench to the bucket
A trench must always be at least as wide as the excavator's own bucket — otherwise the bucket simply cannot pass through.
Trench with vertical walls
Trench with sloped sides, no pipe
Trench for pipe-laying work
b = be + 0.2 m
b = be + 0.3 m
b = D + 2 × 0.3 m
Bucket width, plus a small working allowance
A little extra allowance where the sides batter back
Sized to the pipe diameter D, not to the bucket,
plus clearance on both sides
These three formulas are numbered 27, 27a, and 27b in the standard reference tables — three variations on the same idea: match the trench to what has to pass through it.
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3.4 EXCAVATOR SELECTION
Picking the bucket size itself
A bucket that is too narrow for the job needs far more passes; one that is too
wide won't fit the trench at all — so bucket width is chosen relative to the
excavation width first.
b = (0.6 – 0.7) × b
e
be — the excavator bucket's own width; b — the trench or pit width it has to work
within.
Keeping the bucket at roughly 60–70% of the trench width leaves the excavator room
to manoeuvre without over-sizing the dig.
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Table 3. Bucket volume vs. excavation volume (excerpt)
Excavation volume
Recommended bucket size
Up to 500 m³
0.15 – 0.4 m³
Reading this table
Reference tables like this pair a job's overall excavation volume with a
sensible bucket-size range — a small job doesn't need (or benefit from) an
oversized bucket, and vice-versa. Larger-volume rows continue in the full
reference table beyond this excerpt.
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LECTURE 6 — IN SUMMARY
From a site plan to a chosen excavator
1
Start from the site's grid, plan, and geological cross-section
2
Decide: separate pits, a trench, or one common excavation
3
Calculate topsoil, dig, backfill, and haulage volumes in sequence
4
Compact in layers, by hand near structures and by machine further out
5
Choose an excavator type and bucket size to match the job
Next time, in Lecture 7: earthmoving machinery in action — how excavators, bulldozers, and compaction equipment are organised into a working
crew on site.
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