Estimating Excavation
Introduction
Estimating Excavation
by Deryl Burch
Chapter One
Get Started Right
Construction
cost estimating is demanding work, no matter what type of construction is
involved. But I think estimating earthwork is the hardest of all. Why? Two
reasons. First, excavation has more variables and unknowns. You don't know
what's down there until you start digging. Second, you have to rely on
information from many sources - some of which may not be accurate.
That's
why every earthwork estimator needs special skills:
- The
ability to read plans and specifications
- An
understanding of surveying and engineering practice
- A
facility with mathematical calculations
- The
ability to anticipate environmental and legal issues
- An
abundance of good common sense.
If you
can bring common sense to the task, this manual will show you how to do the
rest.
I'll
help you develop all the skills every good earthwork estimator needs.
Of course,
I can't cover everything on every type of job. But I'll include the information
most earthwork estimators need on most jobs. Occasionally, you'll have a job
that requires special consideration. But if you understand the principles I'll
explain here, you should be able to handle anything but the most bizarre
situations.
In this,
the first chapter, I won't do much more than touch on a few important points you
should understand:
- Why
you have to estimate quantities
- The
importance of plans and specs
- Working
accurately
- Keeping
good records
After
making these points in this chapter, I'll describe a step-by-step estimating
system, from making the site survey to writing up the final cost summary. I'll
teach you a process for making consistently-accurate earthwork estimates.
Part of this process is calculating the cubic yards to be moved.
That's
the heart of every earthwork estimate. I'll cover quantity estimating in detail.
Then I'll explain how to find labor and equipment costs per unit. We'll also
consider soil and rock properties-and how the equipment you use affects bid
prices.
Why
Calculate Quantities?
In the
past, many smaller dirt jobs were bid as a lump sum rather than by the
cubic yard. Dirt contractors based their bids on guesses - what equipment was
needed and how long should it take? They didn't bother estimating soil
quantities. Making estimates this way overcame one problem: most excavation
contractors didn't know how to estimate soil and rock quantities.
I think
those days are over. Fuel and labor costs are too high now. And the competition
is too intense. There's too much risk in "seat-of-the-pants" guesses.
A few mistakes, a couple of surprises and you're going to be looking for some
other type of work. Only the best survive for long in this business. Most of the
survivors know how to make accurate bids by the cubic yard. Fortunately, making
good quantity estimates isn't too hard if you've mastered a few simple skills. I
hope that's why you're reading this page.
I've
found that good earthwork estimators are good at calculating earthwork
quantities. Here's why:
First,
no one's going to do it for you. You have to do it yourself or it's not going to
get done. Many engineers, architects, and even some builders know how to figure
soil and rock quantities. But few take the trouble to do it.
Instead, they depend on you, the earthwork estimator, to do it.
Second,
earthwork contractors who don't bid by the cubic yard usually end up in court.
That can cripple any company. It's common for the actual amount of dirt moved to
be more or less than expected. The best way to protect yourself is to bid by the
cubic yard. If you have to move more dirt than the plans show, you'll get paid
more. It's as simple as that.
Third,
most owners, engineers and architects request excavation bids based on the cubic
yards moved. That's now the accepted procedure for most projects, from
single-family homes to roads and commercial jobs.
General
and Special Quantities
If you
agree that excavation bids should be based on quantity estimates, the next step
should be obvious. We have to start every estimate by figuring the quantity of
soil to be moved.
I
recommend you start the estimate for any project, no matter how large or small,
by dividing excavation quantities into two categories:
General quantities include any work where
you can use motorized equipment such as scrapers, hoes and loaders at their
designed production rate.
Special quantities include anything that
requires special care or lower production rates. Examples are most rock
excavation, nearly all hand excavation, backhoe work around sewer lines,
underground utilities, or existing structures. Naturally, prices for special
quantities are higher than prices for general
quantities.
Keeping these two quantities separate protects
you. Most excavation contracts have a clause that covers extra work.
Unanticipated rock deposits, special soil problems and unusual trenching
problems are extra work that you should be paid extra for. If you've bid a
higher price for special quantities, you'll get paid at that price per cubic
yard for the additional work. Otherwise you could end up chipping out rock at
the price of moving sand.
Calculating
Cubic Yard Cost
Here's the basic formula for costs per cubic
yard:
Labor and equipment cost per hour multiplied
by the hours needed to complete the work, divided by the cubic yards of material
to be moved.
Does that seem simple? It's not. You may know
your hourly labor and equipment costs right down to the last penny. But
estimating the time needed is never easy. And calculating volumes for sloping
and irregular surfaces is demanding work.
Notice several things about the formula for
computing costs per cubic yard. First, it's based on labor and equipment costs
for your business. That's important and I'll have more to say about it
later.
Second, it assumes you know the quantity of soil
or rock to be moved. That's going to take some figuring.
Third, even after you've calculated the cost per
hour and quantity of soil, you're not finished. You need to estimate the time
needed. Usually that's the hardest part. To do it, you have to decide on the
equipment (method) to use.
Of course, the quantity of material (yardage) is
a very important part of our cost formula. But the excavation method (type of
equipment) also has a major influence on cost. The most expensive equipment
(cost per hour) will usually be the most productive (move soil at the lowest
cost). But the machine with the largest capacity isn't always the best choice
for every outhaul. I'll explain why later. For now, just understand that making
good equipment selections will help reduce costs.
Reading Plans and Specifications
Nearly every significant excavation project
that's let out for bid will be based on a set of plans. Plans are scale drawings
that show the finished project. Plans are supplemented with written descriptions
called specifications (specs for short). Specs explain in words what the
plans can't or don't show. Ideally, the plans and specs, read together, should
answer every question about the job. They shouldn't leave anything unclear or
subject to interpretation. The better the job done by the engineer or designer,
the more likely the plans will be clear and complete.
Plan reading is an important skill for every
earthwork estimator. But this isn't a book on plan reading. If you need help
with reading plans, if you don't understand the plans and drawings in this
manual, pay a visit to your local library. They'll probably have several basic
plan-reading texts to choose from.
As an excavation estimator, you're expected to
understand every detail in the plans and specs for the jobs you bid. That's why
they're worth careful study. Read these documents completely. Note everything
that affects your excavation work. Some engineers and architects aren't very
well organized. They may put instructions and notes almost anywhere on the
plans. Read every page carefully, regardless of what you think it's about.
Pay particular attention to notes that spell out
the contractor's responsibility. For example, you may find a note somewhere on
plans that relieves the engineer or architect of responsibility for damage to
utility lines. The note probably says:
NOTE: While every precaution has been taken to
show existing utilities in their proper location, it is the contractor's
responsibility to determine their actual location. No assumption should be made
that no other utility lines fall within the limits of construction.
If you suspect utility lines may be a problem,
ask the utility companies to locate their lines for you. Most will be happy to
do that at no cost. But they may want ample advance notice.
Also pay attention to notes on natural obstacles
(such as rock) or anything that's buried on the site. Is there an abandoned
underground storage tank or old basement in the area to be excavated? The plans
may also mention drainage problems and unsuitable soil deposits, probably in the
cross-section drawings or special provisions of the specs.
Search the plans and specs for everything that
may affect cost. That's always your starting place. But it's not the end of your
search. Many cost items won't show up in either the plans or specs. For example,
you'll have to find out from the city or county building department what permits
will he required. Also, city, county or federal law may set minimums for wages,
employee benefits and insurance coverage.
Here's another pitfall to watch for: Who pays to
have the project staked out by a surveyor or engineer? In most cases, the
designer will pay for surveying - the first time. If you knock over any survey
stakes during actual work, you'll
probably have to replace them at your own expense. Work as carefully around the
stakes as possible. But if job layout makes it impossible to avoid moving
stakes, allow enough in your bid to pay for another survey.
Make
sure you understand how you'll be paid. On larger projects, you're usually paid
per cubic yard, based on the difference between the original soil cross section
and the cross section when work is finished. We'll talk more about cross
sections later in this book.
On many
smaller projects, your payment may be based on the engineer's estimate of
yardage. If that's the case, look for a provision in the specs that gives you an
option to have final cross sections made at your own expense. Experience will
help you decide if a final set of cross sections is to your advantage. But I
recommend that you always take off quantities yourself. Don't assume the plans
are right. Anyone can make a mistake, but you could end up paying the price.
Undercutting
Undercutting
is removing additional dirt
from an area below the finished grade line. There are several situations where
this is necessary. The most common is where a rock ledge is close to, but not
above, the finished grade line. Figure 1-1 shows a typical situation. Most
structures can't be built directly on rock. If the rock weren't there, you would
excavate just to the finished grade line and be done. Because the rock is just
below finished grade, you have to cut deeper. That's the undercut. Then you have
to backfill with suitable material such as compacted dirt. The dirt provides a
buffer between the rock and the foundation.
Figure 1-1
Undercutting for Rock
There's
probably nothing in the specifications that gives you the right to collect for
undercutting and backfill. But it's expensive work and the cost shouldn't come
out of your pocket. Where undercutting may be necessary, include it in your bid
item per cubic yard cut.
Undercutting is also needed for underground
utilities such as storm drains and sanitary sewer lines. Most plans will show
only a designated flow line elevation. Based on the plans and judgment, you'll
have to decide how much and what type of bedding to install below the pipe. Each
cubic yard of bedding requires a cubic yard of undercutting. Figure 1-2 shows an
example. Undercutting may also be required on roads, parking lots and sidewalks
anywhere there's a load on the soil.
Overfilling is the opposite of
undercutting. When backfilling a large area, you can usually bring the backfill
right to grade without doing any cutting away of excess backfill. But in a small
area, it's usually easier to bring the area above the final grade line by 2 to 4
inches, then cut off the excess. This is still called undercutting. Of course,
you can't expect to get paid for removing the 2- to 4-inch excess. But it's
still a cost of the job.
Figure 1-2
Undercutting for Pipe Bedding
Accuracy
Is Essential
Accuracy is the essence of estimating. If you
can't work accurately, you're in the wrong business. But don't get me wrong. I
don't mean that we're going to account for every spadeful of soil on every
estimate. There are times when you can ignore small differences in elevation. On
most jobs these small plus and minus areas will average out to almost nothing.
But a 1-inch mistake in elevation over the whole job can cost you thousands of
dollars. Even 1/16-inch error over a few acres can hurt you.
Here's an example. Assume you're bringing in fill
on a city lot that measures 125 feet by 150 feet. Because of a mistake in grade,
your estimate of imported soil is wrong. It leaves the entire site 1 inch below
the specified finished grade. How much more soil is needed to correct the 1-inch
mistake?
Here's the formula for volume:
Volume (in cubic feet) = length
(in feet) x width (in feet) x depth (in feet)
In this example, you know the length and width in
feet but the depth is 1 inch. To use the formula, convert 1 inch to a decimal
part of a foot. You can either refer to the conversion chart (see Figure 1-3) or
divide 1 by 12, since 1" = 1/12'. Either way, 1 inch equals 0.0833 feet.
Now you're ready to use the formula for volume:
Volume (CF) = 125
x 150 x 0.083
= 1,556.25
How many cubic yards is that? Since there are 27
cubic feet in a cubic yard, divide the cubic feet by 27:
Volume (CY) = 1,556.25/27
= 57.6 CY
Trucking in almost 58 cubic yards of soil won't
be cheap. If imported soil costs you $25 a cubic yard, your 1-inch mistake is a
$1,450 error. That could make the difference between profit and loss on this
job.
Figure 1-3
Inches to Decimal Feet Conversion Chart
| Inches |
Decimal
Feet |
Inches |
Decimal
Feet |
| 1/16 |
0.0052 |
7/8 |
0.0729 |
| 1/8 |
0.0104 |
15/16 |
0.0781 |
| 3/16 |
0.0156 |
1 |
0.0833 |
| 1/4 |
0.0208 |
2 |
0.1667 |
| 5/16 |
0.0260 |
3 |
0.2500 |
| 3/8 |
0.0313 |
4 |
0.3333 |
| 7/16 |
0.0365 |
5 |
0.4167 |
| 1/2 |
0.0417 |
6 |
0.5000 |
| 9/16 |
0.0469 |
7 |
0.5833 |
| 5/8 |
0.0521 |
8 |
0.6667 |
| 11/16 |
0.0573 |
9 |
0.7500 |
| 3/4 |
0.0625 |
10 |
0.8333 |
| 13/16 |
0.0677 |
11 |
0.9167 |
Your
Estimating Procedure
The more
organized and logical your estimating procedure, the more accurate your
estimates will be. If you have the tools, papers and information you need close
at hand, you're off to a good start. Then you can focus your attention and
concentration on producing an accurate estimate. If you're cramped for space,
uncomfortable, and trying to work without all the equipment and information you
need, errors are almost inevitable.
Start by
organizing an efficient work area. It should be large enough so you can lay out
all the plans on a table and still have room to write and calculate. Provide
enough light to make reading comfortable, and keep the work area free of
shadows. This is especially important when working with transparent overlays or
other light-duty paper where you might mistake shadows for lines.
A good
calculator is a must. I recommend buying a calculator with both a digital and
a paper printout. You need the printout to check your figures. Make sure you
have an engineer's scale and drafting triangles for checking and drawing lines,
a small magnifying glass, tape for holding overlays, and the normal collection
of pencils, erasers, and paper.
Although
it's not essential, I like using a light table. You can place a drawing on it,
overlay it with another paper, and see through both of them. It's great for
working with plan and profile sheets, or overlays on grid or take-off sheets.
Later in
the book we'll talk about using a planimeter to take off quantities. Although
it's relatively expensive, a good planimeter will soon pay for itself. Take care
to select one that's sturdy and has all the needed instructions and attachments.
A
computer is even more expensive, but more and more estimators are using one.
There are programs on the market today that can handle anything from simple
calculations to a complete estimating program, with cross sections, quantities
and printouts. But no program can take the place of an estimator who understands
estimating procedures and practices. That's the purpose of this book.
There
are two advantages to using a computer. The first is time. That's
an estimator's most valuable asset, and a computer can help make your time more
productive. Second, a computer makes it easier to keep cost figures for
equipment and labor. Records from past projects and estimates can make current
estimates more accurate.
If you
don't currently have a computer, don't jump in without doing some research
first. There are many computers on the market, tons of software, and hundreds of
dealers. Take the time to make yourself familiar with the options. Talk to
dealers. More important, talk to other estimators who use computers to do their
estimating. Read trade magazines, especially the ads for estimating software. And
don't go out and buy a computer and then look for estimating programs to run on
it. First, choose the estimating program you like, and then buy the
computer that will run that program. Otherwise, you may find the computer you
bought won't run the program you like.
When
you've got your work area and equipment set up to work efficiently, you're on
the path to accurate estimates. To stay on the path, it's important to approach
the work with a logical and organized procedure. That speeds up the work and
reduces mistakes. Let me describe the method that works for me. I think it'll
work for you, too.
When
starting a project, first read all documents describing the job. Take notes on
any situation that's not a normal work requirement. Are there utilities that
must not be disturbed? Do the documents indicate specialized material types from
log borings? Do they stipulate any arrangement for rock on the site? Look for
any special provisions set out by the designer. Then head out for a field visit.
That's the subject of the next chapter.
After
returning from the field, review the documents again, looking for unusual
situations that the site visit brought to your attention. Then make a complete
written outline of all work that needs to be done in the order in which it will
be performed. Set up files for each separate section. Make a list of additional
data such as quad sheets, local conditions, and any other information you need
to gather.
Here's
the order I usually use.
- Consider
any drainage, traffic or work zone protection work that needs to be done.
Are there any on-site streams that must remain open, or roadways to
maintain? These would probably be lump sum items, not items you'd take off
quantities for. Just make sure you don't miss any of these special items.
- After
studying the plans and the site, you should have a good idea if there's
enough fill on the site, or if you'll need a borrow pit. Or will you need a
place to put excess material off-site? Begin now to make arrangements for
needed sites, sampling of material for approval by the engineer, and
purchasing any material that's needed.
- Now
consider the topsoil requirements. Review the material sample, the
requirements for replacement, and availability of storage area on site.
Calculate the amount of usable material and the amount of waste that must be
disposed of.
- Will
there be any special excavation, like rock work or the removal of existing
structures or facilities? Make sure you include all work and any special
equipment you'll need. Will you need to rent equipment? What about rock
drills, blasting material, or cranes?
- Begin
calculating the general quantities with the cut or fill work over the entire
project. Start in the same place and proceed throughout the project the same
way for every estimate. One way to make sure you cover all of the project is
to set up a grid system with a corresponding file system. As you finish work
in each grid, mark it off, file it, and move on to the next grid.
- Next,
calculate all the utility lines, keeping the figures for each area separate.
Be especially careful in estimating the tie-in between new and existing
lines. Allow a little extra time for lines that aren't exactly where the
plans show them to be.
- Then
consider the roads, parking lots, and paved or special drainage ditches.
Again, keep the quantities for each separate. One note of caution: Remember
to consider the base and sub-base when figuring final elevations.
- Buildings,
basements, sidewalks and other similar structures are next. After you've
calculated each structure separately, add them all together to get a
structure total.
- Finally,
calculate the topsoil. And don't forget that if you've used a borrow pit,
you may have to place topsoil there also.
- Now
you're ready to start putting together all that information to come up with
a realistic quantity total for the complete project. Fill out the final
quantities sheet. Remember to attach all worksheets, scratch paper and
calculator printouts so you can recheck your totals.
Now
review your final sheet, looking for potential problem areas. If possible, have
someone else check all your calculations and extensions. If that's not possible,
set the estimate aside and go through it again a few days later. You'll have a
fresh approach that may spot errors or omissions.
The last
step is to go through all the documents and make sure they're in order. Then
file them. Don't throw anything away - not even the scrap paper. Why are those
records valuable? Keep reading.
Record
Keeping
Once
you've learned to read plans carefully and work accurately, there's still one
more important step: record keeping.
Think of
your estimates as accumulated wisdom. Treasure them. Keep them handy. Make sure
they're easy to understand. They should show how each figure was developed. Why?
There are at least four reasons:
First,
planning the work is a big part of every estimator's job. You can't estimate any
type of earthwork without making decisions about equipment. Once you've selected
equipment for estimating purposes, document your choice on the estimate
worksheets.
If your
bid is accepted, you'll probably want to do the work with the same equipment
assumed in the estimate. What if months have gone by and you can't remember how
the figures were developed? You have to start selecting equipment and estimating
costs all over again. If the equipment assumed in your estimate isn't the same
as the equipment actually used, comparison of estimated and actual costs may be
meaningless.
Second,
you're going refer to most estimates many times over months or even years. You
shouldn't have to guess about how each figure was developed. That wastes time
and can exhaust your patience. I've seen estimators who should know better use
the back of an envelope to figure special quantities. After entering the final
cost, they usually discard the envelope. Later, if there's a question about the
estimate, how can you verify the figures? They're gone!
Third,
old estimates are invaluable when compiling new estimates. Every estimate
(especially if you actually did the work) provides a frame of reference for
future jobs - even if labor and equipment costs have changed.
Fourth,
every estimator makes mistakes. That's no embarrassment. But repeating mistakes
is both foolish and expensive. The best way to avoid repeating mistakes is to
preserve every scrap of estimating evidence - in a neat, tidy, well-organized
file. Make notes on what worked and what didn't. Review those estimates and
notes when estimating similar jobs. Save everything. Someday you may want to
write a book. I saved my notes and estimates and wrote a book. You're reading
it.
Using
Public Records
To the
professional estimator, there's no such thing as too much cost information.
Collect all the estimating data you can. It helps if you know where to look for
it. I canvass city and county engineering departments, public works departments
and maintenance departments for whatever information they can provide. They know
about bid prices, soil conditions, abandoned streets, utility lines, sewer and
water problems. Use the resources available from your city and county
government.
Aerial
maps at the county tax office and contour maps from the United States Geological
Survey offer clues to possible water and soil problems.
There
are USGS offices in most states. They're often located in the capitol, or in
cities with universities. Check your local phone book or local engineering
groups for the address of the nearest office. City, state and county highway
departments will have information on soil problems they've found under highways
in the area.
What
If You Don't Have Plans?
Up to
this point, we've assumed that you're bidding the job from plans and specs
provided by an architect or engineer. But you may be asked to bid on a small job
that wasn't designed by an engineer or architect. Then you'll have to create
your own plan. It may also be up to you to determine quantities and prepare a
contract.
In any
case, always figure soil quantities and get a written contract on every job,
large or small. The responsibilities and liabilities are all yours, so plan and
execut
Your Shopping Cart:
