Backfilling with pea gravel
Rich-in-WA
been in the planning stages for some time now. The house is to be
built slab-on-grade. The footprint is essentially level, except for
one corner that sticks out over a fairly steep bank. In this corner,
the footings and stem walls will step down from the nominal two-foot
height to four, six, and eventiallualy eight feet at the corner. I
designed the six and eight-foot sections as cantilevered retaining
walls. The idea is to backfill this corner and then pour a 4" slab
over the whole thing. One of the reasons for going with a slab (vs a
crawlspace) is we want the thermal mass for a hydronic radiant heating
system.
I have a concern with the backfill operation. Specifically I had
someone tell me that for this depth of backfill (~8ft in the corner)
the county might want to see soil compaction testing for each
successive lift. This sounds like a good idea in theory, but I'm a
little concerned with the impact to schedule if I have to have a
geotech out there for several tests. I'd planned to do the
backfilling over one or two days max. Of course the additional cost
of all this is a minor issue as well.
One thought that crossed my mind was using pea gravel (vs crushed
rock or pit run) as this is essentially self-compacting. The stem
walls and bank essentially form a bowl, so there's no worry about the
pea gravel running out. This might allow me to forgo the soil testing
and it would also be easier to place. Pea gravel would cost a bit
more than the other choices, but likely less than testing. I'd still
probably do the last couple feet in pit run to allow for easier
trenching of the under-slab utilities. Supposedly our county doesn't
require testing if the depth is less than two feet. Any experience
with this sort of thing?
Richard Johnson PE
Camano Island, WA
P.S. I did contact the involved building officials, but haven't heard
back yet. I just wanted to hear what others may have done in this
type of situation.
Bob Morrison
> I have a concern with the backfill operation. Specifically I had
> someone tell me that for this depth of backfill (~8ft in the corner)
> the county might want to see soil compaction testing for each
> successive lift. This sounds like a good idea in theory, but I'm a
> little concerned with the impact to schedule if I have to have a
> geotech out there for several tests. I'd planned to do the
> backfilling over one or two days max. Of course the additional cost
> of all this is a minor issue as well.
>
Rich:
Think about the idea of CDF (Controlled Density Fill) provided by your
concrete supplier. It is essentially a lean fly-ash concrete mix that
is flowable. City of Seattle commonly uses it for trench backfill in
street utility excavations.
Pour the stuff in the hole one day and pour your floor slab the next.
no compaction required and the stuff has about 1000 psi compressive
strength.
The material can be dug out with a backhoe if necessary.
--
Bob Morrison
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
Rich-in-WA
Bob,
I have a local redi-mix place working up a bid for CDF. They
thought it was a great idea and said this is becoming more popular as
a backfill material. I mentioned that I'd probably still use Pit Run
for the final lift to make it easier to trench for the under-slab
lines. The redi-mix guy suggested using CDF for the whole thing
because they can adjust the properties so it can be trenched easily
with a pick and shovel.
My main concern is the price may be prohibitive. I found one
reference that CDF in rural locations can run upwards of $40/yard
which is double Pea Gravel and triple Pit Run. I'll know more when
the bid comes back. Of course if I can eliminate the compacting and
testing it might just balance out. Placing it directly from the truck
chute sure would be nice as well.
Rich
Rich-in-WA
>
> I have a local redi-mix place working up a bid for CDF. They
> thought it was a great idea and said this is becoming more popular as
> a backfill material. I mentioned that I'd probably still use Pit Run
> for the final lift to make it easier to trench for the under-slab
> lines. The redi-mix guy suggested using CDF for the whole thing
> because they can adjust the properties so it can be trenched easily
> with a pick and shovel.
> My main concern is the price may be prohibitive. I found one
> reference that CDF in rural locations can run upwards of $40/yard
> which is double Pea Gravel and triple Pit Run. I'll know more when
> the bid comes back. Of course if I can eliminate the compacting and
> testing it might just balance out. Placing it directly from the truck
> chute sure would be nice as well.
>
> Rich
I got the bid back for Controlled Density Fill - $44/Yd delivered
- Ouch. We can get Pea Gravel for $21/Yd, 3/4 Minus Crushed Rock for
$19/Yd, and Pit Run for $13-15/Yd. Even though CDF certainly has the
gee-whiz factor going for it, one of the conventional choices might
just make more sense in this case.
I also talked with the county and the whole issue of what we
"must" do may be a non-issue. Basically, the plan reviewer/inspector
I talked with said they want a GeoTech involved if the foundation
footings are adjacent to a bank with a slope of 33% or greater (Per
UBC Chapter 1806.5). Since the slope in this case is ~25% or less
they won't require a GeoTech to assess the situation. Furthermore,
they consider the slab to be non-structural, so they won't ask for
compaction test results. Let's just hope the inspector who shows up
at our jobsite is on the same page.
This being said, we plan to be in this house for many years. The
last thing we want is any major cracking to occur in the hydronically
heated slab floor. Whatever we choose to backfill with is going be
properly placed and compacted.
Rich
Bob Morrison
> last thing we want is any major cracking to occur in the hydronically
> heated slab floor. Whatever we choose to backfill with is going be
> properly placed and compacted.
>
Rich:
The cost of placing, compacting, testing and possibly re-compacting
gravel fill is the main attraction of CDF. If you add in all those
costs plus give a bonus point or two for ease of placement for CDF you
get a competitive price.
I was working on a utility trench project on Queen Anne Hill in Seattle
where the stuff was a life saver. Turns out the geology is a little
strange. Abut halfway up the north side of Queen Anne the soil is like
beach sand. We started digging a hole for a new sewer and the trench
just growing until it was the width of the street and we still had not
gotten to the required depth. With a 4-inch gas line and 6-inch water
line hanging in mid-air we stopped digging. Filled the hole with CDF
(150 cu. yds), then came back the next day to complete the excavation.
Sides of the new trench stood vertically and we had no more sloughing
problems.
So, as you might imagine I'm a pretty big fan of the product.