United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S8-91/206 Dec 1991
Project Summary
Recommended Foundation Fill
Materials Construction Standard
of the Florida Radon Research
Program
Vern C. Rogers and Kirk K. Nielson
The Florida Radon Research Program
(FRRP), sponsored by the Environmen-
tal Protection Agency and the Florida
Department of Community Affairs, has
developed the technical basis for a ra-
don-control construction standard for
foundation fill materials. Results of
the research conducted under the FRRP
are presented in several technical re-
ports. This report summarizes the tech-
nical basis for the recommended foun-
dation fill materials standard for new
construction in Florida. The recom-
mended standard is first presented, fol-
lowed by a summary of the technical
basis for the standard.
This Project Summary was developed
by EPA'e Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that Is fully
documented In a separate report of the
same title (see Project Report ordering
Information at back).
Introduction
Elevated indoor radon gas concentra-
tions generally come from radon gas that
is formed from radium in the foundation
soils under the structure. One of the most
effective ways to limit indoor radon con-
centrations is to limit the rate of radon gas
generation or its resulting concentration in
the foundation soils. This portion of the
standard defines acceptable limits for the
concentrations of radon gas and its par-
ont radium in the earthen materials under
a structure.
The ease with which the radon gas can
move through soils toward a house founr
datton also affects the amount of radon
entering the house. In this standard, the
ease of radon movement through earthen
materials is characterized in terms of the
soil air permeability coefficient. Therefore,
the acceptable limits on sub-foundation
radium or radon gas also vary according
to the permeability coefficient of the mate-
rial.
The Recommended Foundation
Fill Materials Standard
The recommended foundation fill mate-
rials standard is:
302.1 Natural Foundation Soils
Natural earthen materials under build-
ings, that have relatively uniform radium
and emanation properties with depth, shall
have radium concentrations less than
those given in Figure 302.1. If soil classifi-
cation is used to estimate permeability,
the upper limit in the classification range
should be used to determine the radium
limit. For the purposes of this standard,
soils either shown or demonstrated to con-
tain less than 0.8 pCi/g of radium shall be
considered in compliance with this Sec-
tion. Tests shall be conducted according
to the procedures identified in Section 305.
The acceptable radium concentration in
foundation soils depends on their radon
transport characteristics, principally soil air
permeability. The permeability may be es-
timated from soil textural and moisture
properties using the relation
K-2x 10-5d^exp(-12m4)
where
K - soil air permeability (cm2)
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100
10
I
..
3
I
0.1
Clay,
Silt
Clay-Loam,
Silt-Loam
Loam
Sands
I""*
"Hi
i ml i i mill
""I -I
/a7
Soil Gas Permeability (cm*)
F/gure 302.1 Maximum soil radium concentrations for slab-on-grade buildings
constructed on soils with uniform radiological properties.
d = mass-weighted arithmetic average
diameter of all soil grains passing a
No. 4 sieve (cm)
m = fraction of soil moisture saturation.
For sandy soils it is sufficient to set m»0.
Site-specific measurements of perme-
ability also are acceptable if they are per-
formed with procedures identified in Sec-
tion 305.
302.2 Fill Materials or Layered
Natural Soils
Natural earthen materials under build-
ings whose radiological properties vary
significantly with depth, or fill materials
that are placed directly under the building
or within 10 feet of the building perimeter
shall result in radon concentrations in soil
air that are less than those given in Figure
302.2.
For the purposes of this standard, buikJ-
ing sites shown to have less than 600
pCi/l of radon in the soil gas shall be
considered to be in compliance with this
Section. For planned buildings the radon
measurement will be made at a depth of
at least four feet beneath the free surface.
The measurements shall be made in ac-
cordance with procedures identified in Sec-
tion 305.
303 Foundation Backfill
Materials
Foundation backfill materials shall have
radium concentrations less than 0.8 pCi/
g. Tests shall be conducted according to
the procedures identified in Section 305.
304 Building Materials
All materials used in concrete for the
construction of habitable structures shall
have a radium concentration of 5 pCi/g or
less, as measured in accordance with pro-
cedures identified in Section 305.
305 Testing Procedures
Tests for radium, soil gas radon and
insitu site and fill materials permeability
shall be conducted according to the pro-
cedures of the "Standard Measurement
Protocols, Florida Radon Research Pro-
gram," compiled and edited by Southern
Research Institute, January 15,1990. Test
protocols are as follows: Section 1.6 - Soil
Radium Content/Radon Emanation and
Section 1.1 - Permeability/Soil Radon/Soil/
Fill Sample Collection.
Why the Standard Limits Soil
Radium or Radon for Different
Soil Permeabilities
Radon gas is generated from the radio-
active decay of radium, an element that is
present in virtually all earthen materials.
Elevated soil radium concentrations cause
elevated rates of radon generation, which
in turn cause higher radon gas concentra-
tions in the air spaces in the soil. Thus,
the standard includes a soil radium con-
centration limit. Suction pressures in the
house, due to appliances, thermal gradi-
ents, heating and air conditioning systems
or winds, pull the soil air with its radon
gas into the house. The ease with which
soil air can move through the soil is char-
acterized by its air permeability coefficient
which also is included in the standard.
Even in the absence of soil air movement,
radon migrates from regions of high radon
concentration, such as soil, to regions of
lower concentration, such as the house.
This migration mechanism is called diffu-
sion, and the ease with which radon dif-
fuses through an earthen material is char-
acterized by its diffusion coefficient. The
permeability and diffusion coefficients are
closely related, and exhibit similar trends
with soil type, compaction and moisture.
Thus the permeability coefficient can be
used to specify soil conditions in a way
that also includes the effects of diffusion.
Both effects are included in the analysis
supporting the standard, even though the
standard only refers explicitly to the per-
meability coefficient.
Some regions or house locations may
have natural soils or fill materials with
sufficiently low radium concentrations to
satisfy the soil radium limit of the stan-
dards and yet have a soil layer several
feet deep that has unacceptably high ra-
dium concentration. The radon from the
deeper layer may still enter the house at
excessive rates. A reliable measure of
whether this condition occurs is the radon
concentration in the soil gas under the
slab, or at a depth of at least 1.2 m (4 ft)
from the free surface. The limiting radon
concentrations in the soil gas for layered
systems protects against excessive radon
entry from deeper soil layers with elevated
radium concentrations.
-------
3000
2333
2000
/SB
1000
500
Clay,
Silt
Clay-Loam,
Silt-Loam
Loam
Sands
i i i ii ill
i ml
mil
ii nl
i ii ill
I Mil
1O1*
1O10 1Q> 1
Soil Gas Permeability (cm2)
Figure 302.2 Limiting soil gas radon concentrations for slab-on-grade buildings
constructed on soils with uniform radiological properties.
Radon Entry Rates Into Houses
The rate at which radon enters houses
depends not only on the subslab soil con-
ditions, but also on the house structure
and conditions. This is one of the reasons
that adjacent houses built on the same
soil can have different average indoor ra-
don concentrations.
The standard is based on an allowable
radon entry rate into the house that is
consistent with an indoor radon concen-
tration of 2 pCi/l for many average house
conditions.
The calculations are performed with a
two-dimensional steady-state radon advec-
tion and diffusion code called RAETRAD.
In the calculations, negative house pres-
sure causes an inflow of outdoor air into
the soils near the house and a general
movement of soil air towards the house.
The soil air enters the house through con-
crete joints, cracks around concrete pen-
etrations and other cracks in the concrete.
The soil air contains radon from the soil,
and the radon moves into the house along
with the soil air. Radon also diffuses
through the cracks and through the con-
crete slab.
Indoor radon concentrations can vary
widely over short time periods mainly due
to variations in the house pressure rela-
tive to atmospheric pressure and varia-
tions in the house air changeover or venti-
lation rate. The calculations supporting the
foundation fill materials standard are based
on a reasonably conservative long-term
average negative pressure of 2.4 Pa (0.01
in. H2O) in the house, relative to the atmo-
spheric pressure. Slab-on-grade house
construction is assumed because that is
the dominant construction mode in Florida.
The main radon entry modes are assumed
to be through cracks at the slab-footing
joints, and by radon diffusion through the
entire slab.
Subslab soils range from coarse sand
to fine clay. The smaller particle silts and
clays have higher ambient moisture con-
tents and generally lower permeability and
diffusion coefficients, so that radon gas in
the soil air cannot move as easily to the
entry points into the house. This feature
allows for the higher radium concentra-
tions in the subslab soils shown in Figure
302.1.
Field measurements of soil permea-
bilities, soil air radon, and densities were
made in 16 general locations throughout
Florida. Soil samples were also obtained
to make laboratory measurements of ad-
ditional soil parameters such as: radon
diffusion coefficient (D), radium concen-
tration, radon emanation coefficient, am-
bient moisture, and soil grain size.
Existing, simple diffusion coefficient
models can generally predict radon gas
diffusion coefficients to within 50% for dry
soils (m < 0.4) and to within about a factor
of two for soils with moistures above 0.5
of saturation. The model used for Florida
soils is
D = 0.11 p exp(-6mp-6m14P)
where
D » radon diffusion coefficient (crrfsec1).
Both K and D decrease significantly with
moisture for m > 0.5. Finer grained soils,
such as silts and clays, have higher mois-
tures under normal environmental condi-
tions. Thus, they have lower K and D
values than the sands, so that radon gas
does not move as easily through them.
For a specified radon entry rate into a
house, the silts and clays can have higher
radium concentrations because more of
the radon gas is held in the soil.
Radium concentrations for over 700 un-
disturbed Florida soils averaged 0.6 pCi/g
and ranged from 0.1 to 2.9 pCi/g. Higher
values of 25 to 65 pCi/g have been ob-
served in certain profiles of the Hawthorn
formation or in certain soils disturbed by
phosphate mining. Radon emanation co-
efficients range from 0.1 to 0.45 for most
soils. Emanation coefficients measured for
48 Florida soils averaged 0.33 ±0.11.
&U.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40122
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V. Rogers and K. Me/son are with Rogers and Associates Engineering Corp.,
Salt Lake City, UT 84110-0330
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Recommended Foundation Fill Materials
Construction Standard of the Florida Radon Research Program," (Order
No. PB92-105865/AS; Cost: $17.00, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S8-91/206
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