204 Subgrade Compaction and Proof Rolling
Importance
Over 25 million dollars of
extra work was used to stabilize unstable subgrades during the construction
seasons of 2000 and 2001. This extra work
has been minimized in recent years because of the construction and design
criteria created since that time.
This section will help the
project construct stable subgrades for pavement construction. Proper subgrade treatment ensures a
constructible pavement, enhances pavement performance over its life, and
ensures that the pavement design intent is carried through into the
construction phase. This section is
based on research performed by the Department from the 1960’s through
today. This section should not be used
as the ultimate answer to solve all subgrade problems.
This section is detailed in
such a manner so that construction personnel can easily apply information from
the field and subsurface investigation to provide reasonable adjustments to the
plan subgrade treatment.
Specification
and Plan Requirements
Item 204
requires the top 12 inches of the subgrade to be compacted. Item 204
requires the subgrade to be proof rolled.
If subgrade stabilization or undercutting is designed for the entire
project, then proof rolling is only used to verify the stability of the
stabilized subgrade. If special subgrade
treatment is provided in the plans at spot locations, proof rolling is
specified to identify these areas and then performed afterwards to verify the
undercut stability.
Proof rolling deflections and
soil conditions that are observed during construction determine if the plan
subgrade treatment must be adjusted.
Adjustment of subgrade treatment to fit field conditions is essential and
is the responsibility of the Project Engineer.
Subgrade Correction Prior to Proof Rolling
The Engineer must observe the
effect of heavy equipment, which operates on the subgrade during rough
grading. When rutting and deflection
under heavy equipment indicates unstable subgrade, the Engineer should
authorize the correction. See Elasticity
and Deformation of Soils in Section 203.02 of this manual.
Do not delay the correction
until it can be checked by proof rolling.
Investigate the extent of the problem by using the Investigation section
of this Item. Be aware that the
condition can be improved by time, drainage, and hauling as detailed in the
section, Draining and Hauling, of this item.
If needed, make the
correction by excavating and disposing of unstable soil and replacing it with
suitable material as detailed in the section, Undercut Depth and Stabilization
Determination, of this item.
Drainage and
Hauling
Excess water in fine-grained
soil is the principal cause of unstable soil conditions. The Engineer has a responsibility to ensure adequate
drainage during construction. If the
investigation indicates the need for underdrains or the cleaning of the
existing underdrain outlets, the Engineer must order the work as soon as
possible.
Some examples of these
conditions are as follows:
1. Existing underdrains with clogged outlets on
rehabilitation projects.
2. Free water in the subgrade.
3. Saturated soils of moderately high permeability, such
as sandy silt and silty clay of low plasticity.
4. Ground water seepage through layers of permeable soil.
5. Water seeping into test pits.
6. Water seeping from higher elevations into cut
locations.
7. Water flowing on the top of the rock or shale in
subgrade undercuts.
Note: It is difficult to
remove water from hard clay soils with PI’s greater than 20 with construction
underdrains.
Subgrade stability can be
significantly improved by cleaning out the existing underdrain outlets on
rehabilitation projects and by adding construction underdrains on new or
rehabilitation projects. Once the
underdrain systems are in place and functioning, the drainage system can reduce
the subgrade soil moisture content from 3 percent over optimum moisture to the
optimum moisture content in 6 to 8 weeks.
Moisture contents that exceed 3 percent over optimum must be dealt with
by other means.
For rehabilitation projects,
the Contractor should be instructed to unclog the underdrain outlets
immediately. Try to perform this work in
the time frame listed above. If the project
consists of several phases, instruct the Contractor to perform the outlet
cleaning for the entire project at the same time.
For new or rehabilitation
projects, subgrade stability can be achieved by constructing the plan or
construction underdrains as soon as the water problem is found. On new construction projects, a longer period
of time can be allowed for the underdrain system to work. Opportune times for this work are at the
beginning of construction and before winter shut down.
The plan underdrains should
be placed only when they will not be contaminated by further construction. If contamination is a concern, then
sacrificial or construction underdrains should be used on the project.
Item 605
in the C&MS details the construction of construction underdrains.
Construction underdrains are usually placed in the centerline of the roadway,
but can be placed in other locations too.
They may be placed in the ditch line if water is coming in from a cut
section at a higher elevation. The
porous backfill is extended to the subgrade elevation. The outlets for the construction underdrain
are the same pipe material and backfill as regular underdrains. The underdrains can be outlet to any
convenient location, such as catch basins, manholes, pipe, or ditches. The project should not be concerned with
contamination in the upper portion of the underdrain backfill. Construction underdrains are sacrificial
underdrains that will continue to work throughout the life of the contract and
afterwards, even though the upper portion is contaminated.
In Figure 204.A, the subgrade
is saturated and the soil acts like a waterbed when the subgrade is Proof
Rolled or hauled on. However, once the
underdrains are in place and the soil is loaded, as shown in Figure 204.B, then
the water has a place to go. As the soil
is loaded or hauled on, the water is squeezed out and the subgrade conditions
will improve.
Figure
204.A – Water in the Subgrade without Drainage
Figure
204.B – Water in the Subgrade with Drainage
By placing the drainage
system prior to loading or hauling on the subgrade, the water is given a
location to escape the subgrade system.
If the drainage system is not in place before hauling or loading, the
subgrade will rut or crack and have a detrimental effect on the subgrade and
not improve with loading.
Drainage and hauling can work
together to correct unstable subgrades under the above given guidelines.
Figure 204.C, Shale and Rock
Undercuts, came from Figure 1009-10 in Location
& Design Manual – Volume 2, Drainage
Design. The specification
requirements are detailed in 204.05. Shale and rock are cut 24 inches (610 mm)
below the bottom of the pavement. This
ensures that the pavement gets uniform support and good drainage. When rock is blasted and excavated, the
resulting surface is very rough and tends to collect water. Accumulated water will cause some rock and
shale to deteriorate. By undercutting the rock, we ensure that any water that
collects in the irregular surface does not affect the pavement. In the past, pavement placed on rock and
shale started to develop problems immediately after opening to traffic.
Figure
204.C – Shale and Rock Undercuts
The underdrains in these rock
and shale cuts should extend at least 6 inches (150 mm) into the existing rock
or shale formation. If the underdrains
are too high, the water will accumulate at the rock and soil interface and
cause subgrade instability.
Construction or rock
underdrains can be placed in the ditches and other strategic locations in cut sections
to minimize water coming under the pavement.
Water under the pavement without drainage causes the subgrade to act
like a waterbed. With drainage, the
conditions improve and become more stable.
When to Proof Roll
For areas where subgrade
appears to be stable without undercutting, proof roll after the top 12 inches
(305 mm) of the subgrade meets the compaction requirements and after the
subgrade has been brought to approximate shape within 0.1 to 0.2 feet (30 to 60
mm) required by plan lines.
For areas that are obviously
unstable and require undercutting, do not proof roll unnecessarily to
demonstrate that subgrade correction is required.
The proof rolling should be
done immediately after the subgrade compaction operation, when the moisture
content of the subgrade soil is near the moisture content that was used to
achieve compaction. This minimizes the
subgrade becoming too wet or too dry for an effective proof rolling evaluation. If the subgrade is too wet, the material will
displace and rut. If the subgrade is too
dry, a hard surface crust may carry the proof roller over an undesirable, soft,
wet, underlying material without rutting or deflection, and the unstable
subgrade may not be detected.
Proof rolling may be done
either before or after pipe underdrains are installed. If done after underdrains are installed,
rolling should not be done directly over the underdrains. In C&MS 204.06, proof rolling must be
performed at least 1.5 feet (0.5 m) away from the underdrains because of the
potential damage to the underdrains.
Proof
Rolling
(204.06)
CA-EW-2
Proof Rolling Documentation Form is used to document the proof rolling
operation. It is imperative that the
stations, deflections, weight of the proof roller, and comments are well
documented. Digital photographs of
subgrade distress are highly recommended.
The primary purposes of proof
rolling are to locate unstable areas, check the subgrade compaction, to carry
out the intent of the design, and to provide uniform support for the pavement
structure. Unstable subgrade areas that
are located will be corrected so that the subgrade density can be maintained throughout
the construction. If done correctly, the
pavement design intent will be carried through the construction process.
One trip with a proof roller
is adequate to achieve satisfactory proof rolling results.
An over loaded proof roller
for a soil type may cause satisfactory subgrade to become unstable during proof
rolling. Conversely, unstable areas will
not be found if the proof roller is too light for the soil type.
Selection of Proof Roller Weights
and Tire Pressure
In view of the many variations
which must be expected in Ohio soil and moisture conditions, the Engineer is
given authority to vary the weight and tire pressure of the proof roller to fit
the conditions. The weights and tire
pressures for the different soils are detailed in C&MS 204.06.
It is imperative that the
project chooses the correct load for the type of soil on the project. These loads and tire pressures are soil type
sensitive when evaluating the subgrade.
For A-3, A-4, A-6, and A-7 soils use a 35 ton (32 metric ton) roller
with a tire pressure of 120 psi (820 kPa).
This load and tire pressure is used on most projects because these are
the most common soils found in the State of Ohio.
For granular soils and soil,
rock, and granular mixtures, use a 50 ton (46 metric ton) roller with 120 psi
(820 kPa) tire pressure. However, if the
granular material was placed as part of an undercut to stabilize an unstable
subgrade, then use the weight appropriate for the original subgrade materials
(35 tons).
The goal of proof rolling is
to maximize the load to locate unstable subgrade. These unstable soils could be 3 to 5 feet (1
to 2 m) deep. In rare cases, the
unstable soil may be deeper than 5 feet (2 m).
Close inspection throughout
proof rolling is necessary to observe the rolling effects and to mark unstable
subgrade locations for correction or investigation. Inadequate stability is indicated by deflection,
cracking, or rutting of the surface of the subgrade.
Failure Criteria
When is rutting from the
proof roller an indication of unstable subgrade? Technically, the maximum
allowable rutting or elastic movement of the subgrade is the amount that allows
the subgrade soil to maintain the specified density throughout the construction
process. For example, if subgrade
density can be maintained with 6-inch ruts, then this would be the allowable
maximum. In practice, when the ruts from the proof roller are deeper than 1
inch, then there is usually cause for concern.
Additionally, if the subgrade deflects more than 1 inch with substantial
cracking or lateral movement of the soil, then this is also cause for concern. Elastic deflection is when the subgrade moves
down under the weight of the proof roller and then comes backup (rebounds)
after the proof roller passes.
When rutting and deflections
are less than 1 inch, there is no assurance that overlying pavement
construction will not damage the subgrade compaction. Although subgrade density and stability can
be maintained during proof rolling, the repetitive loading, the hauling of
materials, and the base and pavement construction can still destroy the
subgrade compaction. This may be an
issue on some reconstruction projects.
On reconstruction projects, the following complications can create or
worsen subgrade problems.
1. Water accumulates under the pavement because of poor
drainage or clogged underdrain outlets.
2. Construction time frames are limited.
3. Space limits the ability to dry the material in
place.
4. Once the pavement is removed, all the drainage is
toward the subgrade. This compounds an
already poor drainage situation.
5. Alternate haul routes are limited or not available on
rehabilitation projects.
The failure criteria are used
in this section to determine the locations from which to perform a detailed
analysis. This detailed analysis
consists of methods discussed later in this section, such as rut depth, soil borings,
and test pits. If the subgrade deflects
beyond the failure limits given in this section, and the soil borings and test
pits determine that the subgrade does not need to be undercut, then the
subgrade should be considered satisfactory.
One additional area to evaluate is the moisture content of the
soil. Some soils are more prone to rut
at moisture contents greater than 3 percent below the optimum moisture
content. In fill locations, the moisture
content can be reduced to minimize this problem. If all of the above criteria are met, there
is no reason the subgrade should not perform as anticipated. If there is any debate between the Department
and the Contractor, especially if a warranty is involved, then further
nondestructive or destructive testing can be used to resolve the issue.
In Figure 204.D, the soil has
been compacted in the top foot of the subgrade and the conditions are good for
the top 3 feet (1.0 m). However, there
is a soft layer at a lower elevation.
The soft layer has no detrimental effect on the subgrade density during
the subgrade compaction.
Figure
204.D – Stage 1 Compaction of Subgrade
In Figure 204.E the proof
roller deflects because of the soft soils. The subgrade density may or may not
be affected by the proof rolling. The
loss of subgrade density is proportional to the amount of rutting or elasticity
during proof rolling and subsequent construction operations. The severity of the overall subgrade
condition can be measured by the amount of the deflection and elasticity on the
surface.
Figure
204.E – Stage 2 Proof Rolling
When the proof rolling
deflections exceed the failure criteria; the proof rolling, repetitive loading,
and pavement construction can destroy the top layers of the aggregate base and
subgrade.
In actual field conditions,
this soft layer can be just a few inches thick and at any elevation from the
top 1 foot (0.3 m) to as deep as 5 feet (2 m).
In addition, it may be an indication of an overall soil condition that
is just over optimum for the entire 5 foot (2 m) depth of the subgrade. The test pit excavation is used to identify
the layer, or layers, causing the surface distress. This is further detailed in
the section, Investigation, of this Item.
It is imperative that these conditions are correctly identified.
Crusting is a condition when
the subgrade surface appears to be dry and there is substantial cracking on the
surface with or without rutting. This
indicates a need for further investigation and usually indicates soft or wet
underlying soil at depth with the top foot or so of the subgrade being very
dry.
Variations in the Proof Rolling
Results
You should not be too
concerned with occasional or nominal deflections in excess of the above failure
criteria. If the density is checked, and
the investigation shows that good soil extends throughout the top 5 feet of the
subgrade, then the design intent will be fulfilled and the project can be
constructed. All soils will occasionally
deflect under these loads.
The pavement design is based
on an average CBR. The CBR value was
directly correlated to soil density many years ago. By using the average CBR (Density) value, the
pavement design accounts for a 30 percent, or one standard deviation variation,
in the subgrade strength from the design CBR. Fifteen percent is expected to
exceed this value and 15 percent is expected to be less than this value. Some variation in the subgrade condition is
already accounted for in the pavement design.
Another consideration is the
fact that these proof rolling loads and tire pressures are about 10 times the
final in-place stresses once the pavement is constructed. The proof rolling tire pressures are between
120 to 150 psi (820 to 1030 kPa) and the stresses, once the pavement is
constructed, are about 8 psi (55 kPa) for a thin asphalt pavement and 4 psi (27
kPa) for a thick concrete pavement.
These loads are the largest loads that the subgrade will encounter.
If the project can be
constructed while maintaining subgrade density, the subgrade design intent will
be fulfilled.
The project should not be
concerned with the “Pavement Warranty” issues that Contractors often bring
up. If the project follows these guidelines,
and properly documents the subgrade work, Central Office can defend the
warranty issue.
Once failure is established
based on the proof rolling results, the responsibility for the correction of
the failure should be determined.
Responsibility for the Unstable
Subgrade
If unstable subgrade
locations are found, take compaction tests to determine if the specifications
are met in the top 12 inches (300 mm).
The Engineer should instruct the Contractor to correct any deficiencies
found in these locations.
The
Department is responsible when the unstable subgrade is encountered in:
1.
Cuts.
2.
On reconstruction
projects.
3.
In shallow fill
locations where the unstable material is found under the contract fill.
4. When the unstable material is found at lower elevations
than the project contract work.
Subgrade stability may not be
possible by compacting the upper 12 inches (0.3 m) because of conditions at
these lower elevations.
It is the Contractor’s
responsibility to correct all unstable locations in fills. If the Contractor built the fill correctly,
the proof rolling will do nothing but verify specification work. If the fill fails then the proof rolling will
determine the location of the deficient specification work.
If the Contractor fails to
maintain the subgrade, the Engineer should instruct the Contractor to repair
the failed areas. See C&MS
203.04.A for the Contractor’s responsibility to drain and maintain the
subgrade.
Investigation
Investigate the causes of
failed locations quickly to expedite the corrective treatment. Three pieces of information are needed to
make the most economical subgrade treatment:
1. Rut depth.
2. Soil boring information.
3. Test pit data.
At this point, the rut depth
has already been determined.
Soil Boring Information
For rehabilitation projects
or cut sections, the soil borings can be examined to determine an estimated
undercut depth or stabilization methods.
Evaluate Standard Penetration
Test (SPT) results from soil borings in the failed subgrade locations. The Standard Penetration Test (SPT) is an
indicator of soil consistency or strength and measures the number of blows per
foot (N) required to drive the soil sampler through the soil. The soil data on the boring logs are
presented as the number of blows required to drive each 6-inch (150 mm) increment. The first 6 inches (150 mm) of the run is
ignored because the sampler may not be seated in the borehole or may be driven
through cuttings. For example, standard
penetration data shown as 1/2/3 has an N value of five blows per foot.
When investigating the need
for undercutting or stabilization in failed locations, look at the borings in
those locations in the upper 5 feet (1.5 m) of the subgrade. At each location, pick the lowest N value
when multiple N values are taken in the top 5 feet (1.5 m) of subgrade.
Average the N value along the
failed locations. This value provides
one part of the information needed to determine the undercut depth or
stabilization methods.
Test Pits
Once the soil borings have been
evaluated, construct test pits by excavating 3 to 5 feet (0.6 to 1.5 meter)
into the subgrade using the Contractor’s excavation equipment. Excavate at least two test pits that
represent the failed area. Use judgment
for long areas, usually about two to four test pits per mile is
sufficient. Construct the test pits
across the width of the subgrade in the failed locations. Pick locations with the highest deflections
to evaluate the most severe locations.
Warning: These trenches may
collapse on the construction personnel.
The Department offers an 8-Hour Construction Safety Class to evaluate
the trench collapse risk. In addition,
there is a trench safety class offered by the Bureau of
Workers Compensation, Division of Safety and Hygiene. These classes are given statewide, all year
around. (614-466-5563)
An examination of the soil
and moisture conditions in these test pits provides valuable information to
make the appropriate correction. Once
the pits are excavated, the Engineer must examine the trench sidewalls and the
bottom of the cut.
Record the test pit
information on CA-EW-3
Subgrade Test Pit Investigation form shown in Figure 204.G. The soil conditions vary with depth and must
be quantified. By examining the
sidewalls, the Engineer can determine the soil type, layer thickness, soil
condition, and soil strength by using a hand penetrometer.
Figure
204.F – Form CA-EW-3 Subgrade Test Pit Investigation
The Engineer must field
classify the soil. See 203.02 Materials,
Identifying Soil and Granular Materials in the Field, for help in the
classification.
Added soil conditions are
described on the bottom of the test pit form.
These conditions are stated in commonly-known consistencies, so that the
non-geotechnical reader can relate to the soil conditions. They are listed on the bottom of the
form. No explanation is needed for these
terms.
Hand Penetrometer Readings
A hand penetrometer can be
used to further classify the soil and to estimate its strength. A hand penetrometer can be obtained from a
test lab supply company for less than $100.
Hand penetrometers can be obtained from the following companies:
|
Gilson |
Model # HM-500 |
|
ELE |
Model # E129-3729 |
|
Humboldt |
Model H-4200 |
The exact instructions come with
the hand penetrometer. In summary:
1. Push the hand penetrometer slowly into the soil
perpendicular to the surface.
2. Record the reading when the hand penetrometer
penetrates the soil to the 1/4-inch groove mark.
3. Record the readings to the nearest 0.25 tons per
square foot (tsf).
4. Take at least three different readings in each soil
layer.
Use the CA-EW-3
Subgrade Test Pit Investigation form to record the readings. Average the readings once three readings are
taken for the soil layer. Also, evaluate
the bottom of the test pit; this is extremely valuable information. Average the hand penetrometer readings (HP)
of all the test pits in the failed locations.
Use this number to further evaluate the undercut depth or stabilization
methods.
Consider the following when
evaluating the sidewalls of a trench:
1. Different layers of a natural formation or cut are
more noticeable than fill materials.
2. High hand penetrometer readings may be obtained with
high deflections or rolling at the surface. This is an indication of soft soil
at a lower elevation than 5 feet (2 m) or a subgrade soil that is just too wet.
Undercut Depth and Stabilization
Determination
Once the proof rolling rut
depth (in inches), soil boring information (N), and hand penetrometer readings
from the test pits (HP) are obtained, use the Subgrade Treatment Chart in
Figure 204.H to determine the recommended depth of undercut or chemical stabilization. The input values (rut depth, N and HP) are on
the horizontal axis. The two curves show
the undercut depth with a geotextile and with a geogrid. The chart also shows the
stabilization depth required in inches along the bottom. Note that the results from this chart are
guidelines. The subgrade conditions might require undercuts that are less than
or greater than those shown, because subgrade conditions can be highly
variable.
Figure
204.G – Subgrade Treatment Chart
The subgrade treatment chart
takes into account some variation in test results, the anticipated loading from
the proof roller, and typical truck loading during construction.
Use the rut depth, N values,
and hand penetrometer readings (HP) to draw a vertical line to the curve. The recommended depth of the undercut is
where the vertical line intersects the curve.
For soft and very soft soils, it may be economical to use a geogrid to
reduce the depth of the undercut. The
geogrid restrains the granular material from lateral movement and makes it more
effective. Refer to Supplemental
Specification 861 for using geogrids for subgrade stabilization.
The chart does not recommend
chemical stabilization for soft and very soft soil. This is primarily because of constructability
problems. Although chemical
stabilization does improve the stability of soft and very soft soils, these
soils usually cannot support the equipment used to perform the chemical
stabilization.
It would be rare to see a
perfect alignment in the results from all three inputs. In some cases, one or two of these inputs may
not be available. In other cases, some
judgment is needed to redesign the most economical undercut that will
work. In order of hierarchy, use the
test pit data, then the N values, and then the rut depth. The rut depth is the least reliable indicator
of undercut need because it cannot determine which soil layer is causing the
deflection.
There will be cases where the
N values and unconfined values are all high, but the subgrade is rolling and
cracking, and rut depth is greater than allowable. In this case, use the rut depth as a guide to
redesign the undercut. See the last
example in the example section.
There is an example in Figure
204.I.
Given: Average N
value was 10.
Average HP= 1.5 tsf.
Average Rut Depth was 2 to 4 inches.
Answer: Use
an undercut depth of 12 to 18 inches with a geotextile or chemically stabilize
with 12 to 14 inches of cement or lime.
For very large areas, give serious consideration to the stabilization
method. It will be more cost effective.
Figure
204.H – Example using the Subgrade Treatment Chart
After making the undercut,
this depth may need to be adjusted to meet the actual conditions. See the section, “Implementation during
Construction,” of this manual.
General Rules
On new construction projects,
if all of the unstable material can be removed, and the bottom of the test pits
or cuts are stable, then soil may be used as replacement material. For reconstruction projects, soil is usually
not available in large quantities.
Therefore, soil undercuts are less effective solutions on reconstruction
projects.
If the bottom of the test pit
is unstable when conditions are highly variable or for rehabilitation projects,
use granular material, rock, geotextile, geogrid, or chemical stabilization
rather than soil.
Undercuts should be used in
small locations or in areas where spot locations are identified. Consider chemical stabilization for long
areas greater than one mile.
Only the most unusual cases
require removal to depths greater than 3 feet (1 meter). Seventy five to 90 percent of subgrade
problems can be solved with a 1 foot treatment of granular material and
geotextile or chemical stabilization.
If a project or section of a
project undercut locations are more than 30 percent of the total area, undercut
or chemically stabilize the entire area.
If you do not undercut the entire area, these locations will grow, and
the construction will be inefficient as the construction proceeds. The
Department pays a higher cost at a reduced, final quality by undercutting a
high percentage of the subgrade throughout the project. ODOT would not repair a bridge deck or
pavement with this high a percentage of repairs.
Chemical stabilization
methods speed construction because of the ability to work immediately after a
rain. Estimates indicate that the
construction production is increased by at least 50 percent by using
stabilization methods.
Examples:
The following table shows
some example solutions. The types of
material refer to 703.16.C and Item 206.
|
Given |
Solution |
|
Project with Silty A-4a material with N=15 or HP=2.0 tsf Rut Depth>1" |
12 inches of Granular Material Type B, C or D with
geotextile or 12 inches of stabilization with cement |
|
Project with Deep, weak, and wet A-4a with N = 12 or HP=1.4
tsf Rut Depth = 2" |
18 inches of Granular Material Type B, C, or D, with geotextile
or 12-14 inches of stabilization with
cement |
|
New Construction,
Deep, weak & wet A-4a, A-6 or A-7-6 combination with N = 10 or
HP=1.0tsf. Rut Depth = 4" |
18 inches of Granular Material Type B C or D with geotextile or 14 inches of
stabilization with lime or cement. (Check the PI of the soils. Use the
stabilization type according to the PI’s of the soil.)
|
|
New Construction Jell-O like consistency soil with N = 5 or
HP=0.5 tsf. Rut depth > 6" |
30 inches of Granular Material Type B, C, or D, with
geotextile, or 18 inches of granular material with geogrid and
geotextile, or 16 inches of chemical
stabilization. ( Check the PI of the soil.) |
|
Any Project with
soup like consistency soil with N =
2 or HP=0.25 tsf Rut Depth = Buried equipment |
4 feet of Granular
Material Type B, C, or D, with geotextile, or 3 feet of granular material
with geogrid and geotextile. (Use type
D Granular Material if available) |
|
Reconstruction Project Sandy, A-4a, A-6a soil, PI < 20,
N = 8 or HP=1.0 tsf Rut Depth = 6". (Long Project) |
Cement Stabilized Subgrade 14" deep at 6% |
|
New construction A-7-6 clay soil, PI > 20 N = 11 or HP=1.2 tsf Rut Depth =3".
(Long Project) |
Lime Stabilized Subgrade 12" deep at 5% |
|
Reconstruction Project A-6a silty clay PI < 20, N=30 and HP>4.5 tsf Rut depth > 2" and rolling The key here is the rolling. Probably caused by high
moisture content of the soil at a depth.
If the subgrade is rolling with one pass of a proof roller then the
subgrade condition can rapidly deteriorate during construction. |
16" of Cement at 6% or 2.5 foot undercut
with Granular Material Type B, C, or D, with geogrid and geotextile. Use Type D material if available. |
Type of Undercut Materials
Use Granular Material Types
B, C, D, E, and F. They are generally
cheaper than 304.
Type B is a well-graded
aggregate with the gradations of Items 304, 411, or 617. Type C has a top size of 3 inches and type D
has a top size of 8 inches. Both C and D
are well-graded materials. The larger
top size material will bridge the unstable material better than the smaller
size material.
Use Granular Material Type E
when water levels are high or cannot be drained. The Type E materials are very porous. Always choke the Granular Material Type E
with Granular Material Type B or geotextile fabric.
There is a potential for
piping of soil into the Granular Material Type E as shown in Figure 204.J. In the figure on the left, when the open
graded material is placed on wet, fine-grained soil, the soil pipes into the
open graded material during construction.
In the figure on the right, the geotextile fabric blocks the
fine-grained soil from entering the open graded material. Geogrids will not perform this separation
function.
Figure
204.I – Soil Piping in Open Material
Underdrains cannot be placed
through Granular Material Types D, E, or F.
Use Granular Material Type B in the locations of underdrains. Underdrains can be trenched through
geotextile and geogrid if there is enough material above the geotextile or
geogrid to confine it. Always drain the
undercut to an underdrain, catch basin, or pipe.
The use of 712.09
Geotextile Fabric Type D is recommended in most cases. The cost is around $1.00 per square yard, and
it serves to keep the granular material and underlying soil separated. This
results in better performance of the undercut.
When the depth of the undercut is 24 inches or greater, consider using a
geogrid to reduce the depth of the undercut.
For undercuts 12 to 16 inches deep, place a geotextile in the bottom,
then the geogrid, and then the granular material. For undercuts 16 inches deep and greater,
place the geotextile in the bottom, then half the granular material, the
geogrid, and then the rest of the granular material. For severe situations, you can use multiple
layers of geotextile and geogrids.
Consult with the Office of Geotechnical Engineering in these cases.
Chemical Stabilization of the
Subgrade
Item 206
Chemically Stabilized Subgrade can be used to treat unstable
subgrades. Lime or cement is typically
used, but lime kiln dust is another option.
Lime is used for A-6b
(silty/clay) or A-7-6 (clay) soils which have a plasticity index of 20 or
greater. As a general guideline, use 5 percent lime by dry weight of the soil,
assuming a dry weight of 110 pounds per cubic foot.
Cement can be used to treat
unstable subgrades consisting of A-3 (fine sand, coarse and fine sand), A-2-4
through 7 (gravels), A-4a (sand silt), A-6a (silt and clay), A-6b (silty clay),
or A-7-6 (clay) which have a plasticity index less than 20. As a general guideline, use 6 percent cement
by dry weight of the soil assuming a dry weight of 110 pounds per cubic
foot.
Lime kiln dust can be used
for soils which have a plasticity index in the range of 10 to 20. Consult with
the Office of Geotechnical Engineering when using lime kiln dust.
See Item 206,
Chemically Stabilized Subgrade, of this manual.
Implementation during Construction
Once the type of
stabilization treatment has been chosen, constant monitoring of the construction
is required to adjust the treatment to meet the field conditions. Soil conditions always vary; they vary the
most on rehabilitation projects or in cuts.
If the undercut option is
chosen, the project should monitor the bottom of the cut and evaluate the
condition. Take hand penetrometer
readings at the bottom of the cuts and compare them to the initial test pit or
soil boring information. If the
condition changes from the earlier evaluation of the test pits or the soil
borings, then adjustments to the undercut depth are required.
For undercuts that are 2 feet
deep or greater, give consideration to using geogrid in addition to the
geotextile fabric. The need for geogrid
can be determined by placing approximately half of the undercut depth. Load the undercut with a fully loaded
truck. If the area is unstable, place
the geogrid and continue to fill the undercut.
Once the undercut or
stabilization is complete, proof roll the area to ensure that the final
subgrade meets the rut depth and density requirements as detailed earlier in
section “Failure Criteria.”
Constant vigilance is needed
in order to make the most economical correction. It is easy to over-excavate unnecessarily and
waste money. It is more difficult to
make the right economical choice to stabilize the subgrade and to meet the
design and construction needs.
Documentation Requirements - 204 Subgrade Compaction
and Proof Rolling
1. Materials.
2. Compaction according to S-1015.
3. Lift thickness and roller passes.
4. Equipment used.
5. Type of soils.
6. Verify square yardage.
7. Verify subgrade line and grade.
8. Proof roll and make corrections.
9. Subgrade Test Pit Investigations.
10. Undercut measurements.
11. Document on CA-EW-1,CA-EW-2,CA-EW-3,
CA-EW-8, CA-EW-12 and CA-D-3. Do not
duplicate the information on all forms unless necessary.