2-06 SUBGRADE PREPARATION

(******)

2-06.3 Construction Requirements

2-6.3(1) Subgrade for Surfacing

This section is supplemented with the following:

The subgrade must be suitable, as determined by the Engineer, prior to placement of

geotextile fabric (Note to Engineer: if required include, if not delete) or permeable ballast
base course. All costs for protecting the subgrade from siltation or over-compaction,
including replacing all material that becomes unsuitable while the subgrade is exposed, shall
be considered incidental to the Work and shall not be measured for separate payment.
While the subgrade is exposed it shall be graded to drain to ensure that water does not
pond.

Preparation and compaction of the subgrade, both for permeable and dense pavement,
shall be considered as essential to the construction and all costs thereof shall be incidental
to the Work. Preparation, compaction, maintenance, and all other work related to subgrade
shall not be measured for payment. The subgrade shall be shaped and maintained to drain
at all times during construction, including temporary ditches and modifications to drainage
structures necessary to eliminate standing water on the subgrade.

Add the following new section:

2-06.3(3) Subgrade for Permeable Pavement

[Note to Engineer: Choose one below. Confirm with your geotech if you need further compaction.]

a) Subgrade shall be compacted to 90-92% of maximum dry density according to standard

proctor (ASTM D698) and shall be firm and unyielding prior to placing pavement section
material.
o Subgrade preparation density tests shall be conducted at a rate of 2 tests per
5000 square feet, and 4 tests per 200 lineal feet of roadway.
o Density tests shall also be completed at and near each infiltration test
location (section 2-06.3(4)). Density tests at infiltration test locations shall be
completed at distances of 5, 10, and 15 feet from the infiltration test, parallel
and perpendicular to and within the project alignment. The test results should
then be averaged to represent the density at the infiltration test location, to
which other subgrade density tests may be compared. Engineer shall review
test results and locations for acceptance.
o The subgrade preparation density tests results shall be within + 2 percent of
the average density recorded at the nearest infiltration test location. If
exceeded scarify the subgrade and compact.
o Density tests shall be correlated to tests taken in the Subgrade Infiltration
Test section 2-06.3(4). Percent compaction shall be within + 2 percent of the
average density recorded at the infiltration test locations.

b) Subgrade shall be in a firm and unyielding condition prior to placing pavement section
material.
Contractor shall protect the subgrade as necessary to maintain preconstruction infiltration
rates. A potential procedure to prevent excessive subgrade compaction is as follows:
• Excavation to final subgrade elevation shall occur immediately prior to placing
pavement section materials and paving. If necessary, Contractor may excavate to
an intermediate subgrade elevation established at 12-inches above the final
subgrade elevation. Grading to final subgrade elevation shall be completed by
machinery operating on the intermediate subgrade level or the adjacent non-pervious
pavement subgrade.
• To prevent excessive compaction of subgrade during placement of pavement section
material the following steps should be followed:
o Excavate to subgrade elevation using method by which equipment, including
trucks, are not operated on the final subgrade elevation.
o Scarify subgrade during excavation as stated below.
o Compact subgrade to density specified herein or as directed by the Engineer.
o Install geotextile fabric (Note to Engineer: if required include, if not delete)).
o Back dump the material onto the subgrade from the edge of the installation
and push it out onto the subgrade using low ground pressure equipment.
o Trucks then back dump subsequent loads on top of the previously
dumped/pushed material as the installation progresses.
• Avoid subgrade preparation during wet conditions.

Contractor shall phase the work so as to not compromise or overly compact the subgrade.
Should it be necessary for machinery or trucks to access the final subgrade in certain areas,
Contractor shall protect said areas from over-compaction by placing steel sheets on the
areas access to diffuse point loading. Areas determined to be overly compacted, in the sole
opinion of the Engineer, shall be scarified by the Contractor to a depth specified by the
Engineer and re-compacted, and all costs thereof shall not be measured for separate
payment.

[Note to Engineer: Choose one below. Confirm with your geotech if you need compact .]

a) Prior to installation of the geotextile fabric ((Note to Engineer: if required include, if not
delete)) and permeable ballast, the contractor shall complete subgrade infiltration tests
in conformance with section 2-06.3(4). If testing indicates that the infiltration rate is in
conformance with the design long term infiltration rate and the remaining subgrade areas
are similarly prepared scarification will be at the Engineers discretion.
b) Prior to installation of the geotextile fabric ((Note to Engineer: if required include, if not
delete)) and permeable ballast, the contractor shall scarify the subgrade to a minimum
depth of 4-inches to ensure that subgrade is not sealed, compact the subgrade to 90-
92% maximum dry density condition, and proof-roll by the Contractor and/or probe by
the Engineer to check for areas not in a firm, unyielding condition.
c) Prior to installation of the geotextile fabric ((Note to Engineer: if required include, if not
delete)) and permeable ballast, the contractor shall scarify the subgrade to a minimum
depth of 4 inches to ensure that subgrade is not sealed, compact the subgrade to a
uniform firm and unyielding condition, and proof-roll by a fully loaded dump truck the
Contractor and/or probe by the Engineer to check for areas not in a firm, unyielding
condition.

Loose or disturbed areas identified during proof-rolling shall be over-excavated to firm

bearing and replaced with Gravel Borrow. The Contractor shall notify the Engineer a
minimum of 2 working days prior to scarifying the surface to schedule an inspection.
 Contractor shall submit a Subgrade Preparation Plan to the Engineer for review and
approval prior to beginning excavation work. The Subgrade Preparation Plan shall include
method(s) by which the Contractor plans on treating over-compacted subgrade areas and
methods by which access to private driveways will be maintained.

[Note to Engineer: Add SPP to the ROM. Delete private driveways if not applicable. If applicable
WARN the residents that there will be a steep ramp built and that cars with low clearance may have
issues]

Add the following new section:

2-06.3(4) Subgrade Infiltration Tests

Contractor shall conduct infiltration tests immediately following final subgrade preparation to
verify that the subgrade is not over-compacted. Infiltration tests shall be conducted at a rate
of 1 test per 5000 square feet, and 1 test per 200 lineal feet of roadway. One test per lot for
residential sites, for a minimum of 5 and a maximum of 10. Subgrade infiltration tests shall
be conducted at the discretion of the Engineer. Engineer shall choose the locations of the
infiltration tests.

The tests shall be conducted using the:

[Note to Engineer: Select preferred test method]

Small scale Pilot Infiltration Test ring

The small-scale and large-scale PITs shall be completed at the planned subgrade elevation
as follows:
o Excavate to and prepare the subgrade in accordance with the specifications.
o Excavate a shallow test pit to minimum dimensions of 2 feet wide by 6 feet in
length. Depth of the trench should be at or within 6–inches of the planned
subgrade elevation. The side slopes may be laid back sufficiently to avoid
caving and erosion during the test. However, the side slopes for the depth of
ponding during the test (6”-12”) should be vertical.
o The horizontal surface area of the bottom of the test pit should be 12 to 32
square feet. The pit may be circular or rectangular, but accurately document
the size and geometry of the test pit.
o Install a vertical measuring rod adequate to measure the water depth and it
in half-inch or centimeter increments and install in the center of the pit
bottom. Alternatively, a self-logging pressure sensor may also be used to
determine water depth and drain-down.
o Use a rigid pipe with a splash plate on the bottom to convey water to the pit
and reduce side-wall erosion or excessive disturbance of the exposed
subgrade.
o Pre-soak period: add water to the pit so there is standing water for at least 6
hours. Maintain the pre-soak water level at least 6 inches above the bottom of
the pit.
o At the end of the pre-soak period, add water to the pit at a rate that will
maintain a minimum 6-inch and maximum 12-inch water level above the
bottom of the pit over a full hour. The specific depth should be the about the
same as the maximum designed ponding depth.
 o Every 15 minutes, record the cumulative volume and instantaneous flow rate
in gallons per minute necessary to maintain the water level at the same point
on the measuring rod.
o After one hour, turn off the water and record the rate of infiltration in inches
per hour from the measuring rod data until the pit is empty.
o Fill and repeat the drain down portion of the test. Provide the test results to
the Engineer for review and acceptance.

Large Ring Infiltration Test

The large ring infiltration test shall be completed at the planned subgrade elevation as
follows:

o Excavate to and prepare the subgrade in accordance with the specifications.

Install a large (steel, concrete, or plastic ring) with a minimum internal
diameter of 3-feet and a minimum height of 12-inches. Press the ring into the
soil to firmly seat it and backfill around it with native excavation spoils.
o Install a vertical measuring rod adequate to measure the full ponded water
depth and marked in half-inch or centimeter increments in the center of the pit
bottom. Alternatively. a self-logging pressure sensor may also be used to
determine water depth and drain-down.
o Use a rigid pipe with a splash plate on the bottom to convey over the
exposed subgrade to reduce erosion or excessive disturbance of the
subgrade.
o Pre-soak period: add water inside the ring so there is standing water for at
least 6 hours. Maintain the pre-soak water level at least 6 inches above the
bottom of the pit.
o At the end of the pre-soak period, add water inside the ring at a rate that will
maintain a minimum 6-inch and maximum 12-inch water level above the
subgrade. The specific depth should be the about the same as the maximum
designed ponding depth.
o Every 15 minutes, record the cumulative volume and instantaneous flow rate
in gallons per minute necessary to maintain the water level at the same point
on the measuring rod.
o After one hour, turn off the water and record the rate of infiltration in inches
per hour from the measuring rod data until the water has infiltrated
o Fill and repeat the drain down portion of the test. Provide the test results to
the Engineer for review and acceptance.

[Note to Engineer: Amend the testing to fit your project and provide direction on what to do with the
infiltration test results (such as – Convert it to a long term rate and compare it to the design long term
rate). Provide a remedy if the long term infiltration rate determined from the subgrade infiltration rate
is less than the design long term infiltration rate. Identify an allowable tolerance between the
construction measured rate and the rate the design long term infiltration rate. (why is it different
scarification . Provide for design engineer to review testing and results for conformance with the
design.]
Add the following new section

2-06.4 Measurement

Preparing, maintaining, testing, and restoring (as necessary) subgrade shall not be
measured for payment.

END OF SECTION