2-06 Subgrade Preparation (******)

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