Subgrade Preparation Art. 301.04
Division 300 — Subgrades, Subbases, and Base Courses
SUBGRADE SECTION 301. SUBGRADE PREPARATION
301.01 Description. This work shall consist of preparing the completed or
existing earthwork as an unimproved subgrade prior to constructing the pavement structure, shoulders, or appurtenances.
301.02 Equipment. Equipment shall be according to the following.
Item Article/Section
a.Steel Wheel Roller .............................................................................. 1101.01
b.Tamping Roller ................................................................................... 1101.01
c.Pneumatic-Tired Roller ....................................................................... 1101.01
d.Subgrade Planer ................................................................................. 1103.08
e.Subgrade Machine.............................................................................. 1103.09 CONSTRUCTION REQUIREMENTS
301.03 General. When the contract includes rough grading and surfacing, the
rough grading shall be completed as far in advance of the construction of the surfacing as feasible. When the rough grading has been included in a previous contract, all vegetation shall be removed prior to preparing the subgrade. The subgrade shall be prepared such that after compaction, it will be smooth and conform to the alignment, grades, and cross sections shown on the plans. Surplus excavated material resulting from grading and shaping the subgrade shall be disposed of according to Article 202.03. When additional material is required, it shall be obtained from within the right-of-way when possible and approved by the Engineer. Placement shall be according to Articles 205.04 and 205.05.
301.04 Subgrade Compaction and Stability. The subgrade shall be
compacted to not less than 95 percent of the standard laboratory dry density and shall have a minimum immediate bearing value (IBV) of 8.0. Densities will be determined according to Article 205.06. The IBV will be determined according to Illinois Test Procedure (ITP) 501 or 502. In cut sections, the Contractor shall take the following steps to obtain the required density and IBV values.
a.Step 1. Cut plan ditches, which drain the area, at least to grade. This shall be done at least two weeks prior to Step 2. Art. 301.04 Subgrade Preparation
b.Step 2. Air dry the top 8 in. (200 mm) of subgrade. This procedure shall include at least two 8 in. (200 mm) deep processings utilizing disks or tillers each day for three consecutive good drying days.
c.Step 3. Recompact the layer processed in Step 2 to achieve the required density, or until at least nine passes of a roller which has demonstrated ability to obtain the density on adjacent earth work have been made. Subgrade not meeting the foregoing criteria will be declared unstable by the Engineer and shall be rendered stable through reprocessing and recompacting, and/or more extensive ground treatments. As determined by the Engineer, unstable subgrade material that cannot be rendered stable may be declared unsuitable. Material declared unsuitable shall be removed and disposed of according to
Article 202 — 03, and replaced with material approved by the Engineer according to
Articles 205.04 and 205.05 Where rolling of the subgrade is required, any areas which are inaccessible to a roller shall be compacted by either a mechanical or hand tamper meeting the approval of the Engineer. Equipment of such weight, or used in such a way as to cause a rut in the finished subgrade of 1/2 in. (13 mm) or more in depth, shall be removed from the work or the rutting otherwise prevented. The subgrade will be approved by the Engineer before construction of the pavement structure, shoulders, or appurtenances is started.
301.05 Aggregate Base Course and Aggregate Surface Course, Type A.
The subgrade shall be compacted by rolling with a steel wheel or pneumatic-tired roller. The rolling shall extend at least 12 in. (300 mm) beyond each edge of the proposed base course.
301.06 Aggregate Surface Course, Type B. The subgrade will not have to
be rolled prior to placement of the aggregate surface course, Type B.
301.07 Hot-Mix Asphalt (HMA) Base Course and Pavement (Full-Depth)
and Portland Cement Concrete Base Course and Pavement. The work shall be extended to at least 18 in. (450 mm) beyond each edge of the proposed base course or pavement. When a subbase is being placed under the base course or pavement, the work shall be extended to include the area being covered by the subbase material. Prior to final shaping, the subgrade shall be compacted with a steel wheel or pneumatic-tired roller. Steel wheel rollers shall weigh from 6 to 10 tons (5.5 to 9 metric tons) total and from 200 to 325 lb/in. (35 to 57 N/mm) of roller width. The subgrade shall be brought to true shape by means of a subgrade planer and/or subgrade machine according to the following.
a.Either the subgrade planer or the subgrade machine shall be used when: Subgrade Preparation Art. 301.11
1.Portland cement concrete pavement or base course is constructed on the subgrade or subbase using forms.
2.HMA base course is constructed.
b.The subgrade machine shall be used when:
1.Portland cement concrete pavement or base course is constructed on the subgrade or subbase using the slip form method.
2.Continuously reinforced portland cement concrete pavement is constructed on the subbase or subgrade.
3.HMA pavement (full-depth) is constructed. When portland cement concrete is being placed directly on the subgrade, the subgrade shall be moist, but not muddy, at the time of placing the concrete. If required by the Engineer, the prepared subgrade shall be saturated with water the previous night, or 6 to 20 hours prior to the placing of the concrete. If the subgrade subsequently becomes too dry, it shall be sprinkled again ahead of placing the concrete, in such a manner as not to form mud or puddles of water.
301.08 Gutters, Curbs, and Combination Curb and Gutter. The subgrade
shall be compacted and finished to a firm, smooth surface in a manner approved by the Engineer.
301.09 Drainage. The subgrade shall be kept drained during the construction
of the pavement structure. If earth berms are deposited along the edge of the subgrade, provision shall be made for surface drainage by cutting lateral ditches through the berms.
301.10 Maintenance. The prepared subgrade shall be maintained in a
smooth and compacted condition.
301.11 Method of Measurement. When the contract includes both grading
and paving, this work will not be measured for payment. When the contract includes paving on a pregraded section, this work will be measured for payment as follows.
a.Contract Quantities. The requirements for the use of contract quantities shall be according to Article 202.07(a).
b.Measured Quantities. Subgrade preparation will be measured for payment in units of 100 ft (30 m) in horizontal distances along baselines. No allowance will be made for variable width roadways. No allowance will be made for excavation for removal or placement of any material within 2 in. (50 mm) of the grade and cross section shown on the plans or established by the Engineer. Art. 301.12 Soil Modification
301.12 Basis of Payment. When the contract includes paving on a
pregraded section, this work will be paid for at the contract unit price per unit for SHAPING AND GRADING ROADWAY. Excavation for the removal or placement of material outside the 2 in. (50 mm) tolerance specified in Article 301.11(b) will be paid for according to Article 109.04. Additional drying and compaction beyond the three steps listed in Article 301.04 will be paid for according to Article 109.04. Excavation and replacement of material declared unsuitable, as specified in
Article 301 — 04, will be classified and paid for according to Article 104.02.
SECTION 302. SOIL MODIFICATION
302.01 Description. This work shall consist of constructing a modified soil
layer composed of soil, water, and a modifier.
302.02 Materials. Materials shall be according to the following.
Item Article/Section
a.Type I Portland Cement .......................................................................... 1001
b.Type IS Portland Blast-Furnace Slag Cement ........................................ 1001
c.Water ...................................................................................................... 1002
d.Hydrated Lime ................................................................................... 1012.01
e.By-Product, Hydrated Lime ................................................................ 1012.02
f.By-Product, Non-Hydrated Lime ........................................................ 1012.03
g.Lime Slurry ........................................................................................ 1012.04
h.Fly Ash .................................................................................................... 1010
i.Soil for Soil Modification (Note 1) ....................................................... 1009.01
j.Bituminous Materials (Note 2) ................................................................ 1032 Note 1. This soil requirement only applies when modifying with lime (slurry or dry). Note 2. The bituminous materials used for curing shall be emulsified asphalt RS-2, CRS-2, HFE 90, or HFE 150; rapid curing liquid asphalt RC-70; or medium curing liquid asphalt MC-70 or MC-250.
302.03 Equipment. Equipment shall be according to the following.
Item Article/Section
a.Distributor (Note 1)
b.Rotary Speed Mixer ........................................................................... 1101.06
c.Disk Harrow (Note 2) ......................................................................... 1101.02
d.Lime Slurry Equipment (Note 3) Note 1. The distributor shall be a mechanical type and shall be approved by the Engineer.
Referenced in this section
SUBGRADE PREPARATION in other states
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Source: Illinois Standard Specifications for Road and Bridge Construction, 2022 Edition. Pages 160–163 of 1,217. View official source ↗