1 Scope
NOTE This standard covers grading and subgrade preparation of site surfaces from the completion of mass earthwork through delivery of a tested, accepted subgrade ready to receive aggregate base, slab-on-grade, pavement, or topsoil. (1.1)
NOTE The work begins where bulk cut and fill has established approximate grade and ends at the subgrade surface that subsequent trades build on. (1.2)
NOTE Both rough grading (establishing design contours and elevations) and finish grading (trimming the subgrade to the tolerances required by the next trade) are included; some projects keep these as separate operations but this standard treats them as one continuous scope through subgrade acceptance. (1.3)
1.4The Contractor shall strip and stockpile reusable topsoil before grading the areas beneath it.
1.5The Contractor shall grade all site surfaces to the lines, grades, and contours shown on the drawings.
1.6The Contractor shall scarify, moisture-condition, and recompact subgrade soils to the densities required for the surface they support.
1.7The Contractor shall proof-roll the completed subgrade and correct soft or yielding areas before any base, slab, or pavement is placed.
1.8The Contractor shall fine-grade finished surfaces to the tolerances of the trade that follows.
2 Governing Documents
NOTE The project geotechnical report is the primary governing document for this work; it defines the soil zones, compaction criteria, moisture windows, over-excavation rules, and allowable bearing values for the specific site. (2.1)
NOTE Where the geotechnical report and this standard differ, the geotechnical report governs for soil-specific criteria and this standard governs for procedure, testing, and submittal requirements. (2.2)
2.3The current geotechnical report shall be on site and available to the Contractor and the testing agency throughout grading operations.
2.4The Contractor shall follow the recommendations of the geotechnical report as supplemented by these requirements.
NOTE The parameters stated in this standard are typical defaults for use when a project-specific geotechnical report has not yet established a value; they do not override a value set by the geotechnical engineer of record. (2.5)
3 Referenced Standards
3.1Work shall comply with the latest adopted edition of each of the following unless a specific edition is cited or the geotechnical report pins a different edition.
3.2Where referenced standards conflict, the more stringent requirement shall govern unless the geotechnical engineer of record directs otherwise in writing.
| Standard |
Title |
| ASTM D698 |
Laboratory Compaction Characteristics of Soil Using Standard Effort |
| ASTM D1557 |
Laboratory Compaction Characteristics of Soil Using Modified Effort |
| ASTM D1556/D1556M |
Density and Unit Weight of Soil in Place by Sand-Cone Method |
| ASTM D6938 |
In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth) |
| ASTM D2216 |
Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass |
| ASTM D4318 |
Liquid Limit, Plastic Limit, and Plasticity Index of Soils |
| ASTM D2487 |
Classification of Soils for Engineering Purposes (Unified Soil Classification System) |
| AASHTO T 99 |
Moisture-Density Relations of Soils Using a 2.5 kg Rammer and a 305 mm Drop |
| AASHTO T 180 |
Moisture-Density Relations of Soils Using a 4.54 kg Rammer and a 457 mm Drop |
| AASHTO M 147 |
Materials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses |
| ACI 117 |
Specification for Tolerances for Concrete Construction and Materials |
| IBC 2021 |
International Building Code (Chapter 18, Soils and Foundations) |
NOTE ASTM D698 (Standard Proctor) and ASTM D1557 (Modified Proctor) define two different compaction efforts; a percent-of-maximum-dry-density requirement is meaningless unless the Proctor method is named. (3.3)
NOTE AASHTO T 99 and T 180 are the AASHTO equivalents of ASTM D698 and D1557 and apply where the authority having jurisdiction or a DOT controls access roads or parking that connect to the public right-of-way. (3.4)
4 Submittals
4.1 Action Submittals
4.1.1The Contractor shall submit the following action items for review before grading begins:
- Source, classification (ASTM D2487), and Atterberg limits (ASTM D4318) for any imported select or structural fill
- Proctor curve (ASTM D698 or D1557, with method A/B/C identified) for each soil type to be compacted
- Grading plan and sequence, including topsoil stripping limits and stockpile locations
- Proposed compaction equipment for each soil type (vibratory, sheepsfoot/padfoot, pneumatic)
- Manufacturer data and certification for any separation geotextile
☑ Imported fill source, USCS classification, Atterberg limits
☑ Proctor curve per soil type (method identified)
☑ Grading plan and sequence
☐ Compaction equipment list
☐ Geotextile data and certification
4.2.1The Contractor shall submit the following informational items during the work:
- Field density test reports (ASTM D6938 or D1556) for each lift and test location
- Proof-rolling observation reports identifying any soft areas and corrective action taken
- Over-excavation and replacement records where unsuitable material was removed
- Subgrade acceptance documentation signed by the geotechnical engineer of record
☑ Field density test reports
☑ Proof-rolling observation reports
☐ Over-excavation and replacement records
☑ Subgrade acceptance documentation
5 Quality Assurance
5.1 Testing Agency
5.1.1An independent geotechnical testing agency shall perform all compaction, moisture, and density testing.
NOTE The testing agency shall be engaged and paid by the Owner unless the Contract Documents assign that responsibility otherwise. (5.1.2)
5.1.3The testing agency shall report results to the Owner, the geotechnical engineer of record, and the Contractor within the timeframe established at the preconstruction meeting.
5.1.4Field density testing shall be performed by the nuclear method (ASTM D6938) or the sand-cone method (ASTM D1556).
NOTE The nuclear gauge shall be correlated to the sand-cone method when results are disputed or when the geotechnical engineer requires verification. (5.1.5)
5.2 Subgrade Approval
5.2.1The geotechnical engineer of record shall approve the prepared subgrade before any aggregate base, slab, or pavement is placed over it.
NOTE Self-certification of subgrade acceptance by the Contractor in lieu of geotechnical engineer approval is not permitted; the approval is the Owner's quality-assurance checkpoint and may not be waived by the Contractor. (5.2.2)
5.3 Testing Frequency
5.3.1Field density tests shall be taken at the frequency below, with at least one test per lift in each defined area and additional tests wherever the testing agency observes a change in material or compaction.
6 Topsoil Stripping and Stockpiling
NOTE Topsoil is the organic-rich surface horizon; it is unsuitable as structural fill or subgrade but valuable for finish grading of landscaped areas, so it is stripped and preserved rather than mixed into the cut. (6.1)
6.2The Contractor shall strip topsoil from all areas to be graded, filled, or paved before those operations begin.
6.3Topsoil shall be stripped to the full depth of the organic horizon as encountered, not to a fixed nominal depth, where the horizon is deeper than the value below.
6.4Topsoil shall not be intermixed with subsoil, unsuitable material, or debris during stripping or stockpiling.
NOTE Stockpile height shall be limited because piles that are too tall compact their own lower lifts, exclude oxygen, and kill the soil organisms that make topsoil valuable for reuse. (6.6)
7 Subgrade Soil Material
7.1 Native Soil Subgrade
NOTE Native subgrade is the in-place soil scarified, moisture-conditioned, and recompacted without import; it is the default where the geotechnical report confirms the native material is suitable. (7.1.1)
7.1.2Native subgrade soils shall be free of organic matter, debris, frozen material, and particles larger than permitted by the lift thickness and compaction equipment.
7.1.3Native soils classified by ASTM D2487 as highly organic (PT) or as high-plasticity clay or silt (CH, MH) shall not be used as building pad subgrade unless the geotechnical report specifically accepts them.
7.2 Select and Structural Fill
NOTE Select granular fill is imported clean borrow used to replace over-excavated unsuitable native soil; structural fill is engineered fill placed in controlled lifts to build a building pad to design elevation. (7.2.1)
7.2.2Imported select fill shall classify as GW, GP, SW, or SP by ASTM D2487 unless the geotechnical report accepts a different gradation.
7.2.3Granular subbase fill shall meet the gradation and quality requirements of AASHTO M 147 where used as a working course or under pavement.
7.2.4Structural fill shall be placed only on a subgrade that has been proof-rolled and accepted, and shall be the soil type and gradation specified in the geotechnical report.
● Native soil — scarify, moisture-condition, recompact in place
○ Select granular fill — over-excavate unsuitable native and replace
○ Structural fill pad — engineered fill in controlled lifts to design elevation
○ Geotextile-separated — separation fabric on soft native before granular working course
☑ GW — well-graded gravel
☐ GP — poorly graded gravel
☑ SW — well-graded sand
☐ SP — poorly graded sand
7.3 Geotextile Separation
NOTE A nonwoven separation geotextile is placed on a soft native subgrade before a granular working course to keep the soft soil from pumping up into and contaminating the granular layer. (7.3.1)
7.3.2Where the geotechnical report requires it, separation geotextile shall be placed on the prepared native subgrade before the granular working course.
7.3.3Geotextile shall be overlapped at all seams by the manufacturer's recommended lap, and not less than 12 in., on a prepared surface free of protruding stone or debris.
● Not required
○ Required on soft native subgrade only
○ Required across full building pad
○ Required across full site
8 Compaction Requirements
8.1 Compaction Standard
NOTE The Proctor method is the reference maximum dry density against which field density is measured; naming it (D698 Standard vs. D1557 Modified) is mandatory because the two efforts yield different maximum densities for the same soil. (8.1.1)
NOTE When ASTM D1557 (Modified Proctor) is specified, the test method (A, B, or C) shall be identified because the method depends on maximum particle size and an unspecified method causes testing disputes. (8.1.2)
8.1.3The compaction reference standard for each soil shall be as established by the geotechnical report.
● ASTM D1557 (Modified Proctor)
○ ASTM D698 (Standard Proctor)
○ AASHTO T 180 (Modified)
○ AASHTO T 99 (Standard)
○ Method A
○ Method B
● Method C
8.2 Required Density by Zone
NOTE Compaction shall be specified by zone rather than as a single site-wide value; a uniform high value over-compacts landscape soil so plants cannot establish, while a single low value under-protects the building pad against settlement. (8.2.1)
8.2.2The building pad subgrade shall be compacted to the percentage of maximum dry density below over the depth specified by the geotechnical report, typically the top 12 in.
8.2.3The pavement subgrade shall be compacted to the percentage of maximum dry density below over its upper layer, typically the top 6 in.
8.2.4Landscaped and other non-structural areas shall be compacted to the lower percentage below so as not to inhibit plant establishment.
8.3 Moisture Conditioning
NOTE Soil compacted dry or wet of optimum moisture cannot reach its maximum dry density and settles later; the specification must state an acceptable moisture window and require correction by wetting or aeration when soil falls outside it. (8.3.1)
8.3.2Subgrade and fill soils shall be moisture-conditioned to within the window below of optimum moisture content (ASTM D2216) at the time of compaction.
8.3.3Soil that is too wet shall be aerated, disked, or dried, and soil that is too dry shall be uniformly wetted and mixed, before compaction proceeds.
8.4 Lift Thickness
8.4.2Fill shall be placed in horizontal loose lifts not exceeding the thickness below for the soil type and compaction equipment, and each lift shall be compacted and accepted before the next is placed.
9 Subgrade Preparation
9.1 Scarification and Recompaction
NOTE Scarifying the exposed subgrade and recompacting it removes the loose, disturbed surface left by earthwork and gives a uniform density at the top of the subgrade, which is where settlement matters most. (9.1.1)
9.1.2The exposed subgrade shall be scarified to a uniform depth, moisture-conditioned, and recompacted to the required density for its zone before fill or surfacing is placed.
9.2 Benching of Fill Slopes
NOTE Where fill is placed against an existing slope, the slope is cut into level steps (benches) so the new fill keys into the old ground instead of sliding along a smooth inclined contact. (9.2.1)
9.2.2Fill placed against existing slopes steeper than the value below shall be benched into the existing ground in level steps as the fill is brought up.
9.3 Over-Excavation and Replacement
NOTE When proof-rolling or density testing reveals unsuitable material below grade, the depth of over-excavation and the acceptable replacement material must be defined in advance, or every soft spot becomes a change order and an RFI. (9.3.1)
9.3.2Where unsuitable material is identified, the Contractor shall over-excavate to the depth directed by the geotechnical engineer and replace it with compacted select or structural fill.
9.3.3Over-excavated areas shall be proof-rolled at the base of excavation before replacement fill is placed.
10 Proof Rolling
NOTE Proof-rolling drives a heavy loaded truck over the completed subgrade to find soft or yielding areas that lift-by-lift density testing can miss; it is performed after mass earthwork is complete and before base or slab placement. (10.1)
10.2The completed subgrade shall be proof-rolled in the presence of the testing agency or geotechnical engineer before any base, slab, or pavement is placed.
10.3Proof-rolling shall use a loaded tandem-axle dump truck of the gross vehicle weight below, making at least the number of passes below in a pattern that covers the full area.
10.4Areas that rut, pump, or deflect more than the depth below under a loaded wheel shall be over-excavated and replaced or otherwise corrected as directed by the geotechnical engineer.
11 Finish Grading and Tolerances
NOTE Finish grading trims the accepted subgrade to the elevation and smoothness the next trade needs; tolerances differ by surface because a building pad, a paved subgrade, and a lawn each have different downstream sensitivity. (11.1)
11.2Finished surfaces shall be graded to the tolerances below, measured against the project surveyor's benchmark.
NOTE Where the civil drawing tolerance and the ACI 117 tolerance for soil immediately below a slab-on-ground conflict, the tighter tolerance governs below the slab and the structural slab specification shall be checked to confirm which applies. (11.3)
11.4The finished subgrade shall be free of ruts, humps, depressions, and loose material at the time it is handed off to the next trade.
● ± 3/4 in.
○ ± 1/2 in.
○ ± 1 in.
12 Drainage and Slopes
NOTE Positive drainage on the finished subgrade prevents water from ponding against structures and soaking the subgrade before it is covered; minimum grades are set so even survey tolerance does not create a flat or reverse slope. (12.1)
12.2The finished subgrade shall slope away from buildings at not less than the minimum grade below.
12.3All graded surfaces shall maintain at least the absolute minimum positive grade below so that no area ponds water.
12.4Swales, berms, and drainage courses shall be graded to the lines and inverts shown; their alignment and extent are coordinated on the drawings grading and drainage plan. 12.5Unretained fill side slopes shall not be steeper than the maximum below unless a flatter slope is required by the geotechnical stability analysis.
NOTE Allowable bearing pressure and subgrade modulus are design inputs the geotechnical report provides; the values below are presumptive defaults (IBC 2021 Table 1806.2 lists presumptive bearing values) for use only until the report establishes site-specific numbers. (13.1)
13.2The prepared building pad subgrade shall provide the allowable bearing pressure required by the foundation design, as confirmed by the geotechnical engineer.
13.3The prepared pavement subgrade shall provide the subgrade modulus (k-value) assumed in the pavement design.
15003000
Default: 2000 psf
14 Subgrade Protection
NOTE A subgrade that is fine-graded and accepted can still be ruined if construction traffic from other trades ruts or remolds it, especially in wet weather; the specification has to limit traffic or call for a protective course. (14.1)
14.2The Contractor shall protect the accepted subgrade from construction traffic, ponding water, and weather until it is covered by the next layer.
14.3Construction traffic shall be kept off the finished subgrade, or routed over a granular working course or geotextile-separated layer where traffic cannot be avoided.
14.4Subgrade that is rutted, remolded, softened, or otherwise disturbed after acceptance shall be reworked, recompacted, and re-tested at the Contractor's expense before the next layer is placed.
● Traffic limitation only — keep equipment off finished subgrade
○ Granular working course over finished subgrade
○ Geotextile separation plus granular working course
15 Cold Weather and Frozen Ground
NOTE Frozen soil cannot be compacted to a stable density and thaws to a soft, weak condition, so it is excluded as both a working surface and a fill material. (15.1)
15.2Fill shall not be placed on frozen subgrade.
15.3Frozen soil shall not be used as fill.
15.4Subgrade that has frozen after acceptance shall be allowed to thaw, then re-proof-rolled and re-tested before the next layer is placed.