
Comparison
Engineered fill vs structural fill: How the specs differ
Structural fill names the job: carrying a load. Engineered fill names the method: placed in lifts and tested to a geotech's spec. Fill under a footing usually has to be both.

Quick answer
These two terms overlap, and one load of fill can be both. Structural fill describes what the soil does: it supports a building, wall, slab or pad. Engineered fill describes how it got there: placed in measured lifts, compacted to a percentage of a Proctor maximum and signed off by the geotechnical engineer. Numbers come from the report, not the label.
Engineered fill vs structural fill head to head
The engineered fill vs structural fill question is mostly one of emphasis.
Both terms mean compacted soil with something heavy on top of it. One points at the engineering control over how the soil is placed. Structural fill points at the load and at the material that can carry it. Public specs written by different agencies show how much the two overlap.
Engineered fill: designed for one site's loads
Daly City, California, defines engineered fill in its earthwork spec by the record behind it: fill on which the geotechnical engineer has made enough tests and observations to state that it was placed and compacted to the specification. Native or imported soil both qualify, as long as the engineer approves it.
That makes it site specific, with material limits, lift thickness and the density target set from that site's borings and loads. Soil Connect's guide to engineered fill for stable builds treats it as a mix tailored to precise specifications.
- Defined by: Placement & test record
- Material: Soil the engineer approves
- Spec source: Project geotechnical report
- Sign-off: Engineer's written statement
- Typical use: Pads, slabs, slopes, buttresses
Structural fill: graded material that carries weight
UFGS 31 00 00, the Defense Department's earthwork guide spec, defines structural fill as soil material placed to support buildings, walls, pads and similar facilities. Its material list runs from well-graded gravel (GW) to fat clay (CH) under ASTM D2487, and the spec writer strikes the classes that do not suit the site.
Structural fill dirt bought from a pit should be free of roots, trash and frozen clods, with no rock larger than the spec allows. A delivery ticket does not make it structural. Passing density tests do.
- Defined by: What it supports
- Material: Soil classes the spec allows
- Largest rock: Often half the lift thickness
- Spec source: Project spec or geotech report
- Typical use: Under footings, slabs & walls
Table: Where the two terms put their weight. Most commercial specs ask for both.
| | Engineered fill | Structural fill |
| Term stresses | Control of placement | The load it carries |
| Material limits | Set per site by the geotech | Listed by soil class in the spec |
| Proof of compliance | Engineer's statement backed by tests | Passing tests on every lift |
On many jobs the two meet in the middle. A report will often call the material structural fill and require it to go down as engineered fill. Where a city has adopted the 2021 International Building Code, as Seattle has, Section 1803.5.8 requires a geotechnical investigation wherever shallow foundations bear on compacted fill more than 12 inches deep. That report has to name the fill material spec, the lab test for maximum dry density, the maximum lift thickness, the field density test, the minimum density and how often to test. Section 1804.6 lets fill 12 inches deep or less skip the report if it reaches 90 percent of ASTM D1557 maximum dry density, verified by special inspection.
Trade-offs: structural fill specifications, testing & cost
Tighter structural fill specifications cost more to meet and buy more predictable settlement.
Each added limit shrinks the pool of structural fill material that qualifies and adds a test to the schedule. Both sides of that ledger show up below.
Structural fill: which Proctor test
Every field reading is measured against a Proctor test result. The Proctor compaction method compacts soil samples at rising water contents and plots dry density against moisture, and the peak gives maximum dry density and optimum moisture. ASTM D698 uses a 5.5 lbf rammer dropped 12 inches, 12,400 ft-lbf/ft³ of effort. ASTM D1557 uses a 10 lbf rammer dropped 18 inches, 56,000 ft-lbf/ft³. So 95 percent of a modified maximum is a harder field target than 95 percent of a standard one. UFGS steers spec writers to D698 for most projects and to D1557 for roads, airfields and other heavily loaded areas.
Engineered fill: density & moisture targets
Compacted fill passes when its in-place dry density reaches the specified share of the lab maximum. Numbers move from spec to spec. UFGS 31 00 00 calls for at least 90 percent for cohesive soil and 95 percent for cohesionless soil under structures unless the project says otherwise, with an optional window of 2 percent either side of optimum moisture. Daly City asks for at least 90 percent of the maximum dry density from ASTM D1557 or California Test Method 231-E. Corpus Christi, Texas, asks for 95 percent of standard Proctor on the select material it uses under roadways and in trenches.
Structural fill material: plasticity limits
Plasticity index is the liquid limit minus the plastic limit, and a high PI flags clay that swells when wet and shrinks when dry. Select fill specs cap it. Daly City allows a PI of 15 at most under floor slabs. For roadway, embankment and trench work, Corpus Christi wants sandy clay or clayey sand with a liquid limit under 35 and a PI from 8 to 20. Soil Connect's primer on plasticity index & soil performance explains the liquid and plastic limits behind the number.
Engineered fill: lift thickness
Thin lifts compact more reliably and take more passes. UFGS caps structural fill at 8 inches loose, or 4 inches under hand-operated compactors. Daly City also uses 8 inches but lets the geotechnical engineer change it to suit the equipment and soil. Corpus Christi allows 10 inches loose for its select material.
Testing: frequency & retests
Testing is a schedule cost as much as a lab cost. The default UFGS table, which spec writers edit per job, calls for one field density test per 2,000 square feet of each lift, doubled where hand machines do the compacting, plus two moisture tests a day for each fill type. A failed lift has to be reworked and retested before the next one goes down, and the crew waits.
Clean granular fill: relative density
Clean sand and gravel can fall outside the Proctor test entirely. For soil with less than 5 percent passing the No. 200 sieve, UFGS expresses compaction as relative density under ASTM D4253 and D4254. UFGS also switches coarse soil with more than 30 percent retained on the 3/4 inch sieve to AASHTO T 180, because ASTM D1557 does not apply to it.
Decision guide: reading the geotechnical report before you order
Order from the earthwork section of the geotechnical report, not from a name on a supplier's price sheet.
An estimator should be able to answer each question below from the report before the first truck is booked. Where the report is silent, ask the geotechnical engineer before buying any fill material.
Which fill goes where?
Reports often split fill by zone: select fill under slabs and footings, general fill in yards and berms. Take off each zone as its own quantity, because the two are priced and sourced separately. Soil Connect's geotech reports decoded shows how those material specs move a budget.
What makes it select fill on this job?
There is no national definition. Select fill dirt that meets Corpus Christi's PI band of 8 to 20 can fail Daly City's cap of 15 under a slab. Copy the gradation, liquid limit and PI limits from the report and ask the source for recent results under ASTM D4318 and a sieve analysis before it ships.
Which Proctor method and percentage apply?
Write both on the order, for example 95 percent of ASTM D698. A supplier quoting from a modified Proctor result on a standard Proctor job is comparing different targets.
Can the site's own soil be reused?
Only where the report allows it and the soil tests inside the limits. UFGS treats man-made fill, trash, roots, other organic matter and frozen material as unsatisfactory, so stockpile cut material by type and test it early.
Where does matching import come from?
Request the material type and quantity through the Find Dirt marketplace. A Soil Connect Dirt Specialist will contact you within 1 business day, and Dirt Pros check that the dirt meets your project specifications.
Engineered & structural fill FAQ
Frequently asked questions
Is select fill dirt the same as structural fill?
Not always. Structural fill is a use: soil placed to carry a building, wall or slab. Select fill is a material grade with tighter limits on gradation and plasticity, and many reports name it as the structural fill under slabs and footings. Daly City, for example, adds a maximum PI of 15 for select material under floor slabs on top of its general fill rules. Check which term the report uses for each zone.
What makes compacted fill pass inspection?
In-place dry density at or above the specified percentage of the lab maximum, from the Proctor method the spec names, in lifts no thicker than allowed and from approved material. Many specs add a moisture window at placement. Where the spec calls for proof rolling the subgrade before fill goes down, UFGS uses six passes of a loaded tandem axle dump truck or a 15 ton roller, and areas that rut or deflect get reworked. On work that needs special inspection, 2021 IBC Table 1705.6 has the inspector verify densities and lift thicknesses continuously while compacted fill goes down.
How often is a Proctor test run on fill?
The project spec governs. As one example, the UFGS default runs one moisture-density test per 500 cubic yards of fill and backfill, or whenever the material changes enough to shift optimum moisture or maximum dry density. Spec writers can change that interval. Field density tests run far more often, on every lift. A new borrow source means a new Proctor before its fill is accepted.
Can job site dirt work as structural fill material?
Yes, if it meets the spec and the geotechnical engineer approves it. Daly City's spec, for one, requires engineer approval for all fill, native or imported. Expect gradation and Atterberg limit tests, moisture conditioning and removal of roots and debris. Soil that fails can still go to another site that needs fill, and Soil Connect matches sites with excess dirt to sites that need it.
Select & structural fill guides
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Engineered fill vs structural fill: How the specs differ














































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