Fill Materials: The Good, the Bad and the Ugly, Part 2

By: Bill Leedham, P.Geo, CESA 

Last month I discussed the issue of fill materials, specifically what constitutes fill and why the presence of fill materials at a property can be a potential concern for Environmental Site Assessments. This month I will review some of the potential indicators of fill that you should look for during Phase 1 and 2 ESA work.

How do you identify Fill materials during Phase 1 ESA research?
There are several research parameters to check during the Phase 1 ESA research and prior to Phase 2 ESA field work. Evaluate aerial imagery for evidence of fill piles, filled in ponds, ravines, low areas; review a series of topo maps for possible grading and grade changes; review any prior borehole logs and geotechnical reports for evidence/records/notes of fill or suspected fill. Construction inspection reports can also help, as records of deep foundations or foundation alternatives like piles, helical piers, grade beams etc. can all be indications of looser, or unconsolidated or weaker soils (for bearing capacity), any of which could possibly be due to presence of fill. Phase 1 and 2 ESA and other environmental reports (UST removals, demolition, and remedial reports) can also provide information on possible fill placement. Fill and/or manufactured imported aggregate are likely to be present on a developed property for grading, pavement structure, foundation backfill, and engineered fill. Check historical site records, reports, and interviews with owners, tenants, and neighbours for any recorded or anecdotal evidence of fill placement.

How do you identify Fill materials in the field?
When you are on site look for evidence of unusual grading or built up areas, filled in areas (gullies, ravines, ponds, or former wetlands), grade changes, and other constructed features such as retaining walls. Ask locals, property owners, and drillers/contractors about their knowledge of area development and conditions. A pre-drilling geophysical survey can also identify fill, possible fill, and changes in soil type. During a subsurface investigation like drilling or test pit excavation look for and note soft soils, low Standard Penetration Test (SPT) or ‘N’-values or abrupt changes in ‘N’-values during split-spoon sampling. Don’t hesitate to ask the driller for their opinion (some drillers are better or more experienced than others in how it ‘feels’ while drilling, and can tell if it feels like drilling through fill). Look for any evidence of buried non-native materials – if you encounter buried topsoil, significant rootlets at depth (not always, some rooting does go deep into subsoil), debris, pavement, coal fragments, clinkers, construction materials (glass, wood, bricks, metals); then everything found above level that is most likely fill. Consider how native soils are originally deposited (via glacial deposition, as sediment from water, wind-borne, etc.), and think how that deposition would look in situ. In many soils, not all (e.g. some tills and conglomerate-type soils) there will be distinct layering, sometimes very uniform in consistency with noticeable changes from one homogenous stratum to another. Then consider how fill may get placed at a site – bulk dumping from a truck, graded by a dozer, mechanically transported and placed. In comparison fill may look ‘dirtier’ with several types of soil in a mix or blend (e.g. a heterogeneous mix of visibly different silt, sand, gravel, organics), sometimes oddly mixed with inclusions. Unless it has been compacted it will likely be looser and appear unconsolidated, and could even get softer and looser with depth (native soils often gain strength with depth). Some of the hardest material to distinguish in the field is fill that is comprised of re-worked native soil, such as a large cut and fill grading for a site. If necessary have a more experienced person review with you and directly examine the soil/fill in the field or at least review retained samples with you. The more soil you drill, sample, see, feel and touch; the better you will get at the task.

By:
Bill Leedham, P. Geo., CESA, QP
Bill is the Head Instructor and Course Developer for the Associated Environmental Site Assessors of Canada (www.aesac.ca); and the founder and President of Down 2 Earth Environmental Services Inc. You can contact Bill at info@down2earthenvironmental.ca