18-Env-A6 Solid Waste Engineering and Management · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues; Vesilind, Worrell & Reinhart, Solid Waste Engineering; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); CCME, Guidance Document on Landfill Gas Management; Canadian Environmental Protection Act, 1999.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
(a) Sanitary landfill cross section. A modern engineered cell is built up from a base liner system through successive lifts of compacted refuse to a final cover, with dedicated ancillary systems for leachate, gas and groundwater monitoring layered throughout, as sketched below.
From base to surface, the essential components are: the native subgrade, prepared and graded to receive the liner; the composite liner (compacted clay plus a synthetic geomembrane), which is the primary barrier preventing leachate migration into groundwater; the leachate collection and drainage layer immediately above the liner, which conveys leachate by gravity to a collection sump for removal and treatment; successive compacted refuse lifts, each capped with a thin layer of daily/intermediate soil cover to control odour, vectors, litter and fire risk between working periods; a gas vent/extraction well penetrating the refuse mass to safely collect and route landfill gas; a final cover cap (a low-permeability barrier and drainage layer beneath a vegetated topsoil layer) that minimizes long-term infiltration once the cell is closed; a groundwater monitoring well down-gradient of the cell to confirm the liner is performing as designed; and a perimeter berm defining the cell boundary and containing the active working face.
(b) Trench Method vs. Area Method. Both are standard construction sequencing methods for placing refuse in an engineered landfill; they differ chiefly in whether the waste is placed below or on top of the original ground surface, and in where the daily cover soil comes from.
| Aspect | Trench Method | Area Method |
|---|---|---|
| Refuse placement | Excavated below original grade, into a trench | Placed directly on the (lined) original ground surface, built up in lifts |
| Cover soil source | Native soil excavated from the trench itself | Imported/hauled from an off-site borrow source |
| Site suitability | Requires deep, excavatable soil and a low water table well below the trench floor | Suited to sites with a high water table, shallow bedrock, or soil unsuitable for excavation |
| Liner installation | Lines the excavated trench walls and floor | Lines the prepared original ground surface before the first lift is placed |
| Typical application | Flat sites with deep, well-drained, easily excavated soils | Sites needing above-grade fill (low-lying land, quarries being reclaimed above the water table, or where the area method is combined with a ramp/progressive-slope variant against existing high ground) |
In the trench method, a trench is excavated ahead of the working face, lined, and refuse is placed and compacted in lifts within it; the soil removed during excavation becomes the daily cover for that same trench, which is economically efficient wherever the geotechnical conditions allow deep excavation. In the area method, no excavation into the original grade occurs (often because a high water table or exposed bedrock rules it out); the liner is placed on the prepared original surface, and refuse is built up in lifts to the design height, progressively raising the land surface, with all cover soil hauled in from off-site since none is generated on the site itself. The choice between them is therefore driven primarily by geotechnical and hydrogeological site conditions (depth to water table, excavatability of the native soil), not by preference, and a real facility often combines both methods across different phases as the underlying site conditions change.
