Question 5 of 5: Sanitary Landfill Cross Section and Cover Material

Question 5: Sanitary Landfill Cross Section and Cover Material (25 marks)

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, graded and proof-rolled to receive the liner; the composite liner (compacted clay plus a synthetic geomembrane), the primary barrier preventing leachate migration into groundwater (per Question 1(iii)); the leachate collection and drainage layer immediately above the liner, sloped to convey 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 that controls odour, vectors, blowing litter and fire risk between working periods; a gas vent/extraction riser passing up through the refuse mass to safely collect and route the landfill gas described in Question 1(iv) (or to feed a Question 4(a)-style perimeter interceptor trench for lateral migration); a final cover cap (a low-permeability barrier and drainage layer beneath a vegetated topsoil layer, per Question 1(v)) minimizing long-term infiltration once the cell is closed; a groundwater monitoring well down-gradient of the cell confirming the liner performs as designed; and a perimeter berm defining the cell boundary and containing the active working face beside the site access road.

(b) Importance and usage of cover material. Cover material is soil (or an approved alternative daily-cover product) placed over exposed refuse at the end of each operating period, serving several distinct functions that together are essential to a landfill operating as a controlled, sanitary facility rather than an open dump. Daily cover (a thin layer, typically 150–300 mm, applied at the end of each working day) controls disease vectors (rodents, insects, birds) by denying them access to exposed food waste, suppresses odour and blowing litter, reduces fire risk, improves the appearance and traction of the working face for continued vehicle access, and limits stormwater infiltration into freshly placed, uncompacted refuse. Intermediate cover (a thicker layer, applied where a lift will not be built upon again for an extended period) provides the same functions over a longer duration and additionally supports temporary vegetation and erosion control. The final cover (cap), placed once the cell reaches its design elevation, is the most engineered form of cover material: its low-permeability barrier layer is sized specifically to minimize long-term precipitation infiltration (directly reducing the leachate generation rate analyzed in the field-capacity water-balance method), its drainage layer sheds the water that does reach it before it can pond and force its way through, and its vegetated topsoil layer controls erosion and enables the site's approved post-closure land use. Across all three forms, cover material is the physical interface between the active or closed refuse mass and the surrounding environment, and its adequacy is judged directly against the concerns named in Question 1(i): vectors and odour (operational cover) and long-term infiltration/leachate and settlement protection (final cover).