USCS Classification and Landfill Cover Suitability: 18-Env-A3 Geotechnical and Hydrogeological Engineering, May 2015, Q5

Question 5: USCS Classification and Landfill Cover Suitability (20 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.

Given.

Given data (read from the grain-size distribution curve and Atterberg limits)
Quantity	Symbol	Value
Percent passing #200 sieve (0.075 mm)	fines %	≈ 20%
Percent passing #4 sieve (4.75 mm)	—	≈ 97%
Liquid limit	$LL$	40%
Plastic limit	$PL$	30%

Find. (a) USCS group symbol and group name; (b) suitability of this soil as landfill cover.

Fig. Q5 — grain-size distribution curve, with the #200 and #4 sieve readings marked.

Approach. Split the coarse/fine and sand/gravel fractions from the curve, compute the plasticity index, and route through the USCS flow chart (coarse-grained, more than 12% fines → classify the fines by the plasticity chart).

Grain-size fractions. Fines (passing #200) $=20\%$; coarse fraction $=100-20=80\%$ ($\ge50\%\Rightarrow$ coarse-grained soil). Of the coarse fraction, gravel (retained on #4) $=100-97=3\%$ and sand $=97-20=77\%$; sand $>$ gravel, so the base symbol is S.
Plasticity of the fines. $PI=LL-PL=40-30=10\%$. The A-line at $LL=40\%$ gives $$PI_A=0.73(LL-20)=0.73\times20=14.6\%.$$ Since fines $=20\%>12\%$, classify directly by the plasticity chart: $PI=10
Group symbol and name. Coarse-grained, sand-dominant, more than 12% silty fines below the A-line with $LL<50\%$: $$\boxed{\text{SM}\ \text{--}\ \text{Silty Sand}}$$ (gravel content is only 3%, well under the 15% threshold for a "with gravel" modifier).

(b) A silty SAND (SM) with only 20% fines is comparatively permeable — typical values for an SM soil run roughly $10^{-4}$ to $10^{-6}\ \text{cm/s}$, several orders of magnitude above the $\le10^{-7}\ \text{cm/s}$ usually specified for a low-permeability FINAL cap, so on its own it would let too much precipitation infiltrate and generate leachate if used as a final cover without a compacted clay liner or geomembrane. The same properties make it well suited to DAILY or INTERMEDIATE cover, however: it compacts and handles easily, drains fast enough to avoid ponding and odour/vector nuisance, and retains just enough fines to hold together, resist erosion and support vegetation on an intermediate-cover slope.

Quantity	Value
Fines / Sand / Gravel	20% / 77% / 3%
Plasticity index, $PI$	10%
A-line $PI$ at $LL=40\%$	14.6% (soil plots below it)
(a) USCS group	SM — Silty Sand
(b) Cover suitability	Not suited to a low-permeability final cap; well suited to daily/intermediate cover

Concept & Theory

Topic. Unified Soil Classification System (USCS) for a coarse-grained soil with plastic fines.

Key relations. Coarse-grained if $\ge50\%$ retained on #200; sand vs. gravel by the coarse fraction split at #4; fines $>12\%$ → classify by the plasticity chart, $PI_A=0.73(LL-20)$, silt-like (M) below the A-line vs. clay-like (C) above it, low-plasticity (L) if $LL<50\%$ vs. high (H) if $LL\ge50\%$.

Why this works. USCS is a decision tree built around engineering behaviour, not mineralogy: the #200/#4 splits separate soils by how they carry load and drain (coarse-grained vs. fine-grained, gravel vs. sand), while the plasticity chart separates the fines fraction by compressibility and drainage behaviour using two Atterberg-limit tests, since plasticity — not particle size alone — governs how the finest particles actually behave.

Common pitfall. Applying the $C_u/C_c$ gradation check when fines exceed 12% (it only matters for clean sands/gravels, under 5% fines), or forgetting that the sand/gravel split uses the coarse fraction only, not the whole sample.

Source. Das, Principles of Geotechnical Engineering (9th ed.), Ch. 4 — Soil Classification; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.), Ch. 10 — solid waste / landfill cover design.

Question 5 of 6: USCS Classification and Landfill Cover Suitability

Question 5: USCS Classification and Landfill Cover Suitability (20 marks)