18-Env-A3 Geotechnical and Hydrogeological Engineering · May 2018
Question 2 of 6: USCS Classification and Landfill Cover Suitability
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams May 2018 — 04-Env-A3, Geotechnical and Hydrogeological Engineering — 3 hours, open book. All SIX questions are answered below (the exam marks only the first five as submitted; all six are solved here as a complete study resource). Marking scheme: each question 20 marks, equal value.
Given. Grain-size curve (Figure 1: flat at 100% finer for grain sizes above ≈0.03 mm, falling to about 15% finer at 0.001 mm); liquid limit $LL=40\%$; plastic limit $PL=20\%$.
Find. The USCS group symbol and group name, and whether this soil suits a landfill cover.
[Figure not reproduced: Figure 1: grain-size distribution curve of the inorganic soil (from the exam figure). See the official exam paper.]
Approach. Read the percent passing the 0.075 mm (#200) sieve off the curve to decide coarse- vs fine-grained, then place the Atterberg limits on the Casagrande plasticity chart to get the group symbol.
Coarse vs. fine grained. The curve is flat at 100% finer for every grain size above ≈0.03 mm — since the #200 sieve opening (0.075 mm) sits on that same flat plateau, essentially 100% of the soil passes the #200 sieve. A soil with more than 50% passing #200 is classified as fine-grained, so the group symbol comes from the plasticity chart, not the gradation curve.
Plasticity index and A-line position. $$PI = LL-PL = 40-20 = 20$$ $$PI_{A\text{-line}} = 0.73(LL-20) = 0.73(20) = 14.6, \qquad PI_{U\text{-line}} = 0.9(LL-8) = 28.8$$ Since $PI=20 > 14.6$, the point plots above the A-line (a clay, not a silt), and since $20<28.8$ it is a valid point below the U-line.
Group symbol and name. With $LL=40<50$ (low plasticity, "L") and $PI>7$ plotting above the A-line: $$\boxed{\text{Group symbol } CL, \text{ Group name "Lean Clay"}}$$
(b) Suitability as a landfill cover. A daily/final cover layer needs low hydraulic conductivity (typically $k\le 1\times10^{-7}$ cm/s when compacted) so it sheds infiltration and limits leachate generation, while still being workable and resistant to desiccation cracking. A lean clay (CL) with $PI=20$ is on the low-plasticity side of the clay range: it has enough fines and plasticity to compact to a low-permeability layer, but its moderate $PI$ (versus a high-plasticity CH clay with $PI\gtrsim 30$) makes it far less prone to shrink–swell desiccation cracking on drying — cracks that would otherwise become preferential leachate/gas pathways. Check: this assumes compaction near optimum moisture content on the wet side, as is standard cover-construction practice; an uncompacted or overly dry placement would not achieve the low permeability implied here. On balance, this soil is suitable as landfill cover material.
