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Cylindrical specimen, 95% compaction

Short answer

Assuming the printed wet weight is 30 N, volume is 1.571×10⁻³ m³. Dry unit weight γ_d = 19.10/1.15 = 16.61 kN/m³; void ratio e = 0.565; saturation S = 70.3%. For 95% relative compaction, field dry unit weight must be at least 15.78 kN/m³.

A cylindrical specimen of a cohesive soil of 10 cm diameter and 20 cm height was prepared by compaction in a mold. Taking the specific gravity of soil solid as 2.65 and the wet weight of this specimen as 30 kN [as printed] and water content as 15%, find the following: (i) dry unit weight, void ratio and degree of saturation of this cylindrical specimen; (ii) if 95% of relative compaction is to be achieved in the field, what should be the dry unit weight of compacted soil of the same soil specimen in the field.

Given

D=10D = 10 cm, H=20H = 20 cm, G=2.65G = 2.65, w=15%w = 15\%, γw=9.81\gamma_w = 9.81 kN/m³.

Assumption: the weight "30 kN" is a misprint for 30 N, since 30 kN in this small mould would give an impossible unit weight of about 19,000 kN/m³.

Volume and unit weights

V=π4(0.10)2(0.20)=1.571×10−3 m3V = \frac{\pi}{4}(0.10)^2(0.20) = 1.571 \times 10^{-3}\ \text{m}^3 γ=30 N1.571×10−3=19 099 N/m3=19.10 kN/m3\gamma = \frac{30\ \text{N}}{1.571 \times 10^{-3}} = 19\,099\ \text{N/m}^3 = 19.10\ \text{kN/m}^3 γd=γ1+w=19.101.15=16.61 kN/m3\gamma_d = \frac{\gamma}{1+w} = \frac{19.10}{1.15} = 16.61\ \text{kN/m}^3

Void ratio

e=Gγwγd−1=2.65×9.8116.61−1=0.565e = \frac{G\gamma_w}{\gamma_d} - 1 = \frac{2.65 \times 9.81}{16.61} - 1 = 0.565

Degree of saturation

S=wGe=0.15×2.650.565=0.703=70.3%S = \frac{wG}{e} = \frac{0.15 \times 2.65}{0.565} = 0.703 = 70.3\%

(ii) Field requirement

Taking the specimen's dry unit weight as the maximum (laboratory) dry unit weight:

γd,field=0.95×γd,max=0.95×16.61=15.78 kN/m3\gamma_{d,field} = 0.95 \times \gamma_{d,max} = 0.95 \times 16.61 = 15.78\ \text{kN/m}^3

Answer: (i) γd=16.61\gamma_d = 16.61 kN/m³, e=0.565e = 0.565, S=70.3%S = 70.3\%; (ii) field γd≥15.78\gamma_d \ge 15.78 kN/m³.

✓ Answer checked by Claude Opus 5.5 (high effort)

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