Section 4: Soil and Water Conservation Engineering
Soil and Water Erosion
Subtopic: Biological and engineering measures to control erosion
Question Statement:
Bench terraces are planned on a 15% hill slope. The vertical interval is 2.5 m, and the riser has a slope of 1:1. The earthwork in cutting is equal to the earthwork in filling. The volume of earthwork per hectare (in m³·ha⁻¹) is ________ (Rounded off to the nearest integer)
Official Answer Key:
2655 to 2658
Step-by-Step Solution & Explanation
Step 1 — horizontal interval between successive terraces:
W = VI/land slope = 2.5/0.15 = 16.667 m
Step 2 — deduct the riser projection. The riser is 2.5 m high at a 1:1 slope, so its horizontal projection is 2.5 m. The usable flat bench width is:
b = 16.667 − 2.5 = 14.167 m
Step 3 — cross-sectional area of cutting for a balanced cut-and-fill bench terrace. With the cut depth at the inner edge equal to D/2 over half the bench width:
Area = ½ × (b/2) × (D/2) = b·D/8
= 14.167 × 2.5/8 = 4.427 m³ per metre run of terrace
Step 4 — total terrace length in one hectare:
L = 10,000/W = 10,000/16.667 = 600 m
Step 5 — earthwork per hectare:
V = 4.427 × 600 = 2656 m³·ha⁻¹
Note: omitting the riser projection (Step 2) gives 3125 m³·ha⁻¹, outside the official range — the 1:1 riser data must be used.
SECTION 5: IRRIGATION AND DRAINAGE ENGINEERING
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