In a soil specimen, 51% of particles are passing through 4.75 mm IS sieve and 4% of particles are passing through 75 microns IS sieve. Its uniformity coefficient is 8 and coefficient of curvature is 2. As per IS classification, this soil is classified as
- ASP
- BGP
- CSW
- DGWCorrect
Solution
Concept
Soil classification depends on grain size distribution (sieve analysis) and consistency limits. For coarse-grained soils with <5% fines, classification is based on Cu and Cc values.
Formula
Cu = D60 / D10; Cc = (D30)² / (D60 × D10)
Step-by-step solution
1. Determine Coarse vs Fine: 4% passes 75 microns, so 96% is coarse-grained (>50% coarse, so it is coarse-grained). 2. Determine Gravel vs Sand: 51% passes 4.75 mm, meaning 49% is retained on 4.75 mm. The coarse fraction is 96%. Gravel fraction = 49%, Sand fraction = 96% - 49% = 47%. Since Gravel (49%) > Sand (47%), the soil is Gravel (G). 3. Check Fines: Fines = 4% (< 5%), so classification depends only on gradation (Cu and Cc). 4. Check Gradation: For Gravel to be Well-graded (GW), Cu > 4 and 1 < Cc < 3. Given Cu = 8 (>4) and Cc = 2 (between 1 and 3). 5. Conclusion: The soil is Well-Graded Gravel (GW).
Explanation
The soil is classified as GW because more than 50% of the coarse fraction is retained on the 4.75 mm sieve (making it gravel), the fines content is less than 5%, and both the uniformity coefficient (Cu > 4) and coefficient of curvature (1 < Cc < 3) satisfy the criteria for being well-graded.
Why the other options are wrong
A: SP is Poorly Graded Sand; requires sand > gravel and failing Cu/Cc criteria. B: GP is Poorly Graded Gravel; requires failing the Cu > 4 or 1 < Cc < 3 criteria. C: SW is Well-Graded Sand; requires sand fraction to be greater than the gravel fraction.
Shortcut
If fines < 5%, ignore 'M' or 'C' suffixes. If Cu > 4 (for gravel) and Cc is 1-3, it's always 'W' (Well-graded).
Common mistake
Correct answer: D) GW