What is geotechnical engineering and why is it important in the UK?

Geotechnical engineering assesses ground conditions to design safe foundations, slopes, and retaining structures. In the UK, variable geology and historical land use make thorough site investigation critical to avoid costly failures and ensure compliance with Eurocode 7.

What standards govern geotechnical work in the United Kingdom?

Geotechnical design in the UK follows Eurocode 7 (BS EN 1997) and its National Annex. Ground investigations comply with BS 5930, and SPT testing adheres to ASTM D1586. These standards ensure consistent, reliable results across the country.

How does your firm approach slope stability analysis?

Our firm uses limit equilibrium and finite element methods to evaluate slope stability, considering soil shear strength, groundwater, and surcharge loads. We apply partial factors per Eurocode 7 to deliver safe, economical designs for natural and engineered slopes.

Grain Size Distribution Curve and Gradation Coefficients Calculator

Based on the percentage passing each sieve, this calculator plots the semi-logarithmic grain size distribution curve, interpolates D10, D30 and D60 (diameters corresponding to 10%, 30% and 60% passing) and provides the uniformity coefficient Cu and curvature coefficient Cc. These are central parameters for classifying coarse-grained soils according to BS 5930 (+ BS EN ISO 14688-1/-2), evaluating filters and drains, and verifying the suitability of aggregates for bases and sub-bases according to the Highways Manual of the Ministry of Public Works.

What is the grain size distribution curve and when is it used?

The grain size distribution curve represents the particle size distribution of a soil. D10 (effective size), D30 and D60 are reference points that allow soils to be compared and gradation to be judged. Cu < 4-6 indicates a uniform gradation (little variety of sizes); high Cu plus 1 ≤ Cc ≤ 3 indicates well-graded. It is applied in coarse soil classification, filter design using Terzaghi's criteria, selection of aggregates for concrete, verification of design grading envelopes in highways, and quality control in crushing plants.

Applied Formulas

Uniformity Coefficient: Cu = D60 / D10

Coefficient of Curvature (or Gradation): Cc = (D30)² / (D10 × D60)

Well-graded (gravel): Cu ≥ 4 and 1 ≤ Cc ≤ 3 → GW

Well-graded (sand): Cu ≥ 6 and 1 ≤ Cc ≤ 3 → SW

Logarithmic interpolation of Dx: log(Dx) = log(Da) + ((x − %Pa) / (%Pb − %Pa)) × (log(Db) − log(Da))

where Da, Db are the sizes of adjacent sieves with passing percentages %Pa and %Pb that contain the value x.

Calculation Example

Input data — crushed aggregate, Til Til quarry
Sieve	Aperture (mm)	Cumulative Passing (%)
1"	25.4	100
¾"	19.0	88
⅜"	9.5	58
No. 4	4.75	38
No. 10	2.00	26
No. 40	0.42	14
No. 100	0.15	8
No. 200	0.075	4

D60 falls between ⅜" (58%) and ¾" (88%). Interpolating: log(D60) = log(9.5) + ((60 − 58) / (88 − 58)) × (log(19) − log(9.5)) = 0.978 + 0.067 × 0.301 = 0.998 → D60 ≈ 9.96 mm. D30 falls between No. 10 (26%) and No. 4 (38%): log(D30) = log(2.0) + ((30 − 26) / (38 − 26)) × (log(4.75) − log(2.0)) = 0.301 + 0.333 × 0.375 = 0.426 → D30 ≈ 2.67 mm. D10 falls between No. 200 (4%) and No. 100 (8%): log(D10) = log(0.075) + ((10 − 4) / (8 − 4)) × (log(0.15) − log(0.075)) = −1.125 + 1.5 × 0.301 = −0.673 → D10 ≈ 0.21 mm. With these values Cu = 9.96 / 0.21 = 47.4 and Cc = (2.67)² / (0.21 × 9.96) = 7.13 / 2.09 = 3.41.

Result: D10 = 0.21 mm · D30 = 2.67 mm · D60 = 9.96 mm · Cu = 47 · Cc = 3.4 — gravel with sand, poorly graded (Cc outside 1-3).

Interpretation of Results

Cu = 47 shows a very wide variety of sizes, but Cc = 3.4 falls outside the range 1-3 required by BS 5930 to be considered well-graded. The material has a deficit in the medium range (sizes between No. 4 and ⅜"). Although it contributes to density, it can generate segregation during transport and non-uniform compaction. It is recommended to adjust the dosage in the plant or mix with complementary material.

Reference Standards

BS 1377-2 (+ BS EN ISO 17892-4) — Methods of test for soils for civil engineering purposes — Part 2: Classification tests and determination of particle size distribution
BS 1377-2 (+ BS EN ISO 17892-4) — Methods of test for soils for civil engineering purposes — Part 2: Classification tests and determination of particle size distribution (current)
BS 1377-2 (+ BS EN ISO 17892-4) — Methods of test for soils for civil engineering purposes — Part 2: Classification tests and determination of particle size distribution (hydrometer method for fine fraction)
BS 410-1 — Specification for test sieves — Part 1: Test sieves of metal wire cloth
BS 410-2 — Specification for test sieves — Part 2: Test sieves of perforated metal plate
BSI standard (aggregates for bases and sub-bases)

Frequently Asked Questions

What is the difference between Cu and Cc?

Cu measures the total range of particle sizes (how uniform or heterogeneous the soil is). Cc evaluates the shape of the curve between D10 and D60: if Cc is between 1 and 3, the curve is smooth and continuous; if it is outside, there is a step or gap in the gradation.

Can I obtain D10 if my soil has more than 10% non-sedimented fines?

Not directly. BS 1377-2 (+ BS EN ISO 17892-4) requires hydrometer analysis for the fraction below 0.075 mm when it represents a significant part of the soil. Without a hydrometer, D10 is estimated by extrapolation and loses precision — it is not recommended for formal classification.

Is this curve useful for designing a drainage filter?

Yes, it is the main input. The Terzaghi / USACE criteria require that the D15 of the filter be 4-5 times greater than the D85 of the base soil, and less than 4-5 times the D15 of the base. The complete grain size distribution curve is essential to apply them.

What minimum sieves should my test include?

For BS 5930 classification, at least 3" – ¾" – No. 4 – No. 10 – No. 40 – No. 100 – No. 200 are required. For concrete or pavement aggregates, intermediate sieves are also included according to the specific grading envelope (e.g., ½" and ⅜" for stabilised base).