What standards does your soil mechanics laboratory follow?

Our laboratory operates in accordance with British Standards (BS 1377) and Eurocode 7. All testing procedures are accredited under ISO 17025, ensuring compliance with UK regulatory requirements for geotechnical investigations.

How long does a typical soil testing program take?

Standard classification tests such as granulometry and Atterberg limits are completed within 5–7 working days. Advanced triaxial and consolidation tests may require 2–4 weeks, depending on the number of samples and drainage conditions.

Do you provide services for small-scale projects?

Yes, our firm supports projects of all sizes, from residential extensions to major infrastructure. We offer flexible sampling kits and remote consultation to accommodate smaller budgets while maintaining full technical rigor.

Particle Size Distribution Band Calculator — BSI standard Bases and Subbases

The BSI standard defines specific particle size distribution bands for each type of pavement structural material: granular base CBR ≥ 80, subbase CBR ≥ 40, wearing courses, structural fills, and levelling layers. This calculator compares the particle size distribution curve of a sample against the bands of the relevant BSI standard, identifying on which sieves the sample complies and on which it falls outside. Essential for quality control laboratories in road works and urbanisation projects regulated by the BSI standard.

What is a particle size distribution band?

A band is a range of allowable % passing on each standardised sieve. The band defines the upper (finer, higher % passing) and lower (coarser) limits to ensure adequate mechanical and hydraulic behaviour. Bases with too much fines behave like clay and retain water; those with too much coarse material do not compact and are unstable. The BSI standard uses specific bands according to guaranteed CBR. Other standards include AASHTO M 147 (subbase/base materials) and BS 1377-2 (+ BS EN ISO 17892-4).

Typical bands BSI standard

Stabilised base CBR ≥ 80 (NMPS 50 mm) — the relevant BSI standard:

50 mm (2"): 100 %; 40 mm (1½"): 80-100 %; 25 mm (1"): 60-85 %; 10 mm (3/8"): 40-65 %; 5 mm (No. 4): 30-55 %; 2 mm (No. 10): 20-40 %; 0.42 mm (No. 40): 10-25 %; 0.075 mm (No. 200): 3-10 %

Subbase CBR ≥ 40 (NMPS 63 mm):

63 mm: 100 %; 50 mm: 80-100 %; 25 mm: 55-85 %; 10 mm: 35-65 %; 5 mm: 25-50 %; 2 mm: 15-35 %; 0.42 mm: 8-22 %; 0.075 mm: 3-12 %

Additional criteria: PI ≤ 6 (base); PI ≤ 10 (subbase); Sand equivalent SE ≥ 50 (base), ≥ 30 (subbase)

Physical requirements: Los Angeles abrasion ≤ 30 % (base) or ≤ 40 % (subbase); MgSO₄ soundness ≤ 12 %

Calculation example

Input data — verification of granular base, route G-35 La Araucanía
Sieve	% Passing sample	Band CBR 80	Complies
50 mm (2")	100	100	Yes
40 mm (1½")	95	80-100	Yes
25 mm (1")	78	60-85	Yes
10 mm (3/8")	62	40-65	Yes (at limit)
5 mm (No. 4)	45	30-55	Yes
2 mm (No. 10)	30	20-40	Yes
0.42 mm (No. 40)	22	10-25	Yes
0.075 mm (No. 200)	11	3-10	NO (exceeds)

Additional tests: PI = 4 (complies, ≤ 6); SE = 48 (DOES NOT comply, requires ≥ 50); LA abrasion = 28 % (complies). The material is outside the band only on the No. 200 sieve (11 % vs 10 % maximum) and the Sand Equivalent (48 vs 50 minimum). Interpretation: the sample has a marginal excess of fines which reduces SE below the limit. Options: reject the material, or wash the stockpile to reduce fines (removes particles < 0.075 mm), or blend with cleaner material until reaching 8-9 % passing No. 200 and SE ≥ 55. If accepted with a minor deviation, it must be documented in a protocol of the BSI standard with the inspector's approval.

Result: 2 sieves outside band (No. 200 excess fines, insufficient SE). Material DOES NOT comply with CBR 80; wash or blend.

Interpretation of results

Being outside the band on 1-2 sieves close to the limit is common and acceptable with an improvement plan. If the deviation is large (> 3-5 % of the limit value) or on multiple sieves, the material is unacceptable. The No. 200 sieve (fines) is the most critical because it controls hydraulic behaviour: excess fines causes loss of strength with moisture and pumping under traffic. SE is a proxy for the quality and quantity of plastic fines; low SE means contaminating clay and correlates with swelling and loss of CBR.

Reference standards

BSI standard — relevant section
BS 1377-2 (+ BS EN ISO 17892-4) — Aggregates for mortars and concretes
BS 1377-2 (+ BS EN ISO 17892-4) — Specification for Graded Aggregate Material for Bases or Subbases
AASHTO M 147 — Materials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses
BS 1377-2 (+ BS EN ISO 17892-4) — Sand equivalent

Frequently asked questions

Which band do I choose for my project?

It depends on the required structural quality: CBR 80 for high-traffic asphalt pavement bases; CBR 40 for subbase and low-volume roads; CBR 20 minimum for improved subgrade. The BSI standard specifies the band in the contract technical specifications.

What happens if I do not comply on one of the sieves?

If the deviation is small (± 2-3 % of the limit) and does not affect behaviour, the inspector may accept it with a protocol. If it is larger or on multiple sieves, reject. Solutions: wash, blend with another quarry, sieve to remove extreme sizes. If nothing works, change the material.

Do I always need PI and SE?

Yes for structural pavement material. PI measures fines plasticity (if PI > 6 the fines are active clay, quality decreases). SE measures contamination. Both are mandatory in the BSI standard together with particle size analysis.

How do I test the particle size distribution?

Standard BS 1377-2 (+ BS EN ISO 17892-4): sample quartering, washing on No. 200 sieve to remove washable fines, drying, mechanical sieving with standard series. The cumulative % passing per sieve is reported. The test takes 4-6 hours per sample in a certified laboratory.