What is the purpose of a soil mechanics laboratory?

A soil mechanics laboratory tests soil samples to determine physical and mechanical properties such as strength, compressibility, and permeability. These data are essential for designing foundations, retaining walls, and earthworks, ensuring structures are safe and stable under expected loads.

Which ASTM standards are commonly used in soil testing?

Common ASTM standards include ASTM D1586 for SPT, ASTM D422 for grain size, ASTM D4318 for Atterberg limits, ASTM D698 for Proctor compaction, and ASTM D2850 for unconsolidated-undrained triaxial tests. These ensure consistency and reliability across laboratories.

How does seismic zone affect foundation design?

Seismic zones, defined by ASCE 7, indicate ground motion intensity during earthquakes. Higher zones require stronger foundations, deeper embedment, and sometimes soil improvement to mitigate liquefaction risk. Our laboratory provides dynamic soil properties for seismic analysis.

Gradation Band Calculator — AASHTO Bases and Subbases

The AASHTO M 43 defines specific gradation bands for each type of pavement structural material: granular base CBR ≥ 80, subbase CBR ≥ 40, wearing courses, structural fills, and leveling courses. This calculator compares the gradation curve of a sample against the bands of of of the Manual, identifying on which sieves the sample complies and on which it falls outside. Essential for quality control laboratories in road works and urbanization projects regulated by the highway agency.

What is a gradation band?

A band is a range of allowable % passing on each standardized sieve. The band defines the upper (finer, higher % passing) and lower (coarser) limits to ensure adequate mechanical and hydraulic behavior. Bases with too much fines behave like clay and retain water; with too much coarse material they do not compact and are unstable. AASHTO uses specific bands based on guaranteed CBR. Other standards include AASHTO M 147 (subbase/base materials) and ASTM D2940.

Typical bands AASHTO M 43

Stabilized base CBR ≥ 80 (NMAS 50 mm) — :

50 mm (2"): 100 %; 40 mm (1½"): 80-100 %; 25 mm (1"): 60-85 %; 10 mm (3/8"): 40-65 %; 5 mm (N°4): 30-55 %; 2 mm (N°10): 20-40 %; 0.42 mm (N°40): 10-25 %; 0.075 mm (N°200): 3-10 %

Subbase CBR ≥ 40 (NMAS 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 wear ≤ 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 (N°4)	45	30-55	Yes
2 mm (N°10)	30	20-40	Yes
0.42 mm (N°40)	22	10-25	Yes
0.075 mm (N°200)	11	3-10	NO (exceeds)

Additional tests: PI = 4 (complies, ≤ 6); SE = 48 (DOES NOT comply, requires ≥ 50); LA wear = 28 % (complies). The material is out of band only on the N°200 sieve (11 % vs 10 % maximum) and the Sand Equivalent (48 vs 50 minimum). Interpretation: the sample has a marginal excess of fines that 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 N°200 and SE ≥ 55. If accepted with minor deviation, it must be documented in the protocol of the highway agency / AASHTO with inspector approval.

Result: 2 sieves out of band (N°200 excess fines, insufficient SE). Material DOES NOT comply with CBR 80; wash or blend.

Interpretation of results

Being out of 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 N°200 sieve (fines) is the most critical because it controls hydraulic behavior: 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 CBR loss.

Reference standards

AASHTO M 43
ASTM D2940 — Specification for Graded Aggregate Material for Bases or Subbases
AASHTO M 147 — Materials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses
ASTM D2419 — Sand Equivalent Value of Soils and Fine Aggregate

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 highway agency and AASHTO specify 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 behavior, the inspector may accept it with a protocol. If 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, loses quality). SE measures contamination. Both are mandatory in the Highway Manual along with the gradation analysis.

How do I test gradation?

Standard ASTM D6913: sample quartering, washing over N°200 sieve to remove washable fines, drying, mechanical sieving with standard series. Cumulative % passing per sieve is reported. The test takes 4-6 hours per sample in a certified laboratory.