Field (Visual) Classification of Soils

by

Chuang LinDepartment of Civil and Environmental Engineering

University of Alaska Fairbanks

June 6, 2016

2016 Summer Transportation Institute, Fairbanks, AK

Background and Objectives

Soil types

Characterization of fine-grained soils

Characterization of coarse-grained soils

Outline

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Based on Unified Soil Classification System (USCS)

ASTM Standards

Lab classification: ASTM D2487

Field (visual) classification: ASTM D2488

Background

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Recognition

Description

Classification

Objectives

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Recognition - Soil Types

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Boulder and Cobble

Boulder>12 in (>30.5 cm)

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Cobble12 in – 3 in (30.5 cm – 7.6 cm)

Boulder and Cobble

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Coarse3 in – 0.75 in

(75 mm – 20 mm)

Fine0.75 in – 0.2 in

(20 mm – 4.75 mm)

Gravel

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Coarse0.2 in – 0.08 in

(4.75 mm – 2 mm)Medium

0.08 in – 0.02 in(2 mm – 0.4 mm)

Fine0.02 in – 0.004 in

(0.4 mm – 0.075 mm)

Sand

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AngularityShapeColorOdor

Moisture ConditionConsistencyCementation

Range of the Particle SizeMaximum Particle Size

Only for G or S

Only for G or S

Only for C or M

Description-Soil Properties

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Bulky – most cases

Soil particle shape Flaky – mostly clay mineralsNeedle shaped – less common

AngularBulky particles Subangular

SubroundedRounded

Soil Particle Shape

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Flat or elongated?

Rounded or angular?

Shape and Angularity

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impermeable pores (ip)

permeable pores (pp)

solid (s) MC - wetMC - dry

Moisture Content

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MC - wet

MC - dry

What is soil consistency?A method of describing the degree of cohesion and

adhesion between soil particles as related to the resistance of the soil to deform or rupture

Consistency

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StrongModerate

Strong

If it will crumble or break with finger pressure?

Cementation

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Two Broad Categories

Fine-grained: 50% or more finer than No. 200 sieve

M or C, H or L, portion of coarse-grained

Coarse-grained: less than 50% finer than No. 200 sieve

G or S, W or P, portion of fine-grained

Classification-Flow Charts

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Types of Gradations

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Uniformly graded

Well graded

Gap graded

Open graded

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Visual Determination

Well or dense graded

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Visual Determination

Gap graded

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Visual Determination

Uniformly graded

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Visual Determination

Remember that percent is by dry weight. Gravel Particles overpower gradation

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Textual Determination

When wetted thoroughly, coarse-grained soils have a distinct grittiness. Practice on known samples

Easiest method to differentiate fine-grained soil from fine sand

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Tests to Characterize Fine-Grained Soils

Liquid Limit Evaluation (M or C)

High - Greater than 50

Low - Less than 50

Plasticity Evaluation (H or L)

Plastic Index (PI) above or below A-line

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Liquid Limit Evaluation

With small amount of soil sample that passing No. 40 sieve24

Liquid Limit Evaluation

Water penetration rate: fast or slow?25

Liquid Limit Evaluation

Any hydration heat?26

Liquid Limit Evaluation

How many drops of water to penetrate one tablespoon of soil?27

Dilatency Evaluation

When shaken, dilatent soil releases free water and becomes “livery” in appearance 28

Dilatency Evaluation

When Squeezed, dilatant soil becomes dull29

Toughness Evaluation

Reaching plastic limit requires adding considerable dry soil.

Thread can be re-rolled several times after reaching plastic limit.

Thread requires pressure to form.

Thread can be rolled out to lengths > 2”

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Ribbon Evaluation

Length of ribbon Strength Related to clay content Ribbon more than 2 inches

long indicates clay content is >= 40%

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Shine Evaluation

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Dry Strength Evaluation

Very high: cannot be crushed even on hard surface

High: can be crushed only against hard surface

Medium: can be crushed with considerable pressure

Low: crumbles with some pressure

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Typical Behaviors-CH Soil

High liquid limit High toughness - easily roll out thread to 1/8 inch

diameter to length of several inches Zero dilatency Ribbons to more than 1-1/2 inch High dry strength High Shine

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Typical CH soil

Slickensides

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Typical CH soil

Cracks on Surface36

Typical Behaviors-ML Soil

Low liquid limit Slight to no toughness - if a thread can be formed

to 1/8 inch diameter it can’t be reformed Medium to high dilatency Ribbons only slightly if any Low to very low dry strength No Shine (watch out for mica)

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Typical Behaviors-MH Soil

High liquid limit Medium toughness - if a thread can be formed to

1/8 inch diameter it has low strength Low dilatency Ribbons slightly to moderately Low to Medium dry strength Low Shine (watch out for mica)

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Typical Behaviors-CL Soil

Low liquid limit Medium to high toughness – a thread can be

formed to 1/8 inch diameter with medium toughness

Low to none dilatency Ribbons slightly to moderately Medium dry strength Low to Medium Shine (watch out for mica)

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Distinguishing between CL and MH Difficult

MH

CH

CL

MLPI is the same -

Only difference is in LL

50

PI

LL40

Example Problem

A field sample has the following properties: Fine content 60%, gravel 18% and sand 22% Ribbon test < 1.5 in., high dilatency, water penetrates

into soil relatively fast and no heat generation, low dry strength

What is the Group Symbol and Group Name of this sample?Refer to the flow chart in the handout

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Tests to Characterize Coarse-Grained Soils

Visual Examination Determine whether G or S is predominant Determine if clean or dirty

If Clean Determine if poorly or well graded

If Dirty Determine if fines are plastic or not

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Clean or Dirty?

When wetted a clean coarse-grained soil will not leave a stain on your palm. A dirty soil will leave a stain

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Clean or Dirty?

If a soil sample is dropped in a beaker of water, a cloud remaining after about 30 seconds indicates more than 12 % fines

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Well Graded or Poorly Graded?

A well graded sample has a wide range of particle sizes that are about equally distributed by weight

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Well Graded or Poorly Graded?

A gap-graded sample has a range of particle sizes missing. This sample has coarse and fine but no medium size particles

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Well Graded or Poorly Graded?

A poorly graded sample is predominately one size of particle

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Poorly Graded Sand

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Well Graded Sand

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Evaluation of Plasticity of Fines

The Most Useful Field Tests are Toughness and Dilatency Tests

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Example Problem

A field sample has the following properties: Gravel 65%, sand 18% and fines17% easily roll out threads into 1/8 in. dia. to several inches,

low dilatency, water can hardly penetrates into soil and heat generation, strong dry strength

What is the Group Symbol and Group Name of this sample?Refer to the flow chart in the handout

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Thank you!

Questions?

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