Textural Identification Properties of Soil Topic D

Visual Classification System Based on visual identification of soils Soils divided into three categories: 1.Granular 2.Non-granular (Clays and Silts) 3.Organic Soils Topic D

Granular Soils Proportion Gradation Shape of gravel and course sand grains noted. – Rounded, subrounded, subangular, angular Topic D

Size Ranges of Soil Component Granular Soils Topic D

Non Granular Soils Material passing No. 200 sieve. Degree of plasticity – controlling factor – Clays (plastic) – Silts (non plastic) Occurs in combination with granular. For granular size soils with clay sizes combine granular and cohesive systems. Topic D

Terms to Describe Plasticity Topic D

Clays vs. Silts Identification Tests Differentiate between non-plastic cohesionless silt and plastic clay soils. Topic D

1. Dilatancy Test Shake, squeeze and release wet soil. – Silt material when squeezed Dry dull surface – When released and shaken Glassy wet surface Decreases with increasing clay content. Topic D

2. Dry Strength Test Portion of soil is dried completely. Press material between fingers. Clays – High Strength – Doesn’t break easily – Predominantly clay materials Silts – Very low strength – Breaks apart easily Topic D

3. Stickiness Clays - High degree of stickiness and smooth in the natural state. Topic D

4. Soil Thread Test Estimates degree of plasticity. Differentiates between organic and inorganic materials. Take sample and add water. Roll out threads 1/8” in diameter. Fold and repeat until thread crumbles. Similar to Plastic Limit test Topic D

5. Thread Diameter Test Adjust water content: – For ball to form & flatten when dropped 2 feet. Roll smallest non-crumbled thread The thinner the thread the more plastic Topic D

Relationships for Identification Topic D

6. Dispersion Test Determine the settling rate – Shake sample in a water filled test tube. – Allow particles to settle: a)Sand and Gravel settle out in seconds to minutes b)Pure silts settle out in less than 1 hour c)Clays remain suspended for hours to days Topic D

How Soil Feels Matters Tough – highly plastic, inorganic fat clay Weak – Leaner sandy, or silty clays. Spongy or elastic – Organic and inorganic Topic D

Organic Soils Contain organic matter Affect the engineering properties of soil. Examples-Topsoil, peat, and organic silt. Topic D

Clay Soils Any color More plastic means more effort to pull apart. High plasticity – Smear looks shiny and waxy. Organic Soils Dark gray, black, and brown Have a Hydrogen sulfide smell (stink). Smear looks dull and silty. Low shear strength Respond to dilatancy test. Topic D

Identification of fine-grained soils Topic D

Describing Soil – Color – Predominant Soil Type – Grain Size – Depositional Features – Degree of Saturation/Relative Moisture – Density (granular soils) or consistency (cohesive soils) Topic D

Describing Soil Major Constituent is upper Case Second (20-50%) constituent is adjective modifying major. – Example (70% gravel, 30 % sand)= sandy GRAVEL Topic D

Constituents 10-20% Come after the main constituent with a hyphen. – Example (55% gravel, 30% sand, 15% silt)= sandy GRAVEL-trace to some silt. Topic D

Constituents 10% or less Indicated by “trace” and come after grain size. – Example (50% Gravel, 30% sand, 12% silt, & 8% clay)= sandy GRAVEL, trace to some silt, trace clay. Topic D

Two Components=20-35% Exist as secondary components “Some” is used for one – Example (52% sand, 27% clay, and 21% silt)= clayey SAND-some silt Topic D

Identification of Soils Topic D

Descriptive Terms Depositional Features Topic D

Descriptive Terms Soil Color Confined to a few basic terms. Terms can be combined to give pairs. Useful descriptive terms are helpful. – Example-Rusty or reddish brown. Topic D

Descriptive Terms Density – Granular Soils Based on Standard Penetration Test (SPT) results performed in test borings. – N-Value - Number of blows of a falling weight required to drive a sample spoon a given distance into soil. Topic D

Descriptive Terms Consistency – Non Granular Based on Unconfined Compression Test – Axially loading a soil sample under a constant strain rate until failure is obtained. (Also can be related to pocket penetrometer testing.) Topic D

Other Descriptive Terms Topsoil: Organic soil at surface Peat: Soft organic soils – Lots of decaying vegetable matter. Varved: Alternating thin layers of silt and clay. Glacial Till: Dense with a variety of grain sizes. Topic D

Other Descriptive Terms Interbedded: Hard to differentiate between various soil types. Loess: Wind blown deposits of silt. – immediately under the topsoil. Fill: Artificially placed material. Topic D

Table 1: Abbreviations Topic D

Classification Procedure Granular Soils – Visual estimation of the % of fraction passing through sieves by wet sieving. Cohesive Soils – Use 1 or more of the identification tests. Topic D