SOILS Identify Basic Soils Properties TEC C03-A01

It is the foundation for Why is Soil important ? It is the foundation for all types of pavement systems and building structures.

What is Soil ... is the entire unconsolidated earthen material that overlies and excludes bedrock; plus the water, air, organic matter, and other substances that may be contained therein.

What is Soil ... } Air Voids Water Soil Particles (Solids)

BASIC PHYSICAL PROPERTIES Engineers evaluate soils by the following Basic Physical Properties: GRADATION of sizes of the different particles BEARING CAPACITY as reflected by soils density PARTICLE SHAPES interlock according to shapes; this affects strength An engineer will also consider the effect of water on various types of soil for the project.

….a soil’s properties help determine the engineering characteristics. PHYSICAL PROPERTIES ….a soil’s properties help determine the engineering characteristics. GRAIN SIZE PARTICAL SHAPE GRADATION DENSITY SPECIFIC GRAVITY MOISTURE CONSISTENCY ORGANIC SOIL

} } GRAIN / PARTICLE SIZE + 3” - 3” + #4 #4 + #200 - #200 COBBLES GRAVELS SANDS FINES (CLAY OR SILT) } }

Soils are divided into groups based upon the size of the particles within the soil mass. Size Group Passing Retained on Example Boulders No Max. Size 12 Inch Cobbles 12 Inch 3 Inch Gravel 3 Inch No 4 Lemon to Pea (Coarse ) 3 Inch 3/4 Inch Lemon to Walnut (Fine) 3/4 Inch No. 4 Walnut to Pea Sands No. 4 No. 200 Pea to Powdered Sugar (Coarse) No. 4 No. 10 Pea to Rock Salt (Medium) No. 10 No. 40 Rock Salt to Table Salt (Fine) No. 40 No. 200 Table Salt to P. Sugar Fines No. 200 No Min. Size (Not discernible to eye) (Silt) No. 200 0.002 (0.005)mm (Clay) 0.002 (0.005)mm No Min. Size

GRAIN/PARTICLE SHAPE WHAT ARE THE 2 TYPES? The shape of soil particles influences the strength and stability of a soil. BULKY Gravels, sands and silt fall into the bulky shape ...relatively equal in all three dimensions. Bulky shapes are further subdivided based upon the effects of weathering. They may be angular, subangular, subrounded, or rounded. WHAT ARE THE 2 TYPES? PLATY Particles of clay exhibit a platy shape ... one dimension is very small compared to the other two.

The shape of soil particles influences the strength and stability of a soil. bulky shapes angular subangular subrounded rounded platy shape

GRADATION WHAT ARE THE 2 TYPES? The distribution of the different size particles in a soil mass is the gradation. Well-graded soils have a good range of all representative particle sizes between the largest and the smallest. Poorly-graded soils contain either a narrow range of particle sizes or lack some intermediate sizes. Uniformly graded have a narrow range of sizes. Gap, step, or skip graded have some intermediate sizes missing or not well represented. WHAT ARE THE 2 TYPES?

GRADATION Well-graded Uniformly graded Gap, step, or skip graded

DENSITY Density is how closely packed or loose the soil structure is. A density structure provides interlocking of larger particles with smaller particles filling the voids between larger particles.

SPECIFIC GRAVITY Ratio between the weight-per-unit volume of a given soil sample and the weight-per-unit volume of a similar volume of water at a stated temperature. Specific Gravity of water is 1, Gold is 18. If a soil has a Specific Gravity of 2.72, then it is 2.72 times heavier than water.

MOISTURE Moisture Content is often the most important factor affecting a soils behavior. It is the proportion of the weight of water to the weight of the solid (dry) mineral grains in the soil.

MOISTURE Moisture has its greatest effect on the behavior of fine-grained soils such as silts and clays. Coarse-grained soils with larger voids Are less susceptible to capillary action Hold less water Tend to drain more freely Clay soil particles retard movement of water Vary from liquid to brick hard Impervious to passage of free moisture

Soil Moisture occurs in five types: SURFACE WATER … water from precipitation or runoff. SUBSURFACE WATER … collected or held in pools or layers beneath surface by a restricting layer of soil or rock. GRAVITATIONAL PULL … seeks lower layer & moves through voids until it reaches a restriction. … voids in soil may form tunnels & tubes and cause water to rise in the tubes CAPILLARY ACTION … water that may be present as thin films. ABSORBED WATER & HYGROSCOPIC MOISTURE

Moisture Movement through Soil Capillarity is the ability of a fine-grained soil to absorb water and transmit it in all directions. The water softens and expands the soil. Permeability refers to the rate at which gravitational water will flow through soil. Granular soils offer little resistance to the flow and are most permeable. Fine-grained soils, particularly clay, are least permeable.

PLASTICITY & COHESION Plasticity is a property of fine-grained soils that allows it to be deformed without cracking or breaking. The Plasticity Index (PI) is used to determine weather soil is cohesive. Not all plastic soils are cohesive. (PURE SILT) Soil plasticity is determined by observing the different physical states a soil passes through as the moisture content changes.

Physical States and Atterberg Limits

CONSISTANCY Consistency- four states of consistency for fined grained soils (liquid, plastic, semisolid and solid). The dividing line between these states of consistency are called the Liquid Limit (LL) and the Plastic Limit (PL). All are quantified in terms of water content (w).

SOIL MOISTURE STATES PLASTIC STATE %W INCREASING %W %W PI PL LL LOW %W HIGH %W PLASTIC STATE SOLID STATE SEMI-SOLID STATE LIQUID STATE PI PL LL PI = LL - PL

Between the liquid and plastic limits is the Physical States and Atterberg Limits Liquid State ... soil will flow freely under its own weight ------------------The liquid limit (LL)--------------- Plastic State ... soil will deform under pressure without crumbling ------------------The plastic limit (PL)--------------- Semisolid or Solid State ... soil is no longer pliable and will crumble under pressure Between the liquid and plastic limits is the plastic range. PI = LL - PL

PLASTICITY CHART MH OR OH ML OR OL CH OR OH CL OR OL 10 20 30 40 50 60 7 4 70 80 90 LIQUID LIMIT (LL) PLASTICITY INDEX (PI) “A” LINE “U” LINE MH OR OH ML OR OL CH OR OH CL OR OL CL - ML 16 PI = 0.9 (LL - 8) PI = 0.73 (LL - 20) LL-PL = PI

ORGANICS Soil having a high content of organic material is described as organic soil. Organics are typically very compressible and has poor load-maintaining properties.

Unified Soil Classification System Field Classification Major Divisions Symbol Field Identification Procedures (Base fractions on estimated weights) GW GP GM GC SW SP SM SC ML CL OL MH CH OH Pt Gravels <5% Fines Sands >12% Silts & Clays LL <50 LL >50 Highly Organic Soils Fine-grained Soils Coarse-grained Soils More than half of coarse fraction is larger than No. 4 sieve smaller than More than half of material is larger than No. 200 sieve More than half of material is smaller than Readily identified by color, odor, spongy feel, and frequently by fibrous texture Wide range in grain sizes, all intermediate sizes substantially represented Nonplastic fines or fines with little plasticity (see ML below) Plastic fines (see CL below) Predominantly one size or some intermediate sizes missing Identification Procedures on Fractions smaller than No. 40 sieve Dry Strength Wet Shake Thread or Ribbon None to slight Medium to high Slight to medium High to very high Quick to slow None to very slow Slow Slow to none None Medium Slight High

WHAT SYSTEM DO WE USE? Why classify Soil ... purpose of a soil classification system is to be able to predict the engineering properties of the soil. WHAT SYSTEM DO WE USE?

Unified Soil Classification System The USCS looks at properties such as: Percentages of gravel, sand, and fines. Shape of the grain-size-distribution curve. Plasticity and compressibility characteristics

Soil Classification gives a good indication of … Strength Drainage Resistance to Frost Action Compressibility (Volume Change) Compaction (Constructability)

USCS Letter Symbols Soils are divided into major groups and further subdivided by characteristics. Soil Groups Soil Characteristics G Gravel W Well graded S Sand P Poorly graded M Silt H High compressibility C Clay L Low compressibility Pt Organic (Peat) O Organic (Silts and clays) L Liquid limit under 50 H Liquid limit over 50

USCS Soil Classification Chart PI > 7 & Plots on or above A Line PI < 4 or Plots below PI Plots Limits Plot in Hatched Area USCS Soil Classification Chart Determine % Passing #200 If 50 % or More of Total Sample It is a Fine-grained Soil If Less than 50 % of Total Sample It is a Coarse-grained Soil Passing #4 Plot Atterberg Test Data Determine Cu & Cc If 50 % or More of Coarse Fraction It is a Sand If Less than 50 % of Coarse Fraction It is a Gravel If Less than 5% of Total Sample If Between 5% & 12% If More than 12% Classify with Dual Symbol Cu > 4 & 1 < Cc < 3 Cu > 6 YES NO Classify as GW GP SW SP GM GC GM-GC SM SC SM-SC GW-GM-GC GW-GM GW-GC GP-GM GP-GC GP-GM-GC SW-SM-SC SW-SM SW-SC SP-SM SP-SC SP-SM-SC ML or MH Fines CL or CH CL-ML ML CH MH CL Liquid Limit < 50 > 50 OH OL Liquid Limit (oven dried) Liquid Limit (not dried) < 0.75 If or PLASTICITY CHART 10 20 30 40 50 60 7 4 LIQUID LIMIT (LL) PLASTICITY INDEX (PI) “A” LINE “U” LINE MH OR OH ML OR OL CH OR OH CL OR OL CL - ML 70 80 90 16 PI = 0.9 (LL - 8) PI = 0.73 (LL - 20)

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