Soils Investigation

• Buildings are supported by soil• Engineering properties of soil are highly

variable• Engineers need reliable soil information

for structural and site design

Why Test the Soil?

The knowledge gained from soil investigation allows the engineer to make estimates for:

Bearing Capacity of the soil Settlement of the Foundation (amount and rate) Earth Pressure – both lateral and vertical Drainage characteristics Wastewater Disposal limitations

What Is Soil?

GRAVELGRAVEL SANDSAND

ClayClay SiltSilt

Minerals

• Preliminary Information: USDA Web Soil Survey http://websoilsurvey.nrcs.usda.gov/

• Soil Testing/Analysis– Previous soils investigations on or near the

site– Site inspection and simple soil testing– Soil borings taken at proposed foundation

locations

• Local Building Department or Other Codes and Regulations

Soil Information

http://websoilsurvey.nrcs.usda.gov/

USDA Web Soil Survey

Soi

l Bor

ing

Log

Unified Soils Classification System

• (USCS) is a method for identifying and grouping soils

• First developed by Casagrande for military construction of airfields

Soil Testing

−Coarse-Grained −Fine-Grained −Highly Organic

Only particles sizes smaller than 3 inches are considered in the USC System.

Unified Soil Classification System

• Can vary with moisture content• May indicate the presence of certain

chemicals or impurities

– Dark brown /black may indicate organic material

– Gray, olive green indicates inorganic soils– Red or yellow may indicate iron oxides– Gray-blue or gray-yellow indicates poor

drainage– White to pink may indicate silica, calcium

carbonate, or aluminum compounds

Soil Color

• Coarse-Grained Soils described by grain size

• Fine-Grained Soils described on the basis of their plasticity

Coarse vs. Fine-Grained Soils

13

Gravels range from 3 inches down to the size of peas

Silt and clay can pass through the #200 sieve

Sieve Analysis

Sieves

#40 sieve (similar to window screen)

#4 sieve (¼ in. squares, similar to hardware cloth)

Gravel SandBoulder Cobbles Silt &

Coarse Fine Coarse Medium Fine

>12” 12” 3” ¾” #4 #10 #40 #200 <#200

Clay

NOTE: Particles finer than fine sand (#200 sieve) can not be seen by the naked eye at a distance of 8 inches.

Grain Size (Gradation)

Pea sized Rock salt sized Sugar sized

Baseball sized

Coarse-Grained Soils can be defined by how the particle sizes are distributed within the soil sample.

• Well-Graded Soils provide a good representation of all particle sizes from the largest to smallest.

Graded SoilsGravel and Sand

Coarse-Grained Soils can be defined by how the particle sizes are distributed within the soil sample.

• Poorly-Graded Soils Uniformly Graded – Soil particles are nearly

the same size.

Gap Graded – Contains both large and small particles, but the graduation continuity is broken by the absence of some particle sizes

Graded SoilsGravel and Sand

• Clean soil contains little or no fines (<5%).

• Dirty soil contains an appreciable amount of fines (>12%).

• Silty • Clayey

Clean and Dirty

Gravel and Sand

Fine-grained soils are classified by the plasticity of the soil.

Plasticity refers to the consistency of fine-grained soils as the water content varies.

Plasticity

Clay and Silt

Soils Groups

Soil Type Gradation Plasticity

Gravel – G

Sand – S

Silt – M

Clay – C

Organic – O

Peat – Pt

Well Graded – W

Poorly Graded – P

High Plasticity – H

Low Plasticity – L

These letters are used in combination to indicate soil classifications.

• Coarse-grained soil (granular soil) has more than half of the soil grains visible to the naked eye– If the percentage of GRAVEL and SAND is

greater than 50% of sample, then the soil is a course-grained or granular soil.

SAND if more than half of the coarse grains are smaller than #4 sieveGRAVEL if more than half of the coarse grains are larger than #4 sieve

Sieve Analysis Results

USCS Classification for Coarse-Grained Soils

Fine-grained soils (cohesive soils) contain greater than 50% SILT and CLAY particles. • In addition to the sieve analysis, the following

tests will be performed to determine the plasticity characteristics of the fine-grained soils (in lieu of LL and PL).

Dry Strength – crushing characteristicsDilatancy – reaction to shakingToughness – consistency near plastic limit

Sieve Analysis Results

USCS Classification for Fine-Grained Soils

Soils Test Results for Fine-Grained Soils

FINE GRAINED

SOILS

Dry Crushing Strength Dilatancy Toughness Soils Type

None to slight Rapid Low ML

Medium to high None to very slow Medium to high CL

Slight to medium Slow to none Medium MH

High to very high None High CH

Highly Organic

Soils

Identifiable by color, odor, spongy feel, and/or fibrous texture

OL, OH,and Pt

In lieu of dry strength, dilatancy, and toughness, ATTERBERG LIMITS can be used to classify fine-grained soils.

– Plastic Limit (PL) – lowest moisture content at which soils can be rolled into 1/8 in. dia. thread without breaking

– Liquid Limit (LL) – minimum moisture content at which soil will flow when a small shear or cutting force is applied

– Plastic Index (PI) – difference between the LL and PL

PI = LL - PL

Clay and Silt

Plasticity

USCS Soil Classification Chart

Plasticity Chart

• Sand and gravel – Best

• Medium to hard clays – Good

• Soft clay and silt – Poor

• Organic silts and clays – Undesirable

• Peat – No Good/Avoid

Soil Classifications for Foundations

Soil Type Allowable Bearing(lb/ft2)

Drainage

BEDROCK 4,000 to 12,000 PoorGRAVELS 3,000 GoodGRAVELS w/ FINES 3,000 Good SAND 2,000 GoodSAND W/ FINES 2,000 GoodSILT 1,500 MediumCLAYS 1,500 MediumORGANICS 0 to 400 Poor

Estimated Soil Bearing Capacities

Sowers, G.F. (1979). Introductory soil mechanics and foundations: geotechnical engineering. New York, NY: Macmillan Publishing.Lindeburg, M. R. (1994). Civil engineering reference manual (9th ed.). Belmont, CA: Professional Publications, Inc.Guthrie, P. (2003). Architects portable handbook: First step rules of thumb for building design. NY:

McGraw-Hill.Unified soil classification. Retrieved August 5, 2004,fromhttp://www.hydro.unr.edu/homepages/benson/

classes/hydro/uscs.html

Resources

U.S. Army Corps of Engineers. (1953). The unified soil classification system. U.S. Army Technical

Memorandum, No.3-357. US Army unified soil classification system. Retrieved August 5, 2004, from http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/5- 472/apb.pdf

Resources

Fox, A. (photographer). University of South CarolinaU.S. Army Corp of Engineers. Technical Manual FM5-410 Soils Engineering

Image Sources