soils investigation. buildings are supported by soil engineering properties of soil are highly...
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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
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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