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Soils Investigation. Why Test the Soil?. Buildings are supported by soil Engineering properties of soil are highly variable Engineers need reliable soil information for structural and site design. The knowledge gained from soil investigation allows the engineer to make estimates for:
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Why Test the Soil? • Buildings are supported by soil • Engineering properties of soil are highly variable • Engineers need reliable soil information for structural and site design
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 • Drainagecharacteristics • Wastewater Disposallimitations
Minerals SAND GRAVEL Clay Silt
Soil Information • 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
USDA Web Soil Survey http://websoilsurvey.nrcs.usda.gov/
Soil Testing Unified Soils Classification System • (USCS) is a method for identifying and grouping soils • First developed by Casagrande for military construction of airfields
Unified Soil Classification System • Coarse-Grained • Fine-Grained • Highly Organic Only particles sizes smaller than 3 inches are considered in the USC System.
Soil Color • 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
Coarse vs. Fine-Grained Soils • Coarse-GrainedSoils described by grain size • Fine-Grained Soils described on the basis of their plasticity
Gravels range from 3 inches down to the size of peas Sieve Analysis Silt and clay can pass through the #200 sieve
Sieves #4 sieve (¼ in. squares, similar to hardware cloth) #40 sieve (similar to window screen)
Gravel Sand Boulder CobblesSilt & Coarse Fine Coarse Medium Fine >12” 12” 3” ¾” #4 #10 #40 #200 <#200 Clay Baseball sized Grain Size (Gradation) Pea sized Rock salt sized Sugar sized NOTE: Particles finer than fine sand (#200 sieve) can not be seen by the naked eye at a distance of 8 inches.
Gravel and Sand Graded Soils Coarse-Grained Soils can be defined by how the particle sizes are distributed within the soil sample. • Well-Graded Soils providea good representation of all particle sizes from the largest to smallest.
Gravel and Sand Graded Soils 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
Gravel and Sand Clean and Dirty • Clean soil contains little or no fines (<5%). • Dirty soil contains an appreciable amount of fines (>12%). • Silty • Clayey
Clay and Silt Plasticity Fine-grained soils are classified bythe plasticity of the soil. Plasticity refers to the consistency of fine-grained soils as the water content varies.
Soils Groups These letters are used in combination to indicate soil classifications.
Sieve Analysis Results USCS Classification for Coarse-Grained Soils • 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 sieve GRAVEL if more than half of the coarse grains are larger than #4 sieve
Sieve Analysis Results USCS Classification for Fine-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 characteristics Dilatancy–reaction to shaking Toughness–consistency near plastic limit
Clay and Silt Plasticity 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
Soil Classifications for Foundations • Sand and gravel – Best • Medium to hard clays – Good • Soft clay and silt – Poor • Organic silts and clays – Undesirable • Peat – No Good/Avoid
Resources 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,from http://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
Image Sources Fox, A. (photographer). University of South Carolina U.S. Army Corp of Engineers. Technical Manual FM5-410 Soils Engineering