Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design

Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design Soil improvement and ground modification methods chapter 3 soil mechanics basics, field investigations, and preliminary ground modification design

CHAPTER Soil Mechanics Basics, Field Investigations, and Preliminary Ground Modification Design The first half of this chapter provides a brief overview of soil mechanics fundamentals such as soil strength, compressibility (settlement), and fluid flow (permeability) topics, as they pertain to some of the basic parameters and properties that are used to evaluate the engineering response of soils Also included is a brief discussion of some field and laboratory methods typically used to obtain these values The second half of the chapter is principally dedicated to the information that should be obtained from typical site or field investigations and explorations in order to provide the engineer with the parameters necessary to perform analyses and initiate preliminary ground improvement selection It is from this data (typically contained in boring logs, soil test results, and geotechnical reports) and correlations with soil characteristics that many ground improvement designs are formulated 3.1 SOIL MECHANICS FUNDAMENTALS OVERVIEW Presented here is a brief description of typical soil types and a review of soil mechanics basics that is necessary to understand the fundamentals used in soil improvement and ground modification design This may be elementary for those with a strong background and/or education in geotechnical engineering, but will provide others with the background necessary for understanding the concepts and methods described throughout the remainder of this text 3.1.1 Soil Type and Classification Generally, most soil can be characterized as being made up of either or both of two distinctive types of grains “Rounded” or “bulky” grains have a relatively small surface area with respect to their volume, similar to that of a sphere These soil grains typically have little intragranular attraction (or bonds) and Soil Improvement and Ground Modification Methods © 2015 Elsevier Inc All rights reserved 19 20 Soil improvement and ground modification methods are therefore termed “cohesionless,” referring to lack of tendency to “stick” together Soil with these grain characteristics may also be called “granular.” This soil group includes sands and gravels Clay particles are very different, and are made of very thin plate-like grains, which generally have a very high surface to volume ratio Because of this, the surface charges play a critical role in their intragranular attractive behavior and are termed “cohesive.” As will be discussed in much more depth in later chapters, this difference between grain types has a profound effect on behavior of a soil and the methodology by which improvement techniques can be effective 3.1.1.1 Soil Classification Systems There are a number of different soil classification systems that have been devised by various groups, which vary in definitions and categories of soil type The Unified Soil Classification System (USCS; ASTM D2487) is dominant for most geotechnical engineers, as its soil type designations correlate well with many soils engineering properties Thus, knowing a USCS designation may well be enough for a seasoned geotechnical engineer to be able to envision the types of properties such a soil may possess The USCS will be used as the primary classification system throughout this text Another common classification system, derived for use with roadway materials, is the American Association of State Highway and Transportation Officials (AASHTO) system (ASTM D3282, AASHTO M145) The AASHTO classification designations categorize soil types based on their usefulness in roadway construction applications Another classification system is used by the US Department of Agriculture (USDA) for defining soil categories important for agricultural applications The Massachusetts Institute of Technology also developed a soil classification system in which grain size definitions are nearly the same as the AASHTO Table 3.1 and Figure 3.1 depict grain size definitions by various particle-size classification schemes Soil classifications are typically limited to particle sizes less than about 76 mm (3 in) Soil type and classification usually begins with analyzing the sizes of grains contained, followed by further defining the characteristics of the clayey portion (if any) and/or distribution of grain sizes for the coarser, granular portion (if any) The effect of clay content and characteristics of the clay portion play a very important role in affecting the engineering properties of a soil; therefore, soil types and soil classifications may include qualifiers of the finer-grained portion when as little as 5% of the soil consists of finegrain sizes 21 Soil mechanics basics, field investigations, and preliminary ground modification design Table 3.1 Grain Size Definitions by Various Particle-Size Classification Schemes Particle-Size Classifications Grain Size (mm) Name of Organization Gravel Sand Silt Clay Massachusetts Institute of Technology (MIT) US Department of Agriculture (USDA) American Association of State Highway and Transportation Officials (AASHTO) Unified Soil Classification System (US Army Corps of Engineers, US Bureau of Reclamation, and American Society for Testing and Materials) >2 2-0.06 0.06-0.002 2 2-0.05 0.05-0.002 50 are considered high plasticity, while those with LL < 50 are considered low plasticity A special dual designation of CL-ML is given for soils above the A-line and PI 24 Soil improvement and ground modification methods 70 CH Plasticity Index (%) 60 50 A-Line 40 30 CL MH and CH 20 CL-ML ML and OL 10 ML 20 40 60 80 100 Liquid limit Figure 3.3 Plasticity chart for fine-grained soils 3.1.1.4 Unified Soil Classification System The USCS was originally developed by Casagrande in the 1940s to assist with airfield construction during World War II (Das, 2010) and has been modified a number of times since In order to classify a soil according to the USCS, a number of relatively simple steps must be followed Only one to three simple index tests need to be performed in order to fully classify a soil: a sieve analysis, and/or a LL test, and a PL test In the USCS, soil is generally classified by a two-letter designation (Note: Under special circumstances explained later, a soil may fall in between designations and will be given a dual classification.) The first letter denotes the primary designation and identifies the dominant grain size or soil type The primary designations are G, gravel; S, sand; M, silt; C, clay, O, organic, and Pt, peat (a highly organic soil) The second letter denotes a qualifier that provides further information regarding more detailed information on the makeup and characteristics of the soil Coarse-grained soils are defined as those where more than 50% of the soil is retained on the No 200 sieve According to the USCS, coarse soil grains retained on the No sieve (nominal opening size of 4.75 mm) are defined as gravel while those grains passing the No and retained on the No 200 sieve are defined as sand A coarse-grained soil is defined as gravel or sand depending on the dominant grain size percentage of the coarse fraction of the soil (where the coarse fraction is the cumulative percentage coarser than the No 200 sieve) For example, if more than 50% of the material coarser than the No 200 sieve is retained on the No sieve, then the soil is classified as gravel (G) If 50% or more of the material coarser than the No 200 sieve passes the No sieve, then the soil is classified as sand (S) Soil mechanics basics, field investigations, and preliminary ground modification design 25 For coarse-grained soils (G or S), the second qualifier denotes the type of gradation (P, poorly graded; W, well-graded) or the type of fine-grained soil contained if significant (M or C), so that coarse-grained soils will generally be classified with designations of GP, GW, GM, GC, SP, SW, SM, or SC As mentioned earlier, fine-grained soils (“fines”) become significant to the engineering properties and soil characteristics when as little as 5% by weight is contained According to USCS, when less than 5% fine-grained material is present in a soil, fines are insignificant, and the second qualifier should pertain to the gradation characteristics according to the definitions provided below The definition of well-graded versus poorly graded is a function of various grain sizes as determined by the grain size distributions The definition of well-graded is based on two coefficients determined by grain sizes taken from the gradation curves These are the coefficient of uniformity (Cu) and coefficient of curvature (Cc) If one looks at a gradation curve for a specific soil, there is a grain diameter (size) where a certain percentage of the material grains are smaller This is grain size for a given “percent finer.”For example, if 30% of the grains of a material are smaller than mm, then the grain size for 30% finer is equal to mm This is designated D30 Cu and Cc are defined as: D60 Cu ¼ (3.1) D10 Cc ẳ D30 ị2 D60 D10 (3.2) For a soil to be designated as well-graded, the following must hold true: < C c < and C u ! ðfor sandÞ, C u ! ðfor gravelÞ If either of these criteria fails, then the soil is designated as poorly graded If more than 12% of the soil is determined to be fine grained by sieve analysis, then the second qualifier refers to the type of fines present (C or M), as the soil characteristics and behavior will likely be more affected by the characteristics of the fine-grained material contained than the type of gradation The “type” of fines is determined by classifying the fine-grained portion of the soil, and using the primary designation of those results from the plasticity chart (Figure 3.3), which provides information on the characteristics of the fine-grained fraction For soils that contain between 5% and 12% fines, both the gradation type and properties of the fines may have important contributions to the engineering characteristics of the soil Therefore, a dual classification is used whereby secondary qualifiers for both gradation and type of fines are used in addition to the primary designation for the soil For instance, a soil that is primarily a well-graded sand but contains fines that plot 26 Soil improvement and ground modification methods above the A-line (clay) will be given a dual classification of SW-SC Possible combinations for dual soil classifications would be: GW-GC, GW-GM, GP-GC, GP-GM, SW-SC, SW-SM, SP-SC, and SP-SM Fine-grained soils (those where more than 50% of the soil passes the #200 sieve) are defined according to the plasticity chart shown in Figure 3.3 Most fine-grained soils will have a primary designation based on the LL versus PI values and their relationship to the “A-line” on the chart, with secondary designation as high (H) or low (L) plasticity, determined by whether the LL is above or below 50, respectively Special cases for fine-grained soils are organic (O) designations OL and OH Soils are determined to be organic based on changes in the LL as determined before and after oven drying Other special cases of classification for fine-grained soils occur with low PI and LL values as seen on the plasticity chart (and described previously) AASHTO soil classification of fine-grained soils also uses a variation of a plasticity chart (see ASTM D3282) Table 3.2 provides criteria for assigning USCS group symbols to soils Currently, ASTM D2487 utilizes the group symbol (two-letter designation) along with a group name, which can be determined using the same information gathered for classification designation, but adds a more detailed description that further elaborates on gradation So for a complete classification and description including group name, one must know the percentages of gravel, sand and fines, and type of gradation (all based on sieve analyses), as well as LL and PI for fine-grained portions of the soil Flowcharts for the complete USCS classifications for coarse-grained and finegrained soils are given in Figures 3.4 and 3.5 respectively 3.1.2 Principal Design Parameters In order to develop a plan of approach for designing a practical and economical solution, a geotechnical engineer must first initiate a stepwise process of identifying fundamental project parameters These include: (1) establishing the scope of the problem, (2) investigating the conditions at the proposed site, (3) establishing a model for the subsurface to be analyzed, (4) determining required soil properties needed for analyses to evaluate engineering response characteristics, and (5) formulating a design to solve the problem A number of engineering parameters that play critical roles in how the ground responds to various applications and loads typically need to be determined for each situation Values of each parameter may be evaluated by field or laboratory tests of soils, or may be prescribed by design guidelines Fundamental to applicable analyses and designs are input of reasonably accurate parameters that provide an estimate of response of the ground to expected loading conditions Some of the parameters forming the basis of design applications are reviewed here Table 3.2 Criteria for Assigning USCS Group Symbol (after ASTM D2487) USCS Group Symbol Criteria Coarse-grained soils More than 50% retained on No 200 sieve (coarse fraction) Fine-grained soils 50% or more passes No 200 sieve Highly organic soils Major Classification Gravels More than 50% of coarse fraction retained on No sieve Sands 50% or more of coarse fraction passes No sieve Silts and clays Liquid limit less than 50 Soil Description Specific Criteria Group Symbol Clean gravels Less than 5% finesb Gravels with fines More than 12% finesb Cu ! and Cc 3a Cu < and/or > Cc > 3a PI < or plots below “A” line PI > and plots on or above “A” line GW GP GM GC Clean sands Less than 5% finesc Sands with fines More than 12% finesc Cu ! and Cc 3a Cu < and/or > Cc > 3a PI < or plots below “A” line PI > and plots on or above “A” line SW SP SM SC PI > and plots on or above “A” line PI < or plots below “A” line < PI < and plots on or above “A” line Liquid limit ðoven driedÞ Significant organics < 0:75 Liquid limit ðnot driedÞ Inorganic PI plots on or above “A” line Silts and clays PI plots below “A” line Liquid limit Liquid limit ðoven driedÞ 50 or more Significant organics < 0:75 Liquid limit ðnot driedÞ Primarily organic matter, dark in color, and organic odor Inorganic D60 ðD30 Þ2 ; Cc ¼ D10 D60 Â D10 b Gravels with 5-12% fine require dual symbols: GW-GM, GW-GC, GP-GM, GP-GC c Sands with 5-12% fine require dual symbols: SW-SM, SW-SC, SP-SM, SP-SC a CL ML CL-ML OL CH MH OH Pt Soil mechanics basics, field investigations, and preliminary ground modification design Major Category Cu ¼ 27 28 Soil improvement and ground modification methods Group symbol GW GP GW-GM GP-GC GP-GM GC-GC GM GC GC-GM SW SP SW-SM SW-SC SP-SM SP-SC SM SC SC-SM Group name