CE 360 Exam 1
Specific Surface of Clay
- Kaolinite = 10 - 20 m^2/gram - Illite = 65 - 100 m^2/gram - Montmorillonite = 100 - 800 m^2/gram More surface area = greater interaction with water and therefore greater shrink/swell magnitudes
Stokes Laws Assumptions
- all particles are spheres - no influence between particles - no boundary effects from container - no turbulence (laminar flow conditions) - specific gravity of all spheres is the same
SPT correction for granular soils (N1)60
- blow count would increase with depth because of increase in effective overburden pressure in granular soils (sand, gravel)
Coarse Grained Soils
- can be seen with unaided eye - can be characterized by physical characteristics such as color, shape, and structure - engineering behavior governed by inter particle friction
Engineering Significance
- clays of lower charge imbalance will exhibit a reduced tendency for volume change, decreased compressibility, and increased shear strength - clays of high charge imbalance will exhibit a greater tendency for volume change, increased compressibility, and decreased shear strength
Soil Classification (USCS)
- coarsed grained soils classified by grain size - fine grained soils classified by plasticity - grain size measured by sieve and hydrometer analysis - plasticity measured by Atterberg limits - is a two letter system
Cone Penetration Test (CPT)
- cone is pushed into soil and tip resistance qt is measured - sleeve friction fs and PWP can also be measured (piezocone)
Use for Undisturbed Samples
- consolidation - triaxial compression - unconfined compression
Borehole Pressuremeter Test
- cylindrical prober inserted and expanded radially against borehole - give complete in-situ stress strain strength curve and modulus of elasticity (in linear zone II) - difficult to perform - time consuming
Water (Moisture) Content
- defined as mass of water per unit mass of solids - can be expressed in terms of mass of weight w = (Mw / Ms) * 100% or (Ww / Ws) * 100% Mw = mass of water Ms = mass of soil Ww = Mwg = weight of water Ws = Msg = weight of soil g = gravity (9.81 m/s^2)
Particle Size Analysis (Mechanical Analysis)
- determination of particles sizes is the first step towards classification Two general methods: 1.) sieve analysis (for particle sizes larger than 0.075 mm in diameter, retained on #200 sieve) 2.) hydrometer analysis (for particles smaller than 0,075 mm in diameter, passing #200 sieve)
As specific surface area of a clay mineral increases, it causes:
- greater affinity for water - higher plasticity - increased tendency for volume change - decreased strength - greater potential for engineering problems
Boring Methods
- hand augers for shallow depths (10 to 15ft) - continuous flight augers (solid or hollow stem, soil sampling, SPT tests can be performed) - rotary drilling - wash boring - percussion drilled - geoprobe/powerprobe continuous push
Clay Particles
- have flat, plate like shapes - carry net negative charge on their surface due to isomorphous substation and breaks in structural continuity at particles edges) - greater surface area = larger negative charge
Clay Water
- if limited water is available, all clay particles will flocculate around that limited water and will resist movement - if abundant water is available, the water will be further apart and will be less resistant to movement
Compaction
- is the densification of a soil by the use of mechanical energy - process of expelling air from the soil - improves strength - increases bearing capacity of foundations - increases stability of embankment slopes - reduces compressibility - decreases settlement of foundations - reduces permeability
Specific Gravity Range
- nearly all soils have 2.60 < Gs < 2.80 so tests are rarely performed 2.65 = for clean light colored sand of quartz and feldspar 2.72 = for dark colored sand 2.72 = for sand silt clay mixes 2.65 = for clay
Diffuse Double Layer
- negative surface charge of clay particles attract positive pole of water molecules in an attempt to achieve electrical neutrality - the influence of the negatively charged clay surface decreases with distance, eventually becomes negligible - the layer of attached or bound water is the diffuse double layer
Disadvantages of Soil as a Construction Material
- non homogenous - time and space variation in properties - stress strain curve changes - dynamic loading effects - hard to get uniform characteristics - erosion - weathering - presence of rocks and boulders in weak areas - soil vs rock, where are the boundaries - selection of proper equipment
Non-plastic soils
- non-cohesive soils (silts and fine grained soils passing #40) do not solidify upon drying out and do not pass through the plastic state - they change from viscous liquid state to dry granular material abruptly - if LL for such soils can not be found, then specify soil as non plastic and do not perform PL test (number of blows will always be less than 25)
Suggested Preliminary Spacing of Bore Holes
- one story buildings = 75 - 100 ft - multistory buildings = 50 - 75 ft - highways = 750 - 1000 ft - earth dams = 75 - 150 ft - residential subdivision planning = 200 - 300 ft
Fine Grained Soils (silts and clays)
- particles too small to be define by physical means (ex sieving) - made of two basic building blocks ---- silica tetrahedron ---- alumina octahedron - engineering behavior governed by mineralogy of the particles and surface area
Hydrometer Analysis
- performed on soil passing #200 sieve (0.075mm) - for the silt and clay fractions - sedimentation principles are used to measure the size distribution - procedure involves measured the unit weight of a soil water suspension at known times using a hydrometer - percentage of soil particles still in suspension at a given time can then be calculated - directly indicated density around center of bulb in units of grams of soil solids per 1 liter of soil water suspension
Specific Gravity
- ratio of materials density p (unit weight y) to that of water Gs = (ps / pw) = (ys / yw) pw = 1000 kg/m^2 ps = density of solids ys = psg = unit weight of solids g = gravity (9.81)
Advantages of Soil as a Construction Material
- readily available - relatively low cost construction techniques are simple - semi-flexible - can be modified - durable
Clay Building Block #2: Alumina Octahedron
- six hydroxyl (OH -1) ions surrounding a cation of Al, Fe, or MG - alumina octahedrals combine and share hydroxyl ions to form an octahedral sheet
Surface Area and Role of Water
- specific surface significantly affecting engineering behaviors due to differences in the amount of surface area for water to interact with - water molecules are electrically neutral, but dipolar and therefor attracted to surface of clay minerals to neutralize the charge imbalance
Plasticity Chart
- use PI (y-axis) and LL (x-axis) on chart to classify - if point plots below A-line = silt (M) - if point plots above A-line = clay (C) - if LL >_50, soil has a high plasticity (CH or MH) - if LL < 50, soil has low plasticity (CL or ML)
Use for Disturbed Samples
- visual inspection - Atterberg limits - Mechanical analysis (grain-size distribution) - shear strength tests for sands only - proctor compaction tests
General Remarks of Clay Particles
- volume of water in clay is often equal to or greater than volume of soils - water content or moisture content can be greater than or equal to 100% - dissolved ions disrupt clay/water structure - water is strongly attracted to clay particle surface
Liquid Limit Test
- water content at which a pat of soil in the standard cup and cut by the standard groove will flow together at the base when subjected to 25 blows from a 10mm drop at a rate of 2 blows per second
Consistency Limits
- when clays are wetted they become sticky or cohesive and they are plastic Plastic materials can be deformed: - without rebound - without volume change - without cracking or crumbling - clays are plastic within a certain range of water content
Relative Density Descriptions
0 to 15 = very loose 15 to 40 = loose 50 to 70 = medium 70 to 85 = dense 85 to 100 = very dense
Soil Particle Size
1.) Coarse Gravel (4.75 to 75 mm) 2.) Fine Gravel 3.) Coarse Sand 4.) Medium Sand (all sand between 0.075 and 4.75 mm) 5.) Fine Sand 6.) Silt (0.002 to 0.075 mm) 7.) Clay (0.001 to 0.002 mm)
Typical Steps for Size Characterization
1.) Gather existing information about structure 2.) Review existing geotechnical information about the site 3.) Visual inspection of site 4.) Site investigation
Standard Penetration Test (SPT)
1.) Number of blows to drive the split spoon through three 6 inch intervals is recorded 2.) Blow count, N, or standard pentation number is the number of blows for the final 12 inches (the last two 6 inch intervals) 3.) Soil samples are collected and transported in small glass jars or bags 4.) Repeated at 5 ft intervals - good benchmark test - gives disturbed samples
Soils as 3-Phase Systems
1.) Solid phase - particles derived from rocks, organic materials 2.) Liquid phase - usually groundwater, but could also include: - sea water infiltration due to pumping of wells near the ocean - contaminants, chemicals, or leachate leaking from tanks - petroleum 3.) Gas phase - air - methane, carbon dioxide from decomposition of organic material - petroleum vapors - hydrogen sulfide from decomposition of sulfur
USGC Major Categories
1.) coarse grained 2.) fine grained 3.) organic 4.) peat first letter of classification defines this category
Pycnometer Test
1.) find mass of pycnometer with water = Mpw, with distilled deaired water up to the fill mark 2.) place soil and water in empty pycnometer, remove entrapped air by vacuum or boiling 3.) fill pycnometer up to fill mark and find mass of pycnometer + water + soil = Mpws 4.) determine oven dry mass of soil solids, Ms
Structure in Cohesionless Soils
1.) single grained structure - particles are in stable positions - particles shape and size distribution influence density 2.) honeycombed - very high void rations - collapsible structure - relatively fine sand and silt
Liquidity Index
An index between 0 and 1 which identifies a soils in-situ moisture content (w) in relation to the liquid and plastic limits: - LI = 0 corresponds to w = PL - LI = 1 corresponds to w = LL - if w =~ PL (LI =~ 0) clays are strong - if w = ~ LL (LI =~ 1) clays are weak
Atterberg Limits
As moisture content increases: 1.) Liquid Limit (wL) 2.) Plastic Limit (wP) 3.) Shrinkage Limit Liquid State (#1) - deforms easily, like soft butter Plastic State (#2) - deforms without cracking, stiff putty Semisolid State (#3) - deforms permanently, but cracks, like cheese Solid State (#4) - breaks before it will deform, like hard candy
Relationship Between e and n
Between void ratio and porosity - e = Vv/Vs = Vv/(V-Vv) = (Vv/V)/(1-(Vv/V)) = n/(1-n) - or n = Vv/V = e/1+e
Transported Soils
By Wind: Aeloian - Loess - Dune sand By Water - Alluvial (transported by streams) - Marine (clays in ocean bottom, sand on beaches) - Lacustrine (formed by settling in lakes) - Glacial till (formed by action of glacial ice)
Coefficient of Gradation or Coefficient of Curvature, Cc
Cc = (D30 ^2) / (D60 * D10)
USGC first letter (G, S, M, or C)
Coarse - more than 50% of soil is retained on #200 (R200 > 50%) - G = gravel - S = sand Fine - 50% or more passing #200 (F200 >_ 50%) - M = silt - C = clay - O = organic - Pt = peat
Definition of Soil
Das: - an uncemented aggregate of mineral grains and decayed organic matter (solid particles) with liquid and gas in the empty spaces between the solid particles ASTM: - sediments or other unconsolidated accumulated of solid particles produced by the physical and chemical disintegration of rocks and which may or may not contain organic matter
Structures in Cohesive Soil
Dispersed: - repulsive forces on clay faces (e ~ 0.5 to 1.0) Flocculated: - non-salt +edge to -face attraction (e ~ 2 to 4) - salt water: more parallelism
Relative Density, Dr
Dr(%) =(emax - e)/(emax - emin) * 100% emax = max void ratio in loosest state (min density) emin = min void ratio in densest state (max density) e = actual in-situ void ratio
SPT test: corrected blow count N60
Factors effecting blow count N: - hammer efficiency, borehole diameter, sampling method, length of drill rod Corrected Blow Count: 60 = N(nH/60)nB*nS*nR N = measured blow count nH = hammer efficiency (%) nB = correction for borehole diameter nS = sampler correction nR = correction for rod length
Other In-Situ Tests
Flat plate dilatometer - gives coefficient of earth pressure at rest (K0), elastic modulus, shear strength of sands, silts and clays Geophysical tests (nondestructive) - measurement of mechanical waves (seismic refraction surveys, crosshole, downhole, and spectral analysis of surface wave tests) - give elastic properties, usually small strain shear modulus - Electromagnetic techniques (resistivity, EM, magnetometer, radar) - aids in locating underground cavities, buried objects, utility lines
Specific Gravity Tests
For particles passing the #4 sieve: - ASTM D 854-02 Standard Test Methods for Specific Gravity of Soils Solids by Water Pycnometer For particles larger than #4 sieve: - ASTM C 127, Test Method for Specific Gravity and Absorption of Coarse Aggregate
Dual Symbols for Coarse Grained Soils
If soil has 5 to 12% fines, gravels and sands require dual symbols - combine #1 and one from #2 Possible Combos: - Gravel: GW-GM, GW-Gc, GP-GM, or GP-GC - Sand: SW-SM, SW-SC, SP-SM, or SP=SC
USGC second letter - Coarse Grained
If soil has less than 5% fines (clean gravel or sand): W = well graded - Cu > 4 (for gravels), Cu > 6 (for sands) - 1 < Cc < 3 for anything else P = poorly grade - classification will be GW, GP, SW, or SP If soil has greater than 12% fines: - M = silty (if soil plots below A-line on plasticity chart) - C = clayey (if soil plots above A-line) - classification may be GM, GC, SM, or SC - if Atterberg limits plot in hatched area use the dual symbols GC-GM or SC-SM
Second Letter for Fine Grained Soils
L = low plasticity if LL<50 H = high plasticity if LL>50 Examples: - ML, CL, OL, MH, CH, or OH - if fine grained portion of soil plots un hatched area (4 <_ PI <_ 7) its classification is CL-ML
Weight Relationships
Moisture Content - w = Ww/Ws = Mw/Ms Unit Weight - also called moist unit weight or bulk unit weight - common units are lb/ft^3 kN/m^3 - ym = W/V = total weight/total volume
Clay in Soil
Once clay are present in more than 10%, they exert a marked influence on soil's mechanics behavior: - strength is decreased - deformation is increased - permeability is decreased
Site Grading Operations
Original Topography --> New topography - cuts (excavation, hauling) - fills (compaction) Requirements: - sufficient shear strength for stability - adequate stiffness for settlements - low/high hydraulic conductivity - acceptable properties under dry/wet/freezing conditions
Most abundant element in Earth's crust
Oxygen (46.6%), Silicon (27.7%), Aluminum (8.1%), Iron (5.0%)
Weathing
Process of breaking down rocks by mechanics or chemical disintegration - expansion and contraction due to heating and cooling - wind - water effects, physical and chemical
CPT Pros and Cons
Pros: - fast - continuous profiling - strong theoretical basis Cons: - machinery is expensive - no soil samples - unsuitable for gravels
Plasticity Index
Range of water content over which the soil remains plastic 0 = non plastic 1-5 = slightly plastic 5 - 10 = low plasticity 10 - 20 = medium plasticity 20 - 40 = high plasticity >40 = very high plasticity
Sieve Analysis
Sieve #: - 4 = 4.75mm - 10 = 2.00mm - 20 = 0.850mm - 40 = 0.425mm - 100 = 0.150mm - 200 = 0.075mm
Clay Building Block #1: Silica Tetrahedron
Silicate ion: SiO4(-4) - silica tetrahedrons join to make a silica sheet
Definitions of Rock
Sowers: - any hardened material that requires drilling and blasting or similar methods of brute force for excavation ASTM: - natural solid mineral matter occurring in large masses or fragments
Common Clay Minerals
Tetrahedral and octahedral sheets combine in various configurations to form clay minerals: 1.) kaolinite 2.) illite 3. montmorillonite
Undisturbed Sampling Methods ($)
Thin walled tube (Shelby tube) - pushed hydraulically into soil Piston sampler - prevents soil from entering tube faster than sample is pushed, vacuum helps recovery of soft samples - less disturbance than Shelby tube samples Pitcher Sampler - spring loaded sampling tube retracts inside a rock-coring barrel in stiff clays/soft rocks
Stokes Law
Three forces in equilibrium - buoyant force and drag force going up - weight going down - for falling spheres in a viscous fluid in which the terminal velocity of fall depends on the diameter and density of the spheres
Soil Structure
Two groups: - cohesionless: sands and gravels - cohesive: silts and clays
Shapes of PSD Curves
Uniformly (poorly) graded: - soils contains particles sizes which are primarily within a small bank (also known as well sorted) Well graded: - contains particles of all sizes in good distribution (poorly sorted) Gap graded: - one size of particles is missing from the soil, PSD curve becomes more horizontal in that region
Useful Relationships
Unit Weight, y - y = W/V = (Ws+Ww)/V = (Ws(1+w))/V = (Gs*yw*Vs(1+w))/V = (Gs*yw*(1+w))/1 + e Dry Unit Weight, yd - yd = Ws/V = (Gs*yw*Vs)/V = (Gs*yw)/(1 +e) = y/(1+w) Volume of Water - Vw = Ww/yw = (w*Ws)/yw = (w*Gs*yw*Vs)/yw = w*Gs*Vs Degree of Saturation, S - S = Vw/Vv = (w*Gs)/e - Se = w*Gs (valid for any S)
Volume Relationships
Void Ratio - e = Vv/Vs Porosity - n = Vv/V Degree of Saturation - S = Vw/Vv
Phase Diagram Representation
W = total weight of an element of soil V = total volume of an elemetn of soil Ws = weight of solids Ww = weight of water Wa = weight of air = 0 W = Ws + Ww Vs = volume of solid material Vv = volume of water Va = volume of air Vv = volume of voids = Vw + Va V = Vs + Vv = Vs + (Vw + Va)
Isomorphous Substitution
When cations (other than Si or Al) are present in the tetrahedron or octahedral structure - no substation or replacement actually takes place, rather the mineral is formed with the different cation in the structure - cations of lower valence replace the natural, most common, cations - most common cations are Si, Al, Fe, and Mg
Unit Weight
With water in pores: ym = W/V = (Ws+Ww)/V = (Ws+wWs)/v = (Ws(1+w))/V When soil is dry: yd = Ws/V = ym/(1+w)
Relationship between y, e, w, and Gs
Ww = yw*Vw = w*Ws = W*Gs*yw*Vs Ws = ys*Vs = Gs*ym*Vs Ww = w*Gs*yw Ws = Gs*yw Vs = 1 Vv = e Wa = 0 V = Vs + Vv = 1 + e Gs = ps/pw = ys/yw = (Ws/Vs)/yw
Geotechnical Engineering
application of the principles of soil and rock mechanics to practical engineering problems
Soil Engineering
application of the principles of soil mechanics to practical problems
Rock Mechanics
branch of science that deals with the study of the physical properties of rock and the behavior of rock masses subjected to various types of forces
Soil Mechanics
branch of science that deals with the study of the physical properties of soil and the behavior of soil masses subjected to various types of forces
Sir George Darwin
conducted lab tests to determine the overturning moment on a hinged retaining wall
Albert Atterberg
defined clay size fractions as the percentage by weight of particles smaller than 2 microns in size
Arthur Bell
developed relationships for lateral pressure and resistance in clay as well as bearing capacity of shallow foundations in clay
Karl Terzaghi
developed the theory of consolidation for clays as we know today, known as father of modern soil mechanics
Joseph Boussinesq
developed the theory of stress disturbtion under load bearing areas in a homogenous, semi-infinite, elastic, and isotropic medium
Effective Size, D10
diameter in the PSD curve corresponding to 10% finer by weight
Metamorphic Rock
formed by changing the composition of rocks by heat and pressure without melting
Sedimentary Rock
formed by compaction or cementation of small soil particles
Igneous Rock
formed by solidification of magma, intrusive or extrusive
Shrinkage Limit
maximum water content at which a reduction in water content will not cause a further decrease in volume of the soil mass
Uniformity Coefficient Cu
measure of how uniform soil particles are Cu = D60 / D10
Soil/Rock
naturally occurring combination of minerals
Mineral
solid, homogenous, naturally occurring inorganic chemical element or compound, usually having a crystalline structure two classifications, non-clay minerals and clay minerals
Henri Darcy
studied the permeability of sand filters and defined the term coefficient of permeability of soil
SPT split spoon sampler
the split-spoon sampler is driven into soil by blows of a 140lb hammer dropped 30 inches
Optimum Moisture Content (OMC)
the water content at which a soil can be compacted to a max dry unit weight by a given compaction effort
Charles Coulomb
used the principles of calculus to determine the true position of the sliding surface in soil behind a retaining wall using the laws of friction and cohesion for solid bodies
Plastic Limit
water content of a soil at the arbitrarily defined boundary between the plastic and semi-solid states, where the soil begins to crumble when rolled into a 1/8 inch diameter thread
Residual Soils
weathered in place
