Soil Types & Phase Relationships 1.1 What is a soil? Soil is the accumulation of sediments and mineral particles, typically non-homogenous but not always, influenced by change in moisture content. Differentiated mainly by grain size. Shape/size increase hydraulic and mechanic soil parameters. 1.2 General Definitions: Residual Soil: weathered soil, remaining at original place Alluvial: transported by water Glacial: Transported by glaciers Loess: transported by wind Marine: deposited in salt/brackish water Expansive: large volume changes with addition of moisture Dispersive: loss of cohesion in water Granular: No cohesion REV: Representative Elementary Volume. The sample size which has a size big enough to represent the sample accurately can’t be too small, the bigger the sample size the better. Course-grained samples must be 10x bigger. Within the REV scale, the soil behaviour can be described based on phase relationship parameters. 1.3 Fine Grained Soils Occurs due to weathering of parent rock (mineral), resulting in formation of groups of crystalline particles at colloidal size High specific surface area (high surface area to mass ratio) Surfaces of clay minerals carry residual negative charges, meaning they are less attracted to other particles and can be denser Attraction between clay particles happens because of van der waals bonds Increasing ion concentration leads to net repulsion Net repulsion = face to face orientation, which makes it more dense Net Attraction = face to edge/edge to edge, meaning less close and less dense Absorbed water is held around clay by hydrogen bonding & hydration of cations 1.4 Equations Void Ratio[-] e = n/(1-n); e=Gw/S e= V v V s Effective Unit Weight [kn/m^3] γ ' =γ T Porosity[-] n=1- (ɣD/Gw) n= V v V t = Dry Unit Weight[kn/m ^3] γ D = W s V T Moisture Content[%] w= W w W s Unit Weight of Solids [kn/m^3] γ s = W s V s Degree of Saturation[%] S= V V A + Specific Gravity [kn/m^3] G= W V s Total Unit Weight[kn/m^3] γ w = effect = 9.81(10)kn/m^3 γ= W T V T = Saturated Unit Weight [kn/m^3] γ sat = ( Soil Characterisation & Soil States 2.1 Soil Tests: Moisture Tests: Oven Drying: soil sample taken & measured, then oven dried, measure again. MD = MCDS – MC Mw = MCMS – MCDS w = (Mw/MD)x100% Sieving: soil is placed in sieves, shaken, each different size is measured & graphed on a PSD scale Analysis→ Uniformity Coefficient: Cu = D60/D10, Curvature Coefficient: (D30) 2 /(D10 + DD60) Hydrometer Method: wet dirt, put in tube of water, wait for it to settle, observe the layers of different soils, and take continual readings at different time intervals. 2.2 Atterberg Limits Liquid Limit: LL→ the minimum w at which soil flows (Liquid – plastic) Plastic Limit: PL→ the minimum w at which soil deforms plastically (Plastic – semi-solid) Shrinkage Limit: SL → the w at which soil reduces volume (semi-solid-solid) 2.2.1 Limit Indices Plastic Index: PI → IP = LL – PL Liquid Index: LI → IL = [w-PL]/IP Consistency Index: CI → Ic = [LL – w]/IP Activity: A = IP/[% clay by mass] <1 = low activity 1-2 = intermediate activity >4 = high activity 2.2.2 Atterberg Limit Tests Determine LL - Penetration: drop a machine pin into sample, measure penetration, analyse on log graph. 20 blows Determine SL – Shrinkage: fill sample and measure, then dry sample and measure again, using the equation below: SL= [ m 1 −m 2 m 2 − V 1 −V 2 m 2 ∙ γ w g ] Determine LL – Casagrande Method: mix soil & water in dish, use a U shaped knife and spread/split the soil. Measure gap and see if it reforms, count number of blows delivered by the crank machine, usually at 2 drops/sec, till soil reforms. 25 blows Determine PL – Ellipsoidal (Standard): mix dirt and water, roll into a ball and then roll onto the bench into an ellipsoidal mass until it breaks. Repeat at least two times and use w=PL to find plastic limit. NOTE: PSD & Atterberg Limits are used to determine other properties; erosion, penetration (grouting), hydraulic conductivity, workability and more. Soil Classification & Compaction 3.1 Undefined Soil Classification: G – Gravel S – Sand C – Clay M – Silt O – Organic Soil P – Peat W – Well Graded P – Poor Graded L – Low Plasticity H – High Plasticity Flow charts are used to sort samples of soil into certain categories, the following is an example These classifications are related engineering parameters; strength, compressibility, hydraulic conductivity, workability Applied to dams & roads 1 = highly desirable, 14 = highly undesirable This is an internationally accepted classification system. 3.2 Compaction Increased density due to compaction leads to; Increased shear strength, Reduced compressibility, Decreased porosity, Resistance to shrinkage Compaction depends on soil types, size of crumbs, etc. Proctor Compaction Test: the standard test for compaction. Mix soil & water put in mould, compact the sample, weigh sample as well as mould. Then take out of mould, weigh it and determine the moisture content using moisture cans and ovens. Analysis: γ D = γ T 1+ w w sat =( γ w γ d − 1 G ) ∙S γ D = γ w 1 G + w sat S Plot dry unit weight on y-axis and moisture content on x- axis Draw a smooth connecting curve Also draw a curve for complete saturation 2 tests; standard compaction & modified compaction(larger compaction forces) -Should be noted that the size of crumbs affects the validity of results (<10mm) 3.2.1 Direct Density Measurement Methods Direct Sampling: place hollow cone over soil, insert into soil and remove sample from cone. ρ nat = M V (bulk density) ρ d, nat = ρ nat 1+ w nat (dry density) Substitute Method: Fill jar with sand & determine weight of sand-cone apparatus(W1) Determine weight of sand to fill hole (W2) Dig hole, determine weight of excavation (W3) and moisture content, w. After filling hole with sand determine the weight of the remaining sand and apparatus (W4) Dry Unit Weight: γ D = W d V Weight of sand to fill hole: Ws = W1 – (W2+W4) Volume of hole: V = W3/ γ D Weight of Dry Soil: Wd = W3/(1+w) Balloon Test: Fill cylinder with water, record volume (V1) Excavate small hole, determine weight (W) & moisture content (w) Use pump to invert balloon in order to fill hole Record volume of remaining water in cylinder (V2) Bulk Unit Weight: γ= W V 1 − V 2 Dry Unit Weight: γ D = γ 1+ w 3.2.2 Indirect Density Measurement Methods Standard Penetration Test: drive standard “split”/”spoon” into sample, count number of blows, N, for specific penetration depth. Cone Penetration Test: using standardiesed cone, measure the required thrust to drive the cone into a sampe at a constant rate(10-20mm/second) D r = e max −e e max −e min Plate Load Test: there is a hydrulic pump measuring force as a plate is loaded, measuring the hardness of the ground. 3.3 Dispersion -Chemically supported erosion process -Dispersive clays, disperse in water & are erodible under rainfall (may cause piping fail) -Dispersive soils are common in QLD To test dispersion: Place soil “crumbs”” in water dish, if the water becomes turbid or cloudy around the crumb, then it is dispersive. Emersive Class Number or Pinhole test. Darcy’s Law & Hydraulic Conductivity 4.1 Hydraulic Head/Potential: Hydrostatic pore water pressure, uw, increases with depth below the water table -linear uw = γ w ∙z w where zw = depth below water table -negative pore water pressure occurs above the water table -causes deformations of soil during shrinkage Potential Concept: H=Z+ u w γ w + v 2 2 g H = total head(m) Z = elevation head(m) v = velocity(m/s) v 2 2 g = velocity head(m) For laminar flow: v 2 2 g = 0 (assumed) Hydraulic Head: h=Z+ u w γ w [m] (for laminar flow) -Hydraulic head is a specific measurement of liquid pressure above a datum -Velocity head is due to the bulk motion (kinetic energy) -Elevation Head is due to the fluids weight, the gravitational force acting on a column of fluid 4.2 Darcy’s Law Saturated Seepage for laminar flow -Where seepage is the slow escape of a liquid or gas passing through a porous material. Q = -k ∙ i ∙ A Q, Discharge is the volumetric flow rate per unit time (m 2 /s) k, proportionality factor (given) i, hydraulic gradient combines both change in head and length i= h 2 −h 1 L i crit = 0.222 d e 3 Hydraulic head is a gradient against 2 or more hydraulic head measurements A, cross sectional area (m 2 ) q, specific discharge(Q/A) [m/s] Filter Velocity: This is the apparent velocity of water through soils v f = Q A =−k Δh Δs =−k dh ds Pore Velocity: This is the real velocity of water through pores