Groundwater and Soil Behavior

• Water below the ground surface is known as groundwater or subsurface water.
• Surface water infiltration into the ground and percolation deep into the ground is called recharge.
• Water significantly influences the engineering behavior of most soils, especially fine-grained ones.
• Clayey soil, when dry, can be rock solid but softens and becomes plastic when wet.
• Sandy soil is generally loose when dry but becomes stable when wet.
• Water is a crucial factor in addressing geotechnical engineering challenges.
• Capillarity, swelling, and frost action in soils are examples of water's impact, as is seepage through dams and levees.
• Soil can range from virtually dry to saturated with water.
• A soil is considered partially saturated when voids are not completely water-filled.
• Water in soils is either static or dynamic.
• The groundwater table (GWT) is generally static, as is adsorbed and capillary water.
• Dynamic water flows from areas of high to low energy through interconnected soil voids.
• There are different saturation zones including:
  • Unsaturated (vadose) zone
  • Saturated zone
  • Capillary fringe
• Ground Water Table (GWT) details:
  • Surface where pore water pressure is atmospheric
  • Divides saturated and unsaturated (vadose) zones

Unsaturated Zone

• This is the zone between the land surface and the GWT.
• Pores contain both water and air.
• It can also be called the vadose zone or zone of aeration.

Saturated Zone

• Pores are completely filled with water.
• Water pressure exceeds atmospheric pressure.
• This zone is also known as the phreatic zone.

Capillary Fringe

• The area immediately above the water table gets saturated by capillary action.

Aquifer

• A geological formation that allows storage and transmission of water.
• Typical aquifers include sand, gravel, and fractured rocks.

Aquitard

• This is a geologic formation made of poorly permeable or semi-pervious material.
• Aquitards can store water, but do not freely yield water to wells such as sandy clay.

Aquiclude

• Geological formation composing of relatively impermeable material.
• Aquicludes store water but cannot easily transmit is such as clay.

Aquifuge

• Geological formation composed of impermeable material.
• Aquifuges neither contain nor transmit water like solid unfractured rocks.
• A confined aquifer can result if an impermeable soil layer covers an aquifer and pressure builds.
• An artesian aquifer is confined and its groundwater is under positive pressure.
• Hydraulic conductivity is a key physical parameter that controls the rate of water flow through soil
• Water moves through interconnected voids in soil.
• High permeability occurs when there are larger void spaces in soil.
• Coarse-grained soils like sand and gravel typically have high permeabilities.
• Fine-grained soils like silt and clay typically have low permeabilities.

Common Soil Types and Hydraulic Conductivity

• Clean gravel (GW, GP) has a very good drainage and high hydraulic conductivity (>1.0 cm/s).
• Clean sands and gravel mixtures (SW, SP) have good drainage and medium hydraulic conductivity (1.0 to 10⁻³ cm/s).
• Fine sands, silts, and clays mixtures (SM-SC) have poor drainage and low hydraulic conductivity (10⁻³ to 10⁻⁵ cm/s).
• Silt and silty clay (MH, ML) have poor drainage and very low hydraulic conductivity (10⁻⁵ to 10⁻⁷ cm/s).
• Homogeneous clays (CL, CH) have very poor drainage and practically impervious hydraulic conductivity (<10⁻⁷ cm/s).
• Water flows between two points when a pressure difference exists with flow direction from higher to lower pressure.
• Based on Bernoulli's equation, total head in water under motion is the sum of pressure, velocity, and elevation heads.
• The term containing velocity head can be neglected in porous soil as seepage velocity is small.
• Elevation head references an arbitrary datum, so total head varies with datum position.
• Pressures are defined relative to atmospheric pressure (101.3 kPa at 15°C), known as gage pressure.
• Gage pressure is zero at the groundwater table (free surface).
• Piezometric head equals (hp+Z), and equals the measurement performed by an open standpipe (piezometer).
• Elevation of the water surface in the standpipe is total head.
• Actual height of the water in the standpipe is the pressure head.
• The (gage) pressure is zero at the groundwater table (free surface).
• Fluid pressure variation with depth (hydrostatic pressure distribution) is given by u = γwzw.
• z w is the depth from groundwater level.
• Pore water pressures are measured using water pressure transducers or piezometers.
• In a pore water pressure transducer, water passes through a porous material and pushes against a metal diaphragm with a strain gauge.
• Piezometers are porous tubes that allow water to pass through.
• In a simple piezometer, height of water in the tube from a fixed elevation is measured.
• Pore water pressure is the unit weight of water times the height of water.
• Flow of water in soil is quantitatively analyzed using Darcy's law.
• According to Darcy, the flow rate varies:
  • Directly with the hydraulic head difference.
  • Directly with the soil's cross-sectional area.
  • Inversely with the length over which the hydraulic head difference occurs.
• The coefficient of permeability/hydraulic conductivity (k) has same units as velocity (cm/s or m/s).
• Hydraulic gradient, i, equals the hydraulic head difference divided by the length of the soil sample.
• According to Darcy's Law, q = kiA.
• Average (discharge) velocity, v, represents flow rate divided by gross cross-sectional soil area.
• Actual water velocity (seepage velocity) through the void spaces exceeds the discharge velocity, v.
• Soil permeability depends on:
  • Fluid viscosity
  • Pore size distribution
  • Grain-size distribution
  • Void ratio
  • Roughness of mineral particles
  • Degree of soil saturation
• Structure in clayey soils significantly affects hydraulic conductivity.
• Fine-grained soils with a flocculated structure have a higher permeability coefficient than dispersed structures.
• Permeability correlates with the thickness of adsorbed water layer held to the soil particles.
• Permeability decreases with increasing thickness of the diffuse double layer if other factors stay the same.
• Permeability increases with increasing void ratio and saturation degree.
• Soils with large voids (sands) are generally more permeable than soils with smaller voids (clays).
• Permeability tends to decrease as density increases.
• It is conventional to express hydraulic conductivity, k, at 20°C.
• Laboratory hydraulic conductivity tests are economical and simple.
• Constant-head and falling-head tests are two standard laboratory tests used to determine hydraulic conductivity.
• Constant head tests are used for granular soils.
• The falling head test is used for both fine-grained and coarse-grained soils.
• Permeability determined in a laboratory may not truly indicate in-situ permeability.
• A soil sample tested may not exactly duplicate the in-situ soil structure.
• The flow of water in permeameter is downward vs. horizontal or multidirectional flow in situ.
• Permeameter's smooth walls imply different boundary conditions compared to in-situ soil.
• The gradient might be different in the permeameter versus the field gradient.
• Field tests are typically more accurate over laboratory tests for determining soil hydraulic conductivity
• Field tests undisturbed soil.
• Field tests account for soil stratification, overburden stress, groundwater table location, and other significant variables.
• Pumping, borehole, and tracer tests are generally used for field testing.
• Tracer tests time dye, salt, or radioactive materials travel between wells or borings.
• Average hydraulic conductivity of soil can be measured using pumping tests in wells.
• Many empirical formula relate the coefficient of permeability for uniform sands in a loose state.
• Height of capillary rise in a small-diameter tube is caused by cohesion of water molecules and adhesion to tube walls.
• Hieght of water is inversely proportional to the tube’s diameter
• Capillarity occurs at the water table when water rises from saturated soil into dry or partially saturated soil.
• Void spaces between soil particles form "capillary tubes" through which water rises in soils.
• Clayey soils should theoretically experience highest capilllary rise but have a slow rate.
• Largest capillary rise occurs in soils of medium grainsizes (ex silts)
• Heights of capillary rise can be significant, especially in fine-grained soils.
• Height of capillary and capillary pressures are importnat.
• In equilibrium, the weight of water column must equal the reaction at the meniscus.
• The groundwater table is where capillarity begins with saturated soil going into dry soil surfaces.
• The height of capillary rise relates to the mean diameter if a soil's voids, which relates to effective grain size.
• With soils, total stress and effective stress, should be considered.
• In tube 2, water rises to the surface of the soil.
• Capillary menisci hang on particles, resulting in apparent cohesion.
• Normal stresses and strains:
  • Consider a cube of dimensions x = y = z that is subjected to forces Fx, Fy, Fz normal to three adjacent sides.
• Shear stresses and strains:
  • Consider the XZ plane and apply a shear force F that causes the cube to distort.
• Karl Terzaghi (1883-1963) formulated the pricinple of effective stress in mid-1920’s
• Terzaghi’s Effective Stress is only valid for fully saturated soils
• Volume change and shear strength of soils are a function of effective stress, not total stress.
• The principle of effective stresses applies only to normal stresses and not shear stresses.
• Porewater cannot sustain shear stress so τ = τ'..
• Due to self weight of the overlying soil (geostatic stress) and stress from external loads o applies only to normal stresses and not to shear stresses (as the porewater cannot sustain shear stress). Thus, τ = τ'.
• Porewater pressure (PWP) is isotropic (acts in all directions).
  • Must account for hydrostatic vs dynamic (flowing water)
• Cold Canadian climate results in freezing of near-surface ground for several months of winter almost everywhere.
• Depth of seasonal frost in Canada from minimal to several meters.
• Volumetric expansion of soil caused by freezing creates heaving or movement.
• Heaved structures damage or collapse due to volumetric expansion
• Ontario depth of frozen soil shallow to >2 meters in northern areas.
• For frost action to occur:
  • Must be under freezing temps
  • Source of water
  • Susceptible grain size
• The ground freezes from the surface down, progressing along a front parallel to the surface.
• Ice lenses grow by addition of water to their lower surface
• Spring thaws from the surface down.
• Meltwater gathers under the frost and the subgrade may be saturated after thaw o Roadway with moisture trapped 50% or more loss in bearing capacity.
• Frost heave is form of frost action. Cyclic freezing/thawing of water in soil/rock
• Frost heave is vertcal movement of ground surface.
• Not all soils prone to frost heave.
• Effects of frost heave pronounced in soils that facilitate capillary flow
• Silt and very fine sands frost susceptible
• High water content clays may impede len formation
• Frost heave rare in in clearn coastsands/gracels.
• Frost heave damages roadways/pipelies
• Differential heave is major cause
• Shallow footings and pile foundations crack under influence of differential frost heave
• Frost action in soils:
  • Can implement GWT lowering
  • Remove frost-susceptible soils
  • Use chemical additibes/ or membranes
  • Place foundations to max frost penetration depth.
• Permafrost is ground (soil or rock) that remains at a temperature -0°C or below for at least two consecutive years.
• Permafrost found in cold regions non surface cover
• Common types of ground ice:
  • Segregation -> thin layers of ice.
  • Wedges -> several meters wide an dm deep.
  • Massive -> 10m or more
• Active layer is soil directly beneath ground surface thaws in summer.
• Is generally -5 to 2m thick in Canada
• Colder the climates thinner the soils
• Permafrost engineering is design condition permafrost related engineering.
• Projects increased importance from material excavation. Permafrost impacts structures, construction and maintenance infrastructure.
• It causes greenhouse gas emission and impacts northern ecology.
• Thawas causes potential and has dangerous landslides.