Geology 101 - slopes and soils
Study of soils
Suitability of land Evaluation of hazards Water quality and quantity How carbon storage affects climate Soil erosion Loss of soil fertility affects food security
Shrink-swell potential
Tendency of a soil to gain or lose water Volume change in response to changes in water content
Clay soils don't drain quickly due to
Tiny pore spaces Water being tightly held to electrically charged clay mineral surfaces
Influencers of gravitational and frictional forces on a slope
Topography Rock types/ geologic materials Water content Climate Vegetation Tectonic and volcanic activity
Logging on slope stability
Trees intercept rain and help bind soil together Clear-cutting can increase slope failures, which cause streams to fill w sediment
Climate + topography =
Unstable slopes
Western Iowa
Uplands were plowed Soft,easily erodible loess and associated soils washed down raising elevation of floodplains Channelized meandering rivers Rivers began downcutting to new lowered base level Tributaries started downcutting
Stronger rocks produce what type of cliffs
Vertical
Creep
Very slow flow involving soil or other unconsolidated material Particles in zig-zag fashion
Soil classification - engineering
View soil as any fragmental earth material that can be removed without blasting Physical properties - proportion of gravel, sand slit , clay Construction and land use planning
Porosity
Volume occupied in pore spaces between solid particles Size of solid affects this
Drainage control - mass wasting
Water needs to be kept from infiltrating or running across the slope
Loess
Wind blown sediment Derived from glacial sediments choking Missouri River in Iowa Soft and stable when dry, loses strength when wet
Soil horizons- subsoil
Zone of accumulation Clay minerals and ions accumulate here
Soil horizons - top soil
Zone of leaching Water moving through zone causes chemical weathering in minerals, forming clays or releasing ions
Slope stability maps/ landslide hazard maps
Based on past landslide activity, slope steepness, kinds of Earth materials present Recommendations for land use and engineered structures
Transitional slides
Blocks of rock or soil move quickly along flat, smooth planes of weakness
Slides
Blocks or rock or soil move along a zone of plane of weakness Material may stay together or disinegrate
Rotational slides
Blocks slide along a curved slip plane producing a scarp Common in soil and unconsolidated material
Clay has low permeability since
Can hold a lot of water, but water doesn't easily flow through
Sediment retention basins/ silt fences
Catch storm water runoff and filter out sediment
Ion exchange in soil
Cations released from minerals by chemical weathering Dissolve (+) cations in water will attach to (-) clay surfaces
Soil classification - soil science
Characteristics of soil horizons Soil characteristic related to five soil-forming factors Soils divided into 12 categories called orders Useful for agricultural and land-use purposes
Soil fertility 2
Clay minerals and organic matter supply nutrients through ion exchange
Clay mineral surfaces has a what charge
Electrical
Rotational slides are caused by...
Erosion at the base of slope
Soil erosion
Estimated in terms of tons of soil per acre per year
Universal soil loss equation
Evaluating potential impact of conversation practices that control erosions
Talus
Fallen rock accumulates as talus at the base of the slope
How does material move downslope?
Falls, slides, flows
Older soils
Fewer mineral fragments and nutrients Ion exchange sites on clay particles are occupied by non-essential nutrients Less fértil soil
Lahar
Flow caused when melting snow or heavy rains mix with volcanic ash after pyroclastic eruptions
Fall
Free fall of material (usually rock) from a cliff face
Sinkholes
From wherever caves form, typically in limestone or Dolomites (sedimentary rocks that are soluble in slightly acidic water)
Weaker rocks produce what type of slopes
Gentler slopes
Ways to reduce soil erosion
Grassed waterways and buffer strips - slow down runoff, trap sediment, absorb nutrients
Soil-forming factor: organisms
Help mix and aerate the soil particles Contribute organic matter Release and convert nutrients into forms suitable for plants
Soil structure
How soil particles cling together in aggregates called peds
Soil texture affects
How well or how poorly drained the soil is
Soil-forming factors: parent material
If you build soil, they will look different because different minerals and rocks
Organic matter in soils
Improves soil structure Increase water absorption Increase aeration Increase nitrogen and retention of nutrients
Steep hill slope angles...
Increase the driving forces for movement
Vegetation on a hill slope encourages...
Infiltration of rainwater Provides root systems that help hold soil in place Increases slope stability
Sandy soils drain very quickly due to
Large pore spaces Water only held in place by cohesive forces Low adhesive forces holding water to surface of quartz sand grains
Pore spaces are larger in...
Larger in sandy soil than clay soil But clay soil has more porosity
Conservation tillage
Leaves crop residue on fields, which provides protective cover for soil, helps increase water infiltration, adds organic matter
Reduce the risk of mass wasting by avoiding...
Loading top of slopes Cutting into unstable slopes Placing fill on slopes Removing material from toe of unstable slope
Flows
Loose material mixes with water and moves like a fluid Mud,soil, volcanic, ash, debris
Soil
Loose material on Earth's surface made of mineral and organic matter that is capable of supporting plant life
Soil strength and sensitivity
Measure of how well soil particles stick together when disturbed Function of cohesive and frictional forces between soil particles
Soil compressibility
Measure of soils tendency to compact Soil with finer grain are more compressible (silt and clay) Affects settling of structures Affects ability of soil to absorb water
Land subsidence
Occurs naturally and as result of human activities
"Arching" of soil
Over a solution fracture in limestone can form sinkholese
Withdrawal of fluids - land subsidence
Over pumping groundwater can cause the pore system to collapse, causing land subsidence
Through natural and human processes slopes may become
Oversteepend
Construction on slope stability
Oversteepened slopes Disposal of construction waste material
Soil-forming factors
Parent material Organisms Climate Topography Time
Sediment pollution
Partially fills in stream channels with legacy sediment Fills in lakes and reservoirs Reduces water quality
Formation of soils
Physical and chemical weathering of bedrock and sediments
Cover crops
Planted in the fall to protect bare soil during winter months, reduces runoff and loss of nutrients , adds organic matter
Younger soils
Plentiful rock Release cations through chemical weathering Cations will attach to clay surfaces in the soil Plant roots will exchange H+ for cations they need
Retaining walls- mass wasting
Provide support at the base of oversteepened slope
Iowa daily erosion project
Provides daily estimates of runoff and soil loss within each sub watershed in Iowa Iowa loses 5.5 tons/acre of soil per year Soil redevelops at rate of only half ton/acre per year
Mudflows/debris flows
Quickly moving flows depending on material
Universal soil loss equation based on
Rainfall Runoff Size and shape of slope Soil erodibility Soil cover Erosion-control practices
Soil-forming factor: climate
Rainfall and temperature affect organisms Kinds of weathering Rate of weathering
Excess water
Reduces friction between sediment grains Adds weight to the slope
Soil texture
Relative proportions of clay silt and sand sized particles in soil
Soil compaction
Restricts root development' Decreases soil available water Decreases water infiltration Increases runoff
Loam soil
Retain more water for plant use than sandy soils and give up water more freely than clay soils
Slump
Rotational slide
Type of Earth material affects...
Shape of slope Type/ frequency of downslope movements
Complex slides 2
Slide in the upper part of a slope may transform to a flow at the bottom More common in soil and unconsolidated material
Forest fires followed by heavy rains result in
Slope failures Soil repels water
Ion exchange occurs when
Some cations are removed from clay surfaces by water or plants, and replace by other cations Cations are important nutrients
Covering steep slopes - mass wasting
Stabilize slopes with synthetic mesh, hydro seeding, vegetation
Western Iowa end result
Steep sides of deepened channels collapse Tributaries have increase their length and depth by eroding uphill into soft, erodible loess soils
Angle of repose
Steepest slope granular material can make without collapsing
Protective structures - mass wasting
Barriers, tunnels, chain mesh, sprayed on concrete
Sinkhole process
1. Water table is high and a cavity in limestone is filled with water 2. Water table drops 3. Sediment above cavity is no longer supported and sinks into cavity
Permeability
Ability of a fluid to flow through interconnected pore spaces
Soil fertility
Ability of soil to supply nutrients necessary for plant growth
Soil plasticity
Ability to deform without breaking Increases as clay content increase Water drastically impacts
Expanding or swelling clay minerals...
Absorb water into structure and expand in size and then shrink when water dries out (expansive soils)
Soil-forming factors: topography
Affects infiltration and drainage Erosion Weathering rates Organisms
Soil color
Amount of organic material Presence of different minerals (clay, iron oxides, carbonates)
Rock bolts - mass wasting
Anchor loose rocks in place to prevent rock falls
Dams on slope stability
As reservoir fills, water pressure in rocks on reservoir slopes increase Increased water pressure may weaken rocks and reduce strength of weak rocks Variation in reservoir leves promote slope failures
Complex slides
Combination of two or more types of movement (slide +flow)
Land subsidence is caused by
Compaction of soft sediments Withdrawal of fluids from below surface Collapse of surface rocks and soils over underground void
What on a sloping terrain can cause slope instability
Construction (flat pads cut into sides of hills for building sites and roads)
Conversion of agricultural or forested land to urbanized land
Construction causes increase in sediment production
Minimize soil erosion in hilly or mountainous area
Contour plowing, strip cropping, terracing
Sediment sources of sediment pollution
Crop land Pasture lands that have been overgrazed Urban construction sites Land recently burned or logged, mining
Terracing -mass wasting
Cut the slope into series of benches
Steep hill slope angles increase driving forces for movement and...
Decrease slope stability
Any planar surface tilted in the downslope direction does what to slope stability
Decreases slope stability
Soil-forming factors: time
Depends on other soil-forming factors and geologic events
Rain drops falling on bare soil...
Dislodge clay and organic particles Preferential loss of clay decreases soil fertility