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