Embed Size (px)
Citation preview
Natural Science Geology
David McConnell, 1998 1
Mass WastingDavid McConnell
University of Akron
Table of Contents
Summary
Introduction
Factors Affecting Slope Failure
Gravity – Angle of Repose – Water
Classification of Mass Wasting
Rock Fall – Rockslide – Slump – Debris Flow – Mudflow
Stabilizing Slopes
Exercise
SummaryThis lecture introduces the processes that occur during slope failure(landslides). After reviewing this material you should know:• What is mass wasting?
• Where in the U.S. is there the greatest potential for mass wasting?• What factors are important in determining if landslides will occur?• How are mass wasting processes classified?
• What is the difference between rock fall and a rockslide?• Describe the characteristics of a slump.
• What is the difference between a debris flow and a mudflow?• What steps can be taken to stabilize steep slopes?
go to top of document
Natural Science Geology
David McConnell, 1998 2
IntroductionMass wasting represents the downslope movement of material under the influence ofgravity. The definition excludes material transported downslope by streams, winds, orglaciers. The general term landslide is used to describe all rapid forms of mass wasting.
The national map of mass wasting activity shows areas with greatest potential forlandslides and other mass wasting events in red and pink colors. The areas at greatestrisk are in mountainous regions with relatively steep slopes such as the Appalachians,Rockies and Coastal Ranges (California).
Steep terrain in the Beartooth Mountains, southern Montana
However, the potential for mass wasting is not determined by slope angle alone. Forexample, the highest peaks are in western states but the largest area at risk fromlandslides is in the eastern Appalachian states. Water plays a significant role in masswasting and is much more plentiful in the eastern U.S. than in the West (with theexception of the Pacific Northwest). Finally, miscellaneous factors such as earthquakes,the presence or absence of vegetation, and human activities can also influence thepotential for mass wasting.
The result of a landslide triggered bythe 1994 Northridge earthquake,southern California. Some steephillsides collapsed destroying homes.Image courtesy of USGS geohazardswebsite.
go to top of document
Natural Science Geology
David McConnell, 1998 3
Factors Affecting Slope FailureGravity acts on all objects on the earth’s surface. Gravity canbe divided into two components for objects resting on slopingsurfaces. One component is parallel to the slope (gs) and one isperpendicular to the slope (gp).
On steep slopes (>45 degrees) the component parallel to theslope will be greatest and will act to pull objects downhill. Ongentle slopes the component perpendicular to the slope will begreatest and will act to hold the object in place.
However, gravity alone does not determine if the object willmove downslope. The properties of the surface between theobject and the slope (e.g. friction) and the physical propertiesof the sliding object itself all contribute to the potential formass wasting.
The object is more likely to move if friction between the object and the slope is reduced.In contrast, a slope will be less likely to fail if the cohesion between the grains in thematerial is increased.
For example, no matter how much dry sand is added to a pile, it can never form a slopethat is steeper than ~35 degrees inclination. This angle is termed the material’s angle ofrepose. In contrast, by adding a little water, the cohesion between the sand grains(capillary attraction) increases dramatically allowing us to sculpt sand castles withvertical walls. Irregularly shaped objects may form steeper slopes than dry sand; largeangular blocks may have an angle of repose of ~45 degrees. In contrast, sphericalmarbles are almost impossible to form into a pile with sloping sides.
The addition of excess water to a slope may also be theprecursor for a disaster.• Excess water saturates the material and serves to
reduce cohesion between grains• Water saturated pore spaces will support the weight
of overlying material thus reducing the effect offriction
• Finally, the addition of water may promoteinstability by adding weight to a slope.
Landslide, 1996, Puget Sound, Washington.Image courtesy of USGS geohazards website.
go to top of document
Natural Science Geology
David McConnell, 1998 4
Classification of Mass Wasting
Mass wasting events may be classified by a combination of criteria, including:• Type of material involved: rock or regolith
• Type of movement: fall, slip, or flow• Moisture content: relative volume of water present
Type of MaterialType ofMovement Rock Regolith
Fall Rock fall produces talusslopes of angular rockfragments
------
Slip Rock slide occurs whendownlsope movementoccurs along a plane ofweakness in the rock, e.g.on a bedding surface
Slump occurs when acohesive mass of materialmoves downslope on acurved (concave-upward)surface
Flow
------
Downslope movement ofmaterial in the presence ofwater. Forms sedimentflows such as mudflows anddebris flows.
Rock fall occurs when physical weatheringloosens angular boulders from rocky cliffs inmountainous terrain. The boulders break offand fall downslope producing an apron ofcoarse debris (talus) at the base of the slope.
Talus pile along base of cliff in BeartoothMountains, Montana
Natural Science Geology
David McConnell, 1998 5
Rockslides occur where a coherent mass of rock moves on a planar sliding surface. Thesliding surface is typically a suitably oriented bedding plane or a fracture surface.
The 1925 Gros Ventre slide,northwest Wyoming,occurred when a sandstonelayer moved downslope on anslip plane overlying weakshales. The slide wastriggered by an earthquakeand followed weeks of heavyrains that had saturated theslope. Fifty million cubicyards of rock moveddownslope, crossed the river,and moved 130 meters up theopposite slope. The wholeevent took just three minutesand formed a natural damacross the valley. The damfailed nearly two years latercausing a flood that killed sixpeople. , The rocks exposedby the slide are still visibletoday (see image).
View of Gros Ventre slide (tan areaon slope) with Gros Ventre River inthe foreground.
Rockslides generate relatively thin sheets of rock thatare broken into smaller blocks as they movedownslope. A rockslide in the Bighorn Mountains,north central Wyoming, resulted in the formation of the“Fallen City”, a jumbled collection of blocks, manyover hundreds of meters in width.
Fallen City rockslide, BighornMountains, Wyoming
go to top of document
Natural Science Geology
David McConnell, 1998 6
A slump is the downslope movement ofmaterial on a curved slip surface. Slumpingtypically involves unconsolidated sedimentthat becomes saturated with water. Thecurved sliding surface results in rotation ofoverlying slump blocks. A cliff-like scarp isleft behind at the head of the slump.
Slumps frequently occur in association withheavy rainfall. The La Conchita (California)slump occurred in March 1995 following aseries of heavy rainstorms and warnings bygeologists of the potential for slope failure.Six hundred thousand tons of material buriednine homes and damaged several others.
Property values in the small communityplummeted following the slump and thehomeowners sued the La Cohchita Ranch thatirrigated crops on top of the collapsed slope.The residents claimed that persistent over-watering by the ranch owners had destabilizedthe slope and increased the risk for landslides.The homeowners eventually settled out-of-court for unspecified damages.
Two views of La Conchita slump, California.Images courtesy of USGS geohazards website.
Slumps in unconsolidatedmaterial, Beartooth
Mountains, Wyoming.
go to top of document
Natural Science Geology
David McConnell, 1998 7
Sediment flows occur when there is a relatively large volume of water present in amixture of coarse and/or fine-grained sediment. Rather than moving downslope as acoherent mass (slump, landslide) the material flows downhill as a chaotic mixture. Flowsare differentiated based upon their velocity and the type of sediment involved.
Following a series of storms last January, USGSgeologists flew an aerial survey of the SierraNevada mountains, California, to observelandslide damage. A debris flow approximately100 meters wide and stretching for nearly fivekilometers was observed near Dorrington,California. The flow began near a ridge crest anddropped about 700 m elevation before ending inthe valley of the Stanislaus River.
Debris flows may travel with velocities of 15-50kilometers per hour and involve unconsolidated
regolith (most of which iscoarser than sand).
Dorrington debris flow, 1997,Sierra Nevada Mountains,California. Inset shows close upof debris flow path with road(State Route 4) for scale.Images courtesy of USGS,original photographs by MarkReid, USGS.
House near Farmington, Utah, inundated by debrisflow material following landslides in 1983. Image
from USGS on-line publication Debris FlowHazards in the U.S. (a pdf document).
Highly fluid mudflows incorporate fine-grained sediment and typically followstream channels. These fast-flowinghigh-density flows are common followingvolcanic eruptions producing substantialvolumes of volcanic ash. Mudflowsinvolving volcanic debris are termedlahars.
go to top of document
Natural Science Geology
David McConnell, 1998 8
Stabilizing slopesOne obvious solution to the problem of landslides is to avoid building structures on steepslopes. However, such pragmatic advice may not deter a homeowner in search of aspectacular view and is oflittle use to engineers whomust build roads on steepslopes in mountainous terrain.
The Beartooth highwayclimbs up the side of theBeartooth Mountains,Montana. The route connectsthe northeast entrance toYellowstone National Park(Wyoming) with the city ofRed Lodge, Montana. Masswasting of the steep slopesrequires that the road berepaired every few years.Ongoing efforts to preventslope failures include pouring concrete over the exposed rock to prevent slope erosion orcovering the slopes in chicken-wire to protect drivers from falling boulders. Drainage
systems are also to slopesto remove excessmoisture.
Chicken-wire and concretecover exposed slopes adjacentto roads across the BeartoothMountains.
Elsewhere, efforts to prevent mass wasting haveinvolved supporting the base of the slope to prevent itfrom failing. Wire baskets filled with rocks are oftenused for such retaining walls.
Retaining wall below restaurant on ahillside in Pennsylvania. Insert is a close
up of the contents of the wire baskets.
go to top of document
Natural Science Geology
David McConnell, 1998 9
ExerciseThis weeks exercise examines groundwater and water resources. The exercise can beaccessed through the weekly exercises page (http://enterprise.cc.uakron.edu/geology/natscigeo /Exercises/).
go to top of document
go to Natural Science Geology Home Pagelast update: 3/8/98, David McConnellemail: [email protected]
