MASS MOVEMENT

Pictures from: http://www.geo.duke.edu/geo41/mmo2.htm

What causes mass novement?

Underlaying cause: Material on the slope became loosened, and when pulled by gravity mowe downslope.

Principal factor causing movement:

            -gravity

Principal factors provide resistance:

            -Friction between the slope and loose material

            -Strength and cohesiveness of the loose material

Gravity, friction, slope angle

            The two main factor of the movement is

                        gravity

                        friction

Gravity has two component:

            1. parallel to the slope (Gd) (works to pull material down)

            2. Perpendicular (Gp) contributes to the effect of friction

 

Friction (F) is the force oppeses motion between two bodies in contact

The steeper the slope the larger is Gd, and more likely the material will slide down.

 

Slope composition

1.Solid rocks

            They tend to be completely stable even if they form vertical cliff

            However they can also be site of mass movement if:

            -Tectonic deformation

            -mechanical weathering (frost wedging)

            -sedimentary rocks with bedding planes

            - the rock is soluble (caves-collapse)

            -it is a metamorphic rock and foliated

When the plane of weakness is parallel to the surface of a steep slope the rock will break along that plane.

2.Unconsolidated material

            -If it is dry- the slope is stable, if Gp>Gd.

Gd increases with the steepness of the slope.

-The max. angle of the slope where it is still remaining stable is the so called ANGLE OF REPOSE (for dry sand 30-35)

            -steepest slope in angular, poorly sorted congl. or breccia

Vegetation

            The vegetation especially the deep rooted dense veg. tends to stabilize the             unconsolidated material. (grass has very deep root system)

Water

            Water, more than any other factor is likely cause the stable slope become

            instable and slide. (Few water however tends to stabilize it) and it is also good  

            to the vegetation.

            -LOTS OF WATER however is decrease the friction between the grains (Car             tires on the road when there is a big rain).

SETTING OFF A MASS MOVEMENT

Natural triggers

            Torrential rains (1967 Brasil 1200    died)

            Earthquake (1970 Peru, 1500 milliom m3 ice, water, mud 30,000 died)

            volcanic eruption (1980 Mt. St Helens)

HUMAN INDUCED TRIGGERS

            Missmanagement of water and vegetation (French)

            Oversteepeng, overloading slopes

            Mining miscalculation

            Overirrigated slopes in farming area

            Install leaking septic systems on slopes

            ex. (Family in Los Angeles left the lawn sprinkles on)

            Clear cut forested slope

            Cut into bases of sensitive slopes for home

            blasting of mines, quarriing

TYPES OF MASS MOVEMENT

Material Type of Movement
Fall Slip Flow
Rock

Rockfall
fall.gif (260 bytes)

Rockslide
slide.gif (253 bytes)
Regolith   Slump
slump.gif (279 bytes)
Mudflow,
Debris Flow
flow.gif (262 bytes)

SLOW MASS MOVEMENT

CREEP:

            This is the slowest massmovements, measured in mms or cms/year. This     occur everyvhere even on the most gentle slopes. Mostly on unconsolidated materials. It moves faster on the surface, and slowlier underneath the surface. One of the primary cause of creep is thermal expansion of the underlying material.

On the left is an example of creep(Creep, Marathon, TX (Photo from NOAA)). On the right is a displaced Curb, Photo by DH, 3/4/84).

GOOD INDICATION OF CREEP, WHEN THE TREES ARE NOT UPRIGHT NO LONGER ARE VERTICAL. OLDER GRAVE STONES ARE TILTED, TELEPHONE POLES ARE TILTED AFTER A WHILE.

SOLIFLUCTION:

Special variation of creep in polar region. It is forming on permanently frost rocks, or unconsolidated material, when the summer heat melts just the uppermost layers and they moving downward a bit every year. This is a little bit slowlier, than the other one.

 

RAPID MASS MOVEMENT         

Rapid movements can be as rapid as km per hour, or even ms/second. They are further classified based on the type of movement.

 

FALL

            rocks, or sediment are dislodged and drop from steep or vertical slopes (Can be dangerous)

 

SLIDE

A mass, rock or sediment is dislodged and moves along a plane of weakness, such as fault, fracture and bedding plane. It can be a small displacement of some soil over rocks or could involve a whole mountain side. Could go on from days to years. The material moves along the plane of weakness, which can be identified and it is called slip plane. Nature of slip plane is depend on the local geology (clay, or shale layer, bedding plane, igneous rocks (joint produced by exfoliation)

            SLIP PLANE is usually flat.

Slump

Slide which separates along a concave surface is a slump. They are distiguished by crescent-shaped scarp, that forms in the landscape where the slump has attached. The steep exposed cliff face is the scar afterward. Slumps are usually dont travel too far from the original location.

slump_fig.gif (3709 bytes)LaConchita1_sml.gif (44630 bytes)

1995 La Conchita, California, landslide. Photo by R.L. Schuster

FLOW

mass of rock fragments and sediment moves down slope as a highly viscous                 fluid.

They can be dry or wet. They are more dangerous when wet. They can contain rocks, soil, vegetation, snow and so on. Its velocity is determined by its water content.

 

EARTHFLOW

Relatively dry mass of clayey, silty regolith, and it moves relatively slow 2m/hour. (less dangerouos)

MUDFLOW

            wet, fine grained, consistancy wet concrete to muddy water. It moves in channel like lows so they can move even on very gentle slopes such  as1-2. They develop after heavy rainfall. It is char. in arid regions.

          Dflow1.gif (85888 bytes)Dorrington debris flow, 1997, Sierra Nevada Mountains, California. Inset shows close up of debris flow path with road (State Route 4) for scale. Images courtesy of USGS, original photographs by Mark Reid,

 

DEBRIS FLOW

            SAME AS MUDFLOW WITH LARGER PARTICLES IN IT. DEBRIS      AVALANCHE COMMOM IN BLUE RIDGE AFTER HEAVY RAINS. THEY OCCUR ON STEEP SLOPES, ESP. WHEN THE VEG. COVER IS REMOVED BY FIRE.

 

 

Yungay, Peru's Main Plaza before(left) and after(right) pictures of landslide's destruction.

PREDICTING MASS MOVEMENT

-Open eyes

-looking for evidences of former movement, geology of slopes, angle

water content,

-Avoiding mass movement

-dont build on sensitive slope

            has to be maps showing hazardous areas. (Insurance probably will             not cover)

Before purchasing a property:

1.Go in and check on it!!!!!!!

2. If you like the view, it is a goo news, but do not by it, before taking a closer look. look for exposed rocks, or fresh break which would tell you about the underlying material.

3. Than investigate the whole neighborhood if there is any evidence of a scar, is your site in the middle of a large potential slide mass

4. Drive around the community, and look for tilted trees, poles, so on.

5. Check on the houses around see if they have cracks in the foundation, on the walls?

6. You  can also contact state geological survey!!!

 

PREVENTING SLOPE MOVEMENTS

MUCH CHEEPER than the clean up afterwards!!!!!!!

NONSTRUCTURAL APPROACHES

1. plant fast growing trees (deep root network) Broad leaved specimens (maple, elm, oaks) protect slope from most of the rains.

2.put chemical solutions to increase the strength of the slope materials. Cement injection.

STRUCTURAL APPROACHES

1. Modifying slope, building support for it, or reducing its water content.

2. Removing part of overlying steep slopes.

3. Cutting flat terraces into a slope (where small slides can rest.

4. Walls

5. rocks

6. The best to prevent sensitive slopes

            from water.

            -drainage ditch

            -plastic cover

            -layer of concrete

            -tunnels bringing excess water out from the slope