Mass movements



Mass movements

Different mass movements occur on slopes under different conditions. We'll look at four types; rockfall, mudflow, landslip and soil creep.
Bare rock and scree slope
Rockfall
Rockfall is the rapid, free-fall of rock from a steep cliff face. Rock fragments fall from the face of the cliff because of the action of gravity. This is made worse by freeze-thaw action loosening the rock. Bare, well-jointed rock is very vulnerable to rockfall - water enters the joint, freezes and expands, cracking the rock. A scree slope of fallen rock is formed at the bottom of the cliff.
Arrows pointing downwards, hollow, mudflow, mudflow delta, topsoil, impermeable subsoils
Mudflow
Mudflow occurs on steep slopes over 10°. It's a rapid sudden movement which occurs after periods of heavy rain. When there is not enough vegetation to hold the soil in place, saturated soil flows over impermeable sub soil, causing great devastation and endangering lives.
Sandstone and clay
Landslip
Landslips or landslumps are occasional, rapid movements of a mass of earth or rock sliding along a concave plane. They can occur after periods of heavy rain, when the water saturates overlying rock, making it heavy and liable to slide. Undercutting of a steep slope by river or sea erosion weakens the rock above, also making a slump likely.
Terracettes and soil
Soil creep
Soil creep is a very slow movement, occuring on very gentle slopes because of the way soil particles repeatedly expand and contract in wet and dry periods. When wet, soil particles increase in size and weight, and expand at right angles. When the soil dries out, it contracts vertically. As a result, the soil slowly moves downslope.

Suspended load -- Bedload

Bedload: ensemble des particules en mouvement par roulement, traction et saltation sur le lit d'un cours d'eau. Il s'agit généralement de galets et de sable.
Suspended load: partie de la charge sédimentaire d'un écoulement, transportée en suspension. La charge en suspension est surtout constituée d'argile et de silt.

Engineering geology in hot deserts

In this article the expression "hot deserts" refers to areas characterized by a lack of vegetation cover, non-polar environments, dry, arid or semi-arid deserts.
The low rainfall conditions are essentially due to zonal tropical climates (e.g Sahara) or to orografic effects of mountain ranges (e.g Gobi Desert).




The design of infrastructures in this kind of desert involve specific studies and techniques. Here's some examples of interests and issues of studying the engineering geology :
1) Develop construction techniques that ensure the safety and the durabilitity of structures
3) Delivering water
2) Combatting desertification and the problem of major dunes
3) Prevent the problems dealing with : -Duricrust
                                                          -Metastable soils
                                                          -Karst
                                                          -Metastable soils
                                                          -Salinty....


Wind erosion and near-surface transport :

The bare ground plays a key role in terms of erosion and sediments transport.
It should be noted that the transport phenomena are analogous to the ones we observe inside rivers (exepting with the solution transportation).

However, the differences in density beween the two fluids (air and water) greatly impact the lifting capacity and despite high speeds, only small particles are transported (a few millimeters).
The impact of wind erosion can be observe through the shape of some typical rocks :



Tassili n'Ajjer, Algeria

Barchan Dunes : 



Rocky deserts :


to be continued...