II. DESERT SOILS
51
E. PIPINGS
Piping is mentioned here because it can become a serious economic
problem when man settles on the land and accelerates gully formations
in virgin or undisturbed soil. Piping is a "tunneling erosion" where the
subsoil erodes out from under the surface (Fletcher and Carroll, 1948).
1. Types
In Arizona, some buildings and highways have been threatened with
almost uncontrollable loss of foundations. Fletcher and Carroll (1948)
report three main types of piping: (1) lateral flow of soil along a more
stable subsoil into an open stream bank, (2) flow along cracks of surface
water, and (3) flow taking place when dispersed surface soil is washed
into pore spaces of coarse, poorly graded, upper subsoil. All cause sinking
and caving of the soil in a "tubular" or "pipe-like" depression. Fletcher
et al (1954) describe five conditions necessary to initiate piping: (1)
there must be a source of water, (2) the surface infiltration rate must exceed the permeability rate at some subsurface layer, (3) there must be
an erodable layer immediately above the retarding layer, (4) the water
above the retarding layer must be under a hydraulic gradient, and (5)
there must be an outlet for the lateral flow. Piping is a serious problem
to agricultural loess soils in Central Asia (Glukhov, 1956), in the Niger
Valley of Mali, and parts of the Chad-Logone Valley. Saline and sodic
soils in Australia (Downes, 1956), are frequently eroded by piping. Alluvium where differential particle-size distribution has taken place and
where the subsoils are highly dispersed by soluble salts and exchangeable
sodium are most susceptible to "piping erosion." Earth shrinking and
cracking may initiate conditions for piping formations.
2. Methods of Controlling
Control of piping in building areas may be accomplished by deep plowing, ripping, or chiseling to mix the soil as much as possible to obtain
a uniform texture and packing before construction gets underway. One
of the suggested methods of control in agricultural land is to wet the soils
thoroughly to compact them before cropping. Another is to obtain uniform
soil mixing by use of deep farm tillage equipment.
F. SOIL STRUCTURE
Desert surfaces, as indicated in the earlier discussions, develop a wide
variety of structures as shown Fig. 11. Desert soil surfaces are normally
soft when dry. They may have a thin, platy structure which overlays a
vesicular structure (Jackson, 1957; Lapham, 1932). The platy surface
51
E. PIPINGS
Piping is mentioned here because it can become a serious economic
problem when man settles on the land and accelerates gully formations
in virgin or undisturbed soil. Piping is a "tunneling erosion" where the
subsoil erodes out from under the surface (Fletcher and Carroll, 1948).
1. Types
In Arizona, some buildings and highways have been threatened with
almost uncontrollable loss of foundations. Fletcher and Carroll (1948)
report three main types of piping: (1) lateral flow of soil along a more
stable subsoil into an open stream bank, (2) flow along cracks of surface
water, and (3) flow taking place when dispersed surface soil is washed
into pore spaces of coarse, poorly graded, upper subsoil. All cause sinking
and caving of the soil in a "tubular" or "pipe-like" depression. Fletcher
et al (1954) describe five conditions necessary to initiate piping: (1)
there must be a source of water, (2) the surface infiltration rate must exceed the permeability rate at some subsurface layer, (3) there must be
an erodable layer immediately above the retarding layer, (4) the water
above the retarding layer must be under a hydraulic gradient, and (5)
there must be an outlet for the lateral flow. Piping is a serious problem
to agricultural loess soils in Central Asia (Glukhov, 1956), in the Niger
Valley of Mali, and parts of the Chad-Logone Valley. Saline and sodic
soils in Australia (Downes, 1956), are frequently eroded by piping. Alluvium where differential particle-size distribution has taken place and
where the subsoils are highly dispersed by soluble salts and exchangeable
sodium are most susceptible to "piping erosion." Earth shrinking and
cracking may initiate conditions for piping formations.
2. Methods of Controlling
Control of piping in building areas may be accomplished by deep plowing, ripping, or chiseling to mix the soil as much as possible to obtain
a uniform texture and packing before construction gets underway. One
of the suggested methods of control in agricultural land is to wet the soils
thoroughly to compact them before cropping. Another is to obtain uniform
soil mixing by use of deep farm tillage equipment.
F. SOIL STRUCTURE
Desert surfaces, as indicated in the earlier discussions, develop a wide
variety of structures as shown Fig. 11. Desert soil surfaces are normally
soft when dry. They may have a thin, platy structure which overlays a
vesicular structure (Jackson, 1957; Lapham, 1932). The platy surface