Alluvial fan

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					Alluvial fan
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Alluvial Fan in Death Valley

An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and
spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring
alluvial fans into a single apron of deposits against a slope is called a bajada, or compound
alluvial fan.(AGI, 37-8)

Figure 1. Fan deposits in Death Valley, California.
Owing to the slowing of flow any solid material carried by the water is dropped. As this reduces
the capacity of the channel, the channel will change direction over time, gradually building up a
slightly mounded or shallow conical fan shape. The deposits are in general poorly-sorted.(AGI
37-8) This fan shape can also be explained with a thermodynamic justification: the system of
sediment introduced at the apex of the fan will tend to a state which minimizes the sum of the
transport energy involved in moving the sediment and the gravitational potential of material in
the cone. It can easily be seen that there will be iso-transport energy lines forming concentric
arcs about the discharge point at the apex of the fan. Thus the material will tend to be deposited
equally about these lines, forming the characteristic cone shape. Multiple braided streams are
usually present and active during water flows. Alluvial fans are most likely to be found in desert
areas subject to periodic flash floods from nearby thunderstorms in local hills. Alluvial fans are
very common around the margins of the sedimentary basins of the Basin and Range province of
southwestern United States and northern Mexico.

Plants often are concentrated at the base of alluvial fans and many have long tap roots (30-50
feet) to reach water. The long-rooted plants are called phreatophytes by biologists. The water at
this level is derived from water that has seeped through the fan and hit an impermeable layer that
funneled the water to the base of the fan where it is concentrated and sometimes forms springs
and seeps if the water is close enough to the surface. These stands of bushes cling onto the soil at
their bases and over time wind action often blows away sand around the bushes which form
islands of habitat for many animals.

Figure 2. Cross-section of evolution of fan deposits.
Figure 3. Changes ind sediments along a fan.

                            Figure 4. Debris flow deposits are common on fans.
                     Figure 5. Wider view of debris flow deposits in Taiwan.

  •   American Geological Institute. Dictionary of Geological Terms. New York: Dolphin
      Books, 1962.
Figure 6 - Cross-sectional view of alluvial fan.

Figure 7. Map view of alluvial fan.
Figure 8. Photo of alluvial fan.
Figure 9. Road cut in alluvial fan deposits.
Figure 10. Close-up of alluvial fan deposits. Note particle size from cobble to fine-grained size.
   Evaluation, Design and Mitigation of Project Sites in Collapsible Soil
                       Areas in Western Colorado

                                  Steven L. Pawlak, P.E.

Collapsible Soil Properties

Collapsible soils occur as naturally relatively dry alluvial fans, colluvium and wind-
blown deposits. These soils are typically silt and sand size with a small amount of clay.
Debris fan deposits usually contain from small to large amounts of gravel to boulder size
rock fragments that are suspended in the finer-grained collapsible matrix. Collapsible
soils show relatively high apparent strength (cohesion) in their dry state, but have a low
density, porous structure and are susceptible to large settlements upon wetting. The
severity of the collapse depends on the extent of wetting, depth of the deposit and
loading from the overburden weight and structure. The wetting sources typically consist
of landscape irrigation, poor surface drainage resulting in ponding, utility line leakage,
and intentional ponding such as detention basins and water features.

Evaluation of Collapse Potential

The potential for collapsible soils at a specific project site is initially evaluated based on
the geologic and environmental setting, and our experience in the area. This includes
topographic and geologic maps that identify surficial deposits, their relative age, and
potential geologic constraints. The subsurface conditions are then evaluated by
exploration such as with power auger borings, backhoe pits and open excavations.
Borings are usually preferred because they can extend to depths sufficient to define the
depth of the collapsible soil deposits and they are less disruptive than backhoe pits.

Soil sampling typically consists of driving a "thick walled" sampler such as the 1 3/8 inch
I.D. Split Spoon Sampler (ASTM Method D-1586) or the 2 inch I.D. California Sampler
into the natural subsoils. The California Sampler contains 4 inch long liners which
accept the soil "core" as it is driven into the subsoils. This is typically referred to as a
relatively undisturbed sample. The liner sample is then sealed for later classification and
laboratory testing. In backhoe pits and open excavations, the 2 inch diameter liners
themselves are typically hand-driven into the exposed soils. This is similar to a "thin
walled" sampler and can result in less disruption of the soil compared to the thick walled

Laboratory testing is performed on the samples to evaluate their compression potential
and other physical properties. The compression test consists of extruding the liner
sample into the apparatus confining ring, loading it to 1,000 psf, flooding the sample and
additional loading. An example of a typical stress-strain curve obtained by the test is
shown on Fig. 1. The collapse potential is defined as the change in sample height (h)
upon wetting compared to the original sample height (Ho). The magnitude of collapse
potential is usually rated as low, moderate or high. Other physical properties testing
typically consists of natural moisture content and density, gradation, and liquid and
plastic (Atterberg) limits. It has been demonstrated that the collapse potential of the soil
depends on the composition, gradation, the initial water content and density, and the
loading at the time of wetting. A relationship of density, percent finer than the No. 200
sieve (silt and clay content) and collapse potential is shown on Fig. 2.

The severity of settlement and impact to structures which can result from collapse of the
subsoils depends on several conditions. These include:

                         •   Collapse potential of the subsoils
                         •   Depth of the collapsible soils
                         •   Foundation loading, configuration
                             and depth
                         •   Sensitivity of structure to differential
                         •   Site grading and drainage

The collapse potential and depth of the problem soil are physical constraints for a given
site. The foundation conditions, structure stiffness and other site improvements are
design development considerations and are more controllable.

Predicting settlements due to collapsible soils is difficult due to several factors including
sample disturbance problems, variability of the subsoils, extent of wetting and variable
loading conditions. Settlement estimates are generally made by taking the collapse
potential over the potential depth of wetting. This typically includes loading from the
structure and from overburden weight. Settlements due to structure loading generally
govern, since wetting usually occurs at shallow depth where the foundation loading
stresses are the highest. The settlements typically occur along the perimeter of the
structure and are differential. Relatively severe settlements and building distress have
been experienced where the collapsible soil depth is greater than about 20 feet.
Settlements up to about one foot and severe structural distress have been documented.
In shallower collapsible soil areas, settlements typically do not exceed a few inches.

Design and Mitigation

The identification of the soil collapse and severity of settlement potential is the initial
information needed for the mitigation design. The feasibility of possible mitigation
methods at a given project site depend on the structure conditions and the level of risk
that the project owner is willing to accept. More options will generally be available to
new construction compared to existing structures where there are constraints to
mitigation options. Some settlement and building distress is usually acceptable to
owners of lightly loaded structures such as residences, especially when the risks are
known to be common to the area. Less risk is usually acceptable where there are
heavily loaded or settlement-sensitive structures where the consequences of distress
warrant more effective mitigation measures. Mitigation methods can be divided into the
following groups:
                         •   Structure considerations
                         •   Site features
                         •   Collapsible soil avoidance
                         •   Ground modification

Structure Considerations: Lightly loaded structures such as residences are usually
supported on shallow spread footings. The footings should be lightly loaded with a
bearing pressure of 1,000 psf or less. The building loads should be carried mainly by
heavily reinforced foundation walls in a "box-like" configuration. The foundation design
should limit excess surcharge loading from deep backfills placed against foundation
walls. Stiffened slabs or mat foundations could be used to further reduce differential
settlement potential.

Site Features: Subsurface wetting from shallow sources can severely impact structures
founded on shallow foundations. The foundation backfill should be adequately
compacted and have positive surface drainage to prevent ponding. Gutters should be
provided with drain downspouts that discharge away from the building. Landscape
irrigation should be restricted and, in some cases, essentially eliminated by the use of
xeriscape. Basement foundation drains should be underlain with an impervious liner to
prevent water seepage below the foundation. On-site detention basins or water features
should be lined with an impervious membrane.

Avoid Collapsible Soils: When the collapsible soils are shallow, they can be removed
for bearing on the underlying soils or replaced and compacted to re-establish design
bearing levels. Piles or piers can be used to extend the bearing level to below the
collapsible soils. This alternative is typically considered where the structure is relatively
heavy or settlement-sensitive and the depth to adequate bearing material is
economically feasible.

Ground Modification: Various ground modification methods can be used to prevent or
limit collapse from occurring, or cause the collapse to occur before construction. These
methods include: partial removal and replacement of the collapsible soils compacted;
densification of the collapsible soil in-place, such as by compaction grouting or dynamic
compaction; and pre-wetting of the collapsible soil followed by surcharge loading to
cause settlement before construction.


Collapsible soil deposits are commonly found in western Colorado. These areas can be
successfully developed with the identification, evaluation and appropriate mitigation
designs to limit the settlement potential. The selected mitigation usually requires the
involvement of the owner and their knowledge and acceptance of some risk of
settlement and building distress.

Houston, W.N. and Houston, S.L., 1989, State-of-the-Practice Mitigation Measures for
Collapsible Soil Sites, Foundation Engineering Proceedings Congress, ASCE,
Evanston, Illinois, June 25-29, 1989, p. 161-175.

Houston, S.L. and Others, 1988, Prediction of Field Collapse of Soils Due to Wetting,
Journal of Geotechnical Engineering, January, 1988, p. 40-58.

Mock, R.G. and Pawlak, S.L., 1983, Alluvial Fan Hazards at Glenwood Springs,
Geological Environment and Soil Properties, ASCE Conference Proceedings, Houston,
Texas, October 17-21, 1983, p. 221-233.

Rollins, K.M. and Rogers, G.W., 1994, Mitigation Measures for Small Structures on
Collapsible Alluvial Soils, Journal of Geotechnical Engineering, September, 1994, p.

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