Page 85 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
80 Geotechnical Engineering

Figure 4.7
Filled former river channels (clay plugs) occur at curved outer margins of the extensive point bar
sand deposits. A newer channel has remnants of an oxbow lake. The downstream progress of this
meander was halted by the older clay plug, which caused the cutoff. The smaller oxbow probably
involves a tributary river. At the lower right, patterns on older parts of the floodplain are obscured by
trees and surficial backswamp clay deposits. The lighter area rimming the larger clay plug has a
road and is a natural levee silt deposit. (USDA photo.)

Meander migration is not smooth owing to periodic bank caving and flood surges,
so point bars typically contain arcuate ridges that represent former river margins.
During high river stages the shallow channels across a point bar may be reoccu-
pied and can even develop their own scaled-down meander patterns. Riverboat
pilots called these channels ‘‘chutes,’’ and used them as shortcuts during high
water. The risk was in getting stuck when the river level went down.

Oxbow Lakes and Clay Plugs
If for any reason the downstream progress of a meander is impeded, the next one
upstream may catch up, causing a neck cutoff. This leads to a sequence of events
that is very important in geotechnical engineering. First, the abandoned river
channel rapidly becomes plugged at the ends to create an oxbow lake. Because
the oxbow is isolated from the river, fine sediment carried into the lake during
high river stages is trapped and slowly settles out to form a clay deposit called
a clay plug. Clay plugs are poor foundation materials and are readily identified
from airphotos. Where clay plugs cannot be avoided, special measures such
as temporary surcharging will be required to reduce or control settlement.

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