SUBSIDENCE  AND COLLAPSE

    

A.  INTRODUCTION 

        Land subsidence (the vertical downward movement of the land surface)
        has a variety of causes, including some that are related exclusively to
        human activities (e.g. mining and drainage of organic soils), and some
        that have entirely natural causes (e.g., isostatic adjustments).  
                     


B.  DEEP SUBSIDENCE

       1.  DESCRIPTION

              Deep subsidence is the slow downward movement of land caused by
            the compaction of sediments that occur below the earth's surface.
 
 

       2.  OCCURRENCE

            Deep subsidence occurs in locations where fluids under pressure are
            withdrawn from the subsurface.  This includes groundwater taken from
            confined aquifers and fluids pumped from oil and gas reservoirs.

            Central Valley, California: world's largest area of subsidence (geology)

            Long Beach, CA:  up to 30 feet of subsidence occurred over an oil field

 

       3.  HYPOTHESIS         

            When fluids held under pressure within the subsurface are withdrawn,
            the fluid pressure drops.  If the formation from which fluids were taken
            is composed of compressible sediments, a reduction in fluid pressure
            can cause the overlying formations to slowly subside. 
        

       4.  EVIDENCE         

            Santa Clara Valley, CA:  groundwater levels correlate with subsidence

            Long Beach, CA:  subsidence has been halted by using injection wells 

  

C.  SINKHOLES

        1.  SOLUTION SINKHOLES

              Solution sinkholes form where soluble bedrock (i.e., limestone,
              dolomite, marble, and rock salt) is exposed at the land surface
              and thus subjected to weathering by dissolution.  Surface water
              collects in natural depressions and slowly dissolves a sinkhole.
 
                           

        2.  SUBSIDENCE SINKHOLES

              Subsidence sinkholes are similar to solution sinkholes, except
              that the soluble bedrock is covered by a thin layer of soil and/or
              sediment. Surface water infiltration dissolves cavities where the
              bedrock is most intensely fractured, and the overlying sediment
              gradually moves downward into the expanding cavity.

 

        3.  COLLAPSE SINKHOLES

              Collapse sinkholes form when surface materials suddenly sink
              into a subsurface cavity or cave.  The cavities form slowly over 
              time, as groundwater moves along fractures in soluble bedrock 
              and enlarges them through dissolution, and the actual collapse
              can occur in two different ways:  

              a.  When a cavity gets sufficiently large, the "roof" becomes too
                   thin to support the weight of any overlying rock or sediment,
                   so it collapses into the cavity.  

              b.  Caves are sometimes able to support the weight of overlying
                   sediments because they are filled groundwater.  However, if
                   groundwater levels are lowered, then the overlying sediment
                   will first erode and then collapse into the dewatered cavity.

                   Example:   Lake Jackson drains into the Porter Hole Sink!

                                    (The history of Lake Jackson reveals a pattern)

        

             Collapse sinkholes are a problem in states where caves occur.

         

D.  HYDROCOMPACTION

       Hydrocompaction is notable not for the magnitude of the subsidence that
       occurs but for the fact that much of the western United States has the type
       of geologic conditions which are susceptible to this phenomenon. 

        1.  WHAT IS HYDROCOMPACTION?

              Hydrocompaction is the subsidence of shallow soils and sediments as a
              result of adding water to the land surface.  Typically this occurs in dry
              regions where agriculture relies on extensive irrigation.

 

         2.  WHY DOES IT OCCUR?

              The sediments that are susceptible to hydrocompaction were loosely 
              deposited in an arid or semi-arid environment by processes that left
              them with a very high porosity (> 45%).   As these sediments dry out,
              their high-porosity structure is preserved by clay particles that act as
              "bridges" to cement the larger particles together.  If water is added,
              the clay "cement" loses its strength, and the sediments subside under
              their own weight. 

 

 

 

 

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