25 



COMPACTION EFFECTS 



3 . 1 INTRODUCTION 



Compaction results in the subsidence of ground level due to reduction 

 in the void ratio of the underlying soil, and in coastal areas contributes 

 to a local relative rise in sea level. Reduction in void ratio is often 

 the natural response of a soil to an increase in loading, because an 

 increase in the interstitial stresses between solids is required. An 

 increase in the loading of a soil stratum can be the result of an increase 

 in loading on the ground surface (e.g., building construction or additional 

 sediment deposition), or due to removal of ground fluid (e.g., water, oil, 

 or natural gas) . Compaction occurs in nature as mud is deposited on the 

 beds of rivers and estuaries, and especially in river deltas. Another 

 example is the increase in loading as a barrier island migrates over a 

 stratum of peat, causing the peat to compact and ground level to subside. 

 Because compaction is a time -dependent process, the relative rate between 

 deposition and compaction will determine whether bed elevation increases or 

 decreases. Compaction of a region can also be induced by man, due to 

 1) loading by the weight of structures, 2) the extraction of oil and 

 natural gas, and 3) depletion of the groundwater table due to active 

 pumping or by preventing recharge of aquifers. 



The literature in soil mechanics and foundation design is too replete 

 with articles on the general topic of compaction to review in detail. The 

 proceedings of a symposium "Land Subsidence" held in Tokyo in 1969 (in 

 reference list in section 12) provides a thorough treatment of the causes 

 of compaction, its theoretical description, field measurement techniques 

 and analysis, physical consequences and remedial measures. Much of the 

 subsequent material is gleaned from this collection of studies. The 

 inverse problem (i.e., the effect of sea level rise on compaction and 

 subsidence) has received only limited attention, most recently by Chappell 

 et al. (1982). However, their calculations indicate that a sea level rise 

 on the order of tens of meters would be required for noticeable subsidence 

 of the ocean floor and possible raising of the adjacent land mass. 



