INTRODUCTION 



Seafloor installations may experience large total and differential 

 settlements because of the presence of highly compressible cohesive 

 sediments at many seafloor sites. Further, the potential is high for 

 bearing capacity failures of spread footings on these soft sediments. 

 Because of the likelihood that negatively buoyant seafloor installa- 

 tions will be emplaced in the future at deep-ocean sites with poor sedi- 

 ment properties, means for reducing or eliminating the potential settle- 

 ments or bearing failures have been investigated. One concept that was 

 considered is the preconsolidation of the sediment beneath a foundation 

 prior to emplacement of the supported structure. The technique of pre- 

 consolidation or precompression has been used often to stabilize con- 

 struction sites for terrestrial structures .^ »3, 4 ^he most common method 

 of preconsolidation is the application of a static preload prior to 

 construction. The preload must be left in place for a period of time 

 to permit the settlement to occur. Usually the preload is of greater 

 magnitude than the design structural loading in order to accelerate the 

 rate of settlement and decrease the required consolidation time. An 

 added benefit of preconsolidation is the accompanying increase in soil 

 shearing strength which improves the bearing capacity and lateral load 

 resistance of the foundation. 



Because of the difficulty of using a deadweight preloading method 

 in the deep ocean, the concept that was developed consists of inducing 

 a negative relative pressure in the pore water in the sediment. This 

 increases the particle-to-particle effective stress and causes 

 consolidation. •'-'^ Figure 1 shows the essential features of the concept. 

 The footing encloses a porous filter material within an impervious 

 keying edge or skirt. The pump applies a suction to the filter and 

 induces the negative relative pressure in the sediment. An accumulator 

 between the pump and the interior of the footing maintains the negative 

 pressure and allows the pump to be run intermittently. The flow of 

 water toward the filter will begin to establish a pattern similar to 

 the flow net in Figure 1. In the period of transition from no flow to 

 fully established flow (as represented by the flow net) , the effective 

 stresses in the sediment are increased and consolidation occurs. In 

 effect, a portion of the weight of the water column above the footing 

 is added as a surcharge load to the footing. After the desired degree 

 of preconsolidation is achieved, application of the negative relative 

 pressure to the footing underside may be discontinued, since the soil 

 consolidation process is largely irreversible (about 90% of the induced 

 volume change is permanent) . 



MBL/WHOI 



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