Skirtless Preconsolidating Footing 



Emplacement of a preconsolidating footing with vertical skirts must 

 include expulsion of water trapped by the skirts between the footing 

 base and the sediment surface. Expulsion has been accommodated in sim- 

 ilar full-scale situations by fitting the foundation with vent pipes 

 with flap valves such that water can pass out but not back in. Experi- 

 ence with the models has shown that these vents must have a high flow 

 capacity because otherwise the trapped water will exit around the skirts 

 eroding channels and also may cause the soil beneath the footing to lose 

 strength and flow. This often resulted in excessive footing tilt. 



To eliminate the water exit problem, two attempts were made to 

 eliminate the vertical skirt and replace it with a horizontal skirt 

 extending out to 9-inches diameter (on a 6- inch diameter model) . This 

 modification was unsuccessful on a model scale because of the difficulty 

 of sealing the horizontal skirts (membranes) to the soil surface. 



Applicability to Full-Scale Foundations 



This laboratory model study and the supporting analytical study 

 (see Appendix B) have indicated that the preconsolidation concept 

 applied to seafloor footings can reduce structure settlements to 1/5 to 

 1/20 that of conventional, non-preconsolidated footings; and preconsol- 

 idation can, at the same time, double the ultimate vertical and horizon- 

 tal load carrying capacities. 



Such perforinance improvements are valuable for installations 

 sensitive to large total settlements and/or large tilt, for Instance, 

 equipment emitting a directional beam or equipment intended to locate 

 a sound source. Often seafloor foundation size is dictated by the 

 space available on the launch vessel and/or by the emplacement- line 

 load capacity, and the engineer is faced with the problem of making a 

 too-small foundation adequate. Here, preconsolidation may be of benefit 

 by increasing vertical and/or lateral load carrying capacity. 



The preconsolidating footing also offers another benefit: the 

 resistance of the footing to long-term pull-out loads is considerably 

 improved. Thus, the preconsolidated footing, as long as the "suction" 

 is maintained, will provide Improved overturning resistance and may 

 even be used as a tie-down. 



The seafloor preconsolidating footing concept does have a drawback: 

 the footing and supported structure must normally be placed separately. 

 The footing must be installed first and the preconsolidation pressure 

 applied for a time period on the order of one month prior to application 

 of the structure load. Thus, two separate sea operations are typically 

 required. In addition, the structure emplacement operation requires 

 the capability to mate the structure to the footing on the seafloor. 

 But despite these drawbacks, the preconsolidating footing concept re- 

 mains viable because alternate foundation systems capable of supporting 

 heavy seafloor installations, with little total settlement and tilt. 



15 



