logical selection of these parameters during the design phase, is 

 discussed in the subsequent section. 



Typical Configurations 



A number of typical foundations which have been used on the 

 seafloor are illustrated in Figure 2. The crossed strip footing (see 

 Figure 2a) is used to support a small vertical instrument. The 

 relatively wide spread of the foundation and its large weight (greater 

 than that of the instrument) improve stability against overturning. 

 The downturned leg of the angles acts as a keying edge and prevents 

 any lateral movement. When this edge is embedded completely, the 

 effective thickness of the footing is very small (less than 1/2 inch) 

 and scour should not be a problem. The foundation is made of standard 

 structural steel welded together. All members are oversized to allow 

 for corrosion. The foundation is isolated by plastic from the instrument 

 in order to prevent a galvanic couple and the associated accelerated 

 corrosion. 



The ring strip footing illustrated in Figure 2b is made of a 

 plastic pipe (polyvinylchloride - PVC) with aluminum struts and has a 

 steel anchor at the base. This large concentrated weight at the center 

 of the base, the low weight of the plastic and aluminum, and the wide 

 spread of the ring footing (12-foot diameter) all contribute to the 

 stability against overturning once it is emplaced. In the case of this 

 particular structure, the lifting point is only 3-1/2 inches above the 

 center of the submerged weight (z csw = 30.53 inches and z^ = 34.0 inches). 

 This and the fact that the vertical projected area (A ) is fairly large 

 and the total submerged weight is small (W , = 360 pounds) , increase 

 the possibility of large rotations during lowering, and thus the chance 

 of the structure being landed on its side, unless extreme care and a very 

 slow lowering rate are used. 



The multiple rigid spread footing foundation shown in Figure 2c 

 utilizes a reasonable spread on the individual foundation elements 

 and a fairly large submerged weight (W s ^ = 4000 pounds) to resist 

 overturning moments due to current drag. However, an inclined seafloor 

 would cause problems for a system with a large height to the center of 

 submerged weight (z = 12 feet) such as this. Also, the rigid footings 



(maximum articulation, a = 0) would not be desirable if the microtopography . 

 or surface roughness of the seafloor, were large. This system is made of 

 welded standard structural steel which is painted. 



Figures 2d, 2e, and 2f illustrate a single spread footing with a 

 keying edge, the embedding length of which, d e , is 4 inches. Figure 2d 

 shows this footing elevated on dunnage on dry land. The holes in the 

 upper surface of the footing are vents to allow the water entrapped by 

 the keying edge to escape as the edge embeds. Figure 2e shows the same 

 footing properly deployed on the seafloor in 600 feet of water. In this 

 case, the footing is totally embedded except for the structural backing. 

 This embedment reduces the possibility of scour caused by bottom currents 



