The tops and probably inucn of tUe slopes of submarmine mountains 

 are essentially areas of nonaccumulation because currents remove the 

 fine pelagic sediments. Initial deposits have accumulated in hollows, 

 small erosion channels, and between rock elements (i.e., erosional 

 debris, pillows, etc.). ^^Jhen the surfaces have been filled nearly 

 plane, currents keep them free of additional accumulation.-'-'^ On some 

 older seamounts, rock outcrops are so encrusted with manganese oxide 

 that sampling of the bedrock is difficult.-'--' 



Engineering Significance . The performance of a foundation placed 

 in this environment appears to depend most on the capability to find a 

 reasonably level shelf and then to place the foundation on it: micro- 

 relief slopes of five degrees and possibly up to ten degrees can be 

 accommodated, but not the maximum seamount and ridge slopes of 25 to 30 

 degrees. The problem is one of preventing downslope sliding; it is 

 difficult to obtain purchase on slopes composed of pillows, pillow 

 fragments, hyaloclastite , and intermingled sediments. Fortunately, 

 near-level relief (microrelief less than three feet) does occur along 

 the axes of mid-ocean ridges and rises, possibly in the summit craters 

 of submarine volcanoes, and on wave cut benches and flat-topped 

 seamounts . 



A foundation in these areas will be required to resist large 

 lateral loads from seafloor currents. The use of pointed legs to bear 

 on the rock and engage it is not an ideal solution to the lateral load 

 problem because such legs may penetrate between surficial pillows and 

 pillow fragments or through sediment possibly resulting in greater 

 deviation from level than that with a surface bearing plate. Legged 

 foundations with integral bearing plates, as the AFAR towers. Figure 

 2, should serve much better. 



Manganese Precipitates 



Occurrence . Manganese deposits occur in varying degree and form in 

 most oceans and in water depths from 25 to 20,000 feet. Significant 

 manganese coatings may be found on rocks of older seamounts. Manganese 

 deposits also occur as nodules and as "pavements" apparently over un- 

 consolidated sediments. Spherical nodules range upward in size to about 

 20 cm (eight inches) as spheres, but flatter ones do occur ranging up 

 to one meter (three feet) in diameter." Nodule densities range from 

 one or two per square meter to a tightly packed cobblestone mass.-'-" 

 Pavements of manganese concretion appearance are found, the most notable 

 on the Blake Plateau. Thicknesses have not been measured, but tabular 

 manganese slabs in the area were 20 to 30 cm thick and one-half to one 

 meter across.-'-' 



Engineering Significance . Seafloor installations in nodule areas 

 should employ spread footing type foundations, ° although special con- 

 sideration may have to be given to the problem of lateral load 



