(4) Subsidence of the filter materials by loss of fines while 

 developing a filter or by decomposition from seawater exposure and currents 

 created by wave action. 



(5) Subsidence of the island core due to additional consolida- 

 tion of the core from its own load and from superimposed loads. 



(6) Subsidence of the island core due to loss of material 

 through the revetments by wave pumping action. 



(7) Subsidence of the island due to consolidation from earthquake 

 shocks. 



(8) Subsidence in the vicinity of pipeway risers due to consolida- 

 tion of the core above elevation -20 feet, caused by material lost from 

 this zone when a sheet-pile cofferdam was used to excavate down to the pipe- 

 way flanges to extend the pipeway risers to the working elevation. 



(9) Subsidence in the vicinity of the conductor pipes due to 

 consolidation of the bottom material near the conductor pipe bottoms, 

 caused by material lost from this zone during construction. This loss 

 occurred when the interiors of the conductor pipes were excavated and 

 pumped dry to allow survey of their alignment. 



Of these sources, it was anticipated that only the subsidence of 

 revetments due to wave action or erosion, or a nearby major earthquake 

 would produce any significant settlement after construction. 



b. Level Check Surveys - A program of releveling has been conducted 

 to monitor elevations at selected points on the. is land. The surveys of 

 these points have all been referred to a bench mark on the pile-supported 

 wharf structure at the east end of the island. Because the piles supporting 

 the wharf penetrated to the siltstone formation, the wharf structure was 

 expected to show no subsidence unless area subsidence occurred. 



At less frequent intervals than for the intraisland checks, surveys 

 have been made to relate the wharf structure to the onshore level net 

 maintained by the Earth Sciences Laboratories of the National Oceanic and 

 Atmospheric Administration (ESL) . These level surveys to the onshore ESL 

 net and releveling of the ESL net have shown no significant elevation 

 changes for the island bench mark or the ESL net in the vicinity of the 

 island. 



Figure 9 shows the location of the island level check points; Figure 

 10 shows settlements measured during the latter part of the construction, 

 from April to mid- August 1958. During this period, settlements ranged up 

 to 0.4 feet and generally conformed to construction activities. Most of 

 the settlement can be attributed to elastic compression of the core, 

 revetments, and foundation materials as additional weight was added to 

 the island. 



32 



