Removal of sand prior to grouting . - In the reaches of the jetties 

 opposite beaches above MLLW in elevation, as much as a foot of sand was 

 found to be stacked on top of the jetty core. This sand was removed at 

 each hole by flushing with sea water at high pressure. A piston pump 

 of l|0 g.p.m. capacity was provided for this purpose. 



Power . - The collection of equipment used by the contractor was 

 powered several ways. The mixer and pump, and the elevating belt were 

 driven by electric motors. Compressed air drove the wagon drill and 

 mine hoist. Water pumps were driven by gasoline engines. Electric 

 current was supplied by a $0-kw portable generator, and compressed air 

 by a 600-c.f.m. portable air compressor. 



EVALUATION 



The method herein described, for sealing the Mission Bay jetties, 

 is believed to have been successful in stopping the passage of sand into 

 the navigation channel. Surveys through November 19f>9 indicate no further 

 incursion of sand. Surveys as well as visual inspection show that a 

 shoulder of sand along the channelward toe of the jetty, much in evidence 

 before sealing, has disappeared since the supply of intruding sand has 

 been cut off. Wave action and tidal currents have carried this surplus 

 sand to the channel slopes in deeper water. 



There is little doubt regarding the permanence of the work. Specimens 

 of the grout immersed in sea water for 8 months show no indication of 

 disintegration from sulphides or other chemicals contained in sea water. 

 The very low porosity of the concrete seems to exclude water entirely. 

 Tests cylinders cast during construction tested 2,200 p.s.i. at 100 days 

 and the hardness of the concrete is estimated to be above 3 on the Mbhs 

 scale. Serious abrasion by water-carried sand is not likely as, on the 

 channel side, the attacking waves should be clear water, and on the beach 

 side, sand thrown into the interstices by the waves should remain in 

 sufficient quantity to form a cushion over exposed surfaces which would 

 prevent serious erosion of the concrete. For best results, work of this 

 type should be done during favorable tides and moderate sea action. 



Cost . - It has been calculated that the work under the experimental 

 contract cost about $6? per linear foot of jetty, and the final work, 

 about $Ul per linear foot, with combined costs amounting to about $U5> 

 per linear foot. An average of 1.06 cubic yards of grout per linear foot 

 was placed and the average spacing of holes was 6.3 feet. 



In the case of Mission Bay, any shoaling in the outer entrance 

 channel results in nuisance and hazards to the navigation of small craft 

 because shoals cause chaotic and breaking waves. Between 195>5> and 1958 

 shoaling took place at an annual rate of U5,000 cubic yards. The only 

 conclusion compatible with other facts was that all of this sand was 

 finding its way into the channel through the jetties. To maintain project 

 depth without sealing the jetties at this shoaling rate would require 

 dredging annually at a cost of about $U0,000. If this shoaling has been 

 prevented or even reduced to minor quantities, the cost of sealing the 

 jetties will be justified. 



