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 Corrosion, deterioration, subsidence and failure of bulkhead.
Symptoms:
Collection of Facts: To determine the extent of bulkhead deterioration, field
inspections were conducted by the Navy at many Naval Stations. Some important
findings were:
(1) Corrosion of sheetpiling occurred mainly in the low tide range,
between El. 0.0 to -3.O ft.
(2) Evidence of deterioration was seen on the side faces and webs of
the Z-sections, where honeycomb type cavities and large holes
had developed. In most cases, there was a clear and consistent
pattern. A band of heavy corrosion appeared in the low tide
range, from below the mean low water line to 3 ft below.
(3) Below the low tide range and down to the mud line, corrosion
appeared to be relatively light. Corrosion in the upper tide
range and splash zone was moderate.
(4) Anchor-rod and wales generally showed very little corrosion.
Fig. 2 shows the condition of an anchored bulkhead after 25 years in
service. Fig. 3 shows the corrosion on a sheetpile 28 years of age. As a
result of the field inspection findings, the Navy has designed and constructed
replacement bulkheads by driving a new sheetpile in front of the old sheetpile
and connecting the two by a tie-rod. This design method was used recently at
the U. S. Naval Station, Annapolis, Maryland. In 1942, a bulkhead was built,
consisting of 232 section sheetpiling, double channel wales, steel tie-rod,
and timber pile anchorage. In 1976, it was found that deterioration had
occurred in the intervening 34 years. The damaged bulkhead was replaced by
installation of new 227 sheetpiles in front of the old sheetpiles. Fig. 4
shows the actual bulkhead replacement.
Solution: Rather than replace corroded sheetpiles, it may be more economical
to drive new sheetpiles a short distance, say, 3 to 4 ft, in front of the
existing sheetpiles. To calculate the supporting effect of the old sheetpile,
the method of stability analysis can be modified. Fig. 5 shows a scheme of
the bulkhead replacement. The modified stability analysis and design
procedures are presented as follows:
(1) Calculate the active earth pressure, Pa, to a depth of Dmin
which is about 20% shorter than the depth D.
(2) Assume 60% of the anchor-rod's yield strength will be available
to resist an overturning force, or use the anchor rod pull force
calculated from the free-earth support method. If earthquake
induced dynamic loading is to be included in the design, then
75% of the anchor-rod yield strength may be used.
(3) Calculate the weight of backfill, W, placed between the new and
existing sheetpiles.
(4) Calculate the passive earth pressure, Pp, from the assumed
penetration depth D' which is equal to Dm i n.
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