There can be few deep sea ports that never need dredging. Until very recently Long Beach in California, together with its rival neighbour Los Angeles, defied the laws of hydraulics and never clogged up.
Or so it seemed. In fact the seabed did silt up. It's just that it was sinking at about the same rate. 'The port was within an oilfield area, ' explains Ari Steinberg, programme management consultant for a major expansion project currently under way.
'As oil came out, the ground went down.' The main channel is 23.4m deep - enough for even the biggest ships.
Although 'nodding donkeys' throughout the area still pump oil, these days the relatively shallow reservoirs are re-injected with water to prevent uncontrolled ground settlement. As a result, Long Beach has been forced to begin dredging its channels.
The new work is to ensure that even the new generation of 'super post Panamax' container ships can berth at two new earthquake-resistant wharves now under construction. Long Beach is the world's sixth biggest container port, or taken with Los Angeles, the third largest. Trade is growing, and a new rail corridor through the city, the Alameda project, will soon increase it further. It must offer the 17m water depth required for the 6,600 container monsters to dock fully laden when their draught is 14.6m.
Two areas are up for development - Pier S and Pier T. The former is undergoing a relatively straightforward soil remediation and infill job to create new high ground. Pier T is an old US Navy facility, now demolished and with a major rebuild under way. Along with it goes a large-scale deepening of the west basin.
'The Godot Construction Company is doing the work on Pier S, ' says Steinberg. 'The old 1920's oilfield there caused the land to sink 8m or so and it is pretty dirty. We have to clean it up and infill with some 2.6M. m 3of extra soil.
'Basically, we are sandwiching the old stuff. We move it to one side and stockpile and then bring in fill to above ambient groundwater level. The old stuff is stabilised with cement above that and then we cap it with clean material.'
About 800 truck movements take place a day, he says and, 'we have probably cornered the market in this part of California for dirt. It is not 'dirt cheap' any more - it is $8 a ton'.
Further contracts will be let next year for wharf construction around a couple of reserved areas for continued oil extraction. But on Pier T the big works are already well under way.
The old Navy yard here was a 'mini city' until the early 1990s, says Art Wong, a spokesman for the port. 'There were bowling alleys, cinemas and hotels.' There were also three large drydocks and support buildings which have now been leased out to a private operator. Meanwhile, the rest of the area has seen complete demolition.
The 112ha area behind the sea frontage is being graded and infilled by local company Excel Paving, which will also install drainage, fuel lines and other services under its $37.3M contract which started last May.
But a contract twice the size is a joint venture between Manson Construction & Engineering and Connolly Pacific. This £44M work is for dredging and construction of the wharf frontages, an initial 800 length first, with a doubling later. Work is checked by consultant CH2M Hill.
Dredging is being done mainly with a cutter suction unit discharging slurry via a floating pipeline to shore. A barge mounted crane with a grab supplements the activities. The West Basin will be deepened to 15.3m with 17m at wharfside.
According to a geotechnical investigation by Diaz-Yourman & Associates, 'dredge material varies considerably in consistency and material types from clean (free of fines) sands, silty sands, silts, to clays. Only limited zones make good beach replenishment and hydraulic fill materials.
Local beaches have benefited from some 100,000m 3but most of the 2.7m 3from the basin floor must be disposed of.
One other barge has a Clyde Model 32 crane on board fitted with a Demag D100 piling hammer. It is steadily driving seven rows of vertical piles to make up the wharf wall; these are octagonal section precast concrete units, 600mm in diameter and up to 30m to 40m length.
Piles are needed to resist possible liquefaction of loose sands in the upper ground. Los Angeles is a Zone Four seismic area, the USA's most severe category, which means Richter 7 or 8 earthquakes can be expected, and a 9 cannot be discounted. Design criteria are for peak ground acceleration of 0.22 G for operation and a contingency level of 0.53.
The soils in the wharf area are fine sands and silty sands and there are fine-grained silt/clay layers that are weaker than the sands. 'A dense sand bearing layer is encountered at elevations near -27m mean-lower-low-water (MLLW), ' says Diaz, and piles bear here.
But some piles meet earlier resistance in dense sand layers, causing refusal. About a quarter of the piles have had problems, and a jetting technique is being used to get them through.
To stiffen the structure, a double row of raked batter piles is driven as well as the seven plumb rows. A Manitowoc 4600 crane, owned like the barge crane by the contractor himself, tracks along the shoreline with a Demag D80 hammer for this work. The whole will be tied together with a pilecap to form the wharf loading area, running on to the paved stacking sectors behind.
Exactly when two 400m long wharf extensions will be added to the initial 800m is not yet clear, says Wong, because the port is still finalising a tenant for the area.
A deal with the mainland China Ocean Shipping Company fell through after a two-year political wrangle when Congress passed an order blocking the lease. A Korean firm is the likely replacement.