within 10 years is conservative. Presently no 
medical facilities on the Gulf Coast are available 
and attuned to the needs of divers. Three to five 
diver deaths occur each year in the Gulf. 
k. Bottom Activities Many activities already are 
taking place in the shallow bottom areas. Tunnels 
built on the surface in sections can be economical 
where the conditions are favorable, as in protected 
waters with unconsolidated sediments on the 
seafloor. Sections of tunnel several hundred feet in 
length are fabricated on shore, floated to the site, 
lowered into a dredged trench with a suitably 
prepared foundation bed. The sections are joined 
together with the aid of divers, and the trench is 
backfilled. The Trans-Bay Tube now under con- 
struction for the San Francisco Bay Area Rapid 
Transit System is an example. It has tunnel 
sections about 48 feet wide and 22 feet high, 
located 135 feet below sea level at the deepest 
point. As advances in procedures for working 
under water are made, open-cut tunneling may be 
extended to deeper, less-protected offshore sites. 
Tunneling in soft ground follows the same 
general procedure as tunneling in rock, except 
drilling and blasting are not required, and critical 
attention must be given to temporary lining. For 
very soft and wet ground, especially for undersea 
sites, shield tunneling methods are used. While the 
technology of shield-driven tunnels is well along, 
continued improvements will increase advance- 
ment rates and reduce costs. 
Site preparation can be accomplished by pile 
driving and caisson sinking—standard operations in 
conventional construction work, particularly in 
the nearshore regions. In the offshore petroleum 
industry, piles and pipe caissons are being driven 
from barges in water depths exceeding 300 feet. 
The most commonly used drivers are pneumatic 
hammers, sometimes combined with such supple- 
mentary means as jetting and drilling. Vibratory 
and sonic drivers are used occasionally and may 
have considerable growth potential. 
In the nearshore zone, concrete can be placed 
underwater from the surface using either drop- 
bottom buckets or tremies.* Bags filled with 
concrete or grout intrusion of emplaced aggregates 
are sometimes used. Methods for trenching to bury 
SFunnel-like devices lowered into the water to deposit 
concrete. 
VI-114 
pipelines and cables include plowing, hydraulic 
jetting, combined jetting and suction dredging, and 
use of shaped charges. 
Protecting offshore oil and gas pipelines from 
damage by dragging anchors, soil movement, and 
underwater currents has required improved deep 
burying techniques. Large trenching barges can dig 
ditches 12 feet deep and 5 feet wide in water 200 
feet deep, using a special dredging sled pulled 
along the pipeline. Towed sea plows on sleds are 
used to bury seafloor cables. A sea plow of 
sophisticated design was used to bury more than 
100 miles of transatlantic cable in waters from 120 
to 900 feet deep to safeguard it against damage 
from fishing vessels. 
Dredging, a well-established construction and 
mining technique in shallow waters, has been used 
to deepen navigation channels, remove overburden 
for foundations, excavate open-cut type tunnels 
and outfalls, mine and place fill materials, and 
recover placer and seafloor deposits. The dredges, 
however, are severely limited in capacity and 
cannot be considered for major seafloor construc- 
tion or mining in deep water. 
Construction and mining require moving large 
amounts of material. Commercial mining requires 
production rates of thousands of tons per day to 
be economically justifiable. Capacities of this 
magnitude to 2,000-foot depths may be achieved 
by developing improved hydraulic or airlift 
dredges. 
2. Future Needs 
Improved cements that will set rapidly in low 
temperature sea water and concretes more resist- 
ant to deterioration in sea water are needed. As 
demand grows for concrete foundations in waters 
to 2,000 feet. major improvements will be needed 
in placing concrete from surface barges. Concrete 
mixing and emplacement on the seafloor using 
underwater plants or equipment may be required 
in the more distant future. 
There will be a need to extend trenching and 
dredging operations to greater depths and deeper 
cuts. Devices such as mobile breakwaters and 
pneumatic curtains to shield operations may be 
needed. The ability to observe and monitor under- 
water would help greatly to increase operating 
efficiency, but a major breakthrough in observa- 
tion techniques in turbid water will be required. 
