APPENDIX B: REVIEW OF SLOPE PROTECTION SYSTEMS USED IN THE ARCTIC
Sand bag slope protection
yielding armor gaps near the waterline. Due to the
Since 1972, sand bag slope protection has been
50% overlap bag placement configuration, there
used effectively in the offshore islands built along
were abundant bags that could slide down slope
the Canadian and the Alaskan coasts of the south-
to cover the voids and provide slope protection
ern Beaufort Sea. Gadd (1988) described the design
for the life (ca. 510 years) of the island.
and the development of slope protection by the use
Sand bag slope armor has proven to be effec-
of sand bags. He discussed the placement of sand
tive protection for short-lived offshore structures
bag armor on two artificial islands: Resolution Is-
in the Arctic. However, it can be damaged by ice
land in 2.4-m water depth, and Seal Island in 12-m
during both summer and winter, and also by ultra-
water depth. Both these islands had a design life
violet radiation affecting the strength of bag fab-
of three years. Because of the limited design life
ric. Periodic repairs should be conducted to pre-
and also unavailability of large stone at the sites,
vent progressive deterioration and to prolong the
sand bag slope protection was considered to be the
functional life of a sand-bag slope armor system.
only viable option.
Resolution Island required about 100,000 m3 (3.5
Concrete mat slope protection
106 ft3) of gravel that was delivered to the site by
As mentioned above, the sand bag armor is
barges from a gravel pit onshore. The 1.5-m3 (53-
susceptible to damage from ice impacts and loss
ft3) bags were filled from a barge-based bagging
of fabric strength from exposure to ultraviolet ra-
plant and were placed end-to-end fashion (no over-
diation. Moreover, the filling and placement of
lapping) with a 60-ton crane at a maximum rate of
sand bags are time-consuming and labor-intensive
30 bags per hour. After completion of construction
operations. Because suitable quarry sites are scarce
in 1980, annual inspection of slopes above and be-
along the Beaufort Sea coast, articulated concrete
low water were conducted. Damaged bags were
mats have been selected as an effective and eco-
replaced in 1981 and 1984. Because of shallow
nomical slope protection in the Arctic since 1980.
water and resulting mild environment, damage
Leidersdorf (1988) has given a summary of Arctic
due to wave action alone was not observed. The
experience with concrete mat armor installed at
principal cause of bag loss was due to ice abrasion
six sites along the Alaskan Beaufort Sea coast. Po-
and puncture, followed by fill loss due to wave
tential advantages of concrete mat armor are 1)
action on torn bags.
low weight per unit area because of interblock link-
Seal Island was constructed in 1982 about 17 km
ages, 2) resistance to damage from ice impacts and
(10.5 miles) northwest of Prudhoe Bay. The envi-
low temperatures, 3) ability to accommodate
ronmental forces of moving ice and summer wave
changes in the subgrade without major fill loss,
action were larger at this sites than those at any
and 4) modular placement and removal. Leiders-
previous site in Alaska. To construct the island core,
dorf (1988) discussed hydraulic, ice, and material
about 535,000 m3 (18.9 106 ft3) of gravel was trans-
design considerations; fabrication and installation
ported to the site from an onshore quarry in win-
procedures; and additional research needed to
ter over an artificially thickened floating ice road.
understand their failure modes, hydraulic stabili-
About 17,100 gravel bags, each 3 m3 (106 ft3) ca-
ty, and long-term durability. The capital costs of
pacity, were used to protect the slope. Based on the
installing a concrete mat armor are higher than
results of model testing, the bags were arranged in
those for sand bag armor, but the maintenance
a 50% overlap configuration above 6 m (20 ft)
costs are comparatively lower. Hybrid slope pro-
level to enhance their stability. Bags were produced
tection systems, consisting of sand bag armor and
on-site at an average rate of 400 bags per day. Float-
concrete mat armor, have been used to cut down
ing ice damaged the sand bags located between
the construction cost. On Northstar Island, the
elevations +1.5 m (+5 ft) and 6 m (20 ft). The ice
concrete mat extended from +1.5 m (+5 ft) to 6 m
would tear the bag fabric, allowing the gravel fill
(20 ft) levels, and sand bag armor was used above
to escape. During two late summer storms, many
and below the concrete mat armor (Leidersdorf
bags were removed by the 2-m-high wave action,
1988).
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