Field studies of structure damage

ft, but actual damage estimates would have to be

Some structures were observed to sustain dam-

augmented by an analysis of structural integrity

age during icebreaking operations when the ice

on a site-by-site basis. The 1-ft criterion was only

cover was so violently disrupted that chunks of

meant to define river areas where reasonably well

ice were thrown against them. While most dam-

built structures could be exposed to damaging

age was minor, three of the docking structures

ship effects and to compare the damage poten-

monitored had portions substantially destroyed,

tial of various fleet mixes.

and a boat house was so severely damaged dur-

In forming their estimate of the potential for

ing the opening of navigation in March that it

vessel-related structural damage, Hodek et al.

later collapsed.

(1986) considered both vertical and horizontal

In that study (USACE 1974) arrays of survey

dynamic loadings. For the areas of St. Marys River

stakes were laid out at six sites to monitor ice

considered in the report, they felt that dynamic

movement in the offshore direction. At one site

horizontal loadings would be small for cases with

the ice was found to have moved as much as 30 ft

a continuous ice sheet (such as those that com-

offshore over a period of 40 days, but typical ice

monly occur during the extended season). If ves-

movements were on the order of 10 ft or less. Some

sel passage were sufficient to break the ice cover

lateral movement was also observed at three sites.

into individual pans (as documented in USACE

In connection with some of their recordings of

1974), the potential for damage by horizontal forces

water level fluctuations, they also measured the

would increase. For vertical ice forces, they felt

relative motion of the ice sheets on either side of

that gravity structures, such as rock-filled tim-

an active crack. These data indicated that the off-

ber cribs, would not be damaged by vertical forces.

shore ice sheet could move at least several tenths

The rest of their analysis was limited to uplift forces

of a foot offshore during vessel passage (appar-

on vertical piles. They concluded that all piles,

ently in response to the drawdown-induced move-

unless properly engineered and installed, will be

ment of water) and that the ice cover did not fully

subjected to tensile loads in excess of their ca-

return to its original horizontal position when the

pacity and will eventually move upward.

water returned to its prior elevation.

It is postulated here that this mechanism re-

sults in a gradual horizontal jacking of the ice

Field studies of

cover towards the navigation channel, since the

structure damage

The first field study of shore structure dam-

water in the open shore crack can partially or

age during winter navigation was conducted by

totally refreeze before the next ship passage. Start-

the Detroit District on the St. Marys River begin-

ing from this new position the next event will

ning in 1972 (USACE 1974). They selected sev-

leave the offshore ice sheet slightly farther from

eral docks in two reaches of the river for moni-

its original location. Repeated cyclings of this

toring: near Six Mile Point on Lake Nicolet and

backing process could lead to the large horizon-

Johnson's Point on Neebish Island. Some of the

tal motions recorded over the winter season and

docks monitored were pile supported, while others

lead to increased ice volumes in the vessel track.

were constructed with rock-filled timber cribs or

If the crack were to pass through a structure, this

steel piling. Monitoring was limited to visual

horizontal jacking could incrementally pull the

observations and photographs. Their conclusions

structures apart.

were that water level fluctuations and ice floes

A follow-up study by the consulting firm of

can cause structural damage and that navigation

Dalton, Dalton, Little and Newport, Inc. (1975)

in ice can be a contributing factor.

concluded that although ship passage during ice-

The pile-supported portions of structures were

covered conditions can contribute to structure

found to be the most susceptible to damage, but

damage, a major factor is the inadequate design

even timber crib structures were not immune. Jack-

and construction of many structures on the river.

ing of finger piers due to vertical ice forces in-

They concluded that control of vessel speed was

duced by natural water level fluctuations and

not important since they considered ship-gener-

drawdown was common, and several structures

ated waves to be of minimal importance. Instead,

were damaged as the result of differential ice move-

they recommended improved structural design

ments along active cracks that passed through

to withstand natural forces and consideration of

them. Water levels were high enough that some

removable, floating structures.

structures were damaged by ice in contact with

Carey (1980) reviewed the potential for win-

the horizontal members of the superstructure.

ter-navigation-related shore structure damage on