A Strandberg1, P Spencer2, G Strandberg3, and U Embacher4
As presented at the International Conference and Exhibition on Performance of Ships and Structures in Ice, Banff, AB, Canada, 17-20 September 2012
Spring extended-season ice road operation near Prudhoe Bay, Alaska, Snap Lake, NWT and Attawapiskat, Ontario is studied in this paper. Spring season operation at each of these sites is presented with a focus on engineering design and monitoring of conditions relative to accepted standards for safe operation. Ice profile temperatures and borehole jack strength measurements are a key parameter discussed in this paper to ensure safe operation. Date of ice road closure during a particular year or years of operation is presented for each site.
Extending the operation window of an ice road into the spring season requires procedures that preserve the ice road surface and structural integrity. Topics on extending spring season ice road operation window presented in this paper are as follows:
A case study of three cold region locations in Alaska and Canada (Attawapiskat, Ontario, Snap Lake, NWT and near Prudhoe Bay, Alaska) is presented in this paper to highlight spring season ice road procedures for prolonged operation.
Location Climate Summary
The three cold region locations are as follows;
These three sites as depicted in Figure 1 have varied severity of winter/spring season cold as reflected in the FDD for each site. The FDD for the three locations varies from 2,500 to 4,800 C-days as given in Table 1. The freezing-degree-days (FDD units °C-day) equals the cumulative sum absolute of the daily average temperature below 0°C. One FDD equals (0°C-TAVE)*1day where TAVE is the average daily temperature below 0°C. The starting date of cumulative summation was selected to produce the maximum FDD index number over the winter season.
The FDD values for the three locations are from Lobacz (1986) and Environment Canada Climate Normals (2012). Table 1 presents average daily temperatures at Attawapiskat, Yellowknife, Snap Lake and Kuparuk. The Kuparuk weather station which is near the offshore Alaska man-made island addressed in this paper was employed for site daily max and min average temperatures (Wunderground, 2012). Snap Lake daily max and min average temperatures were calculated as Yellowknife max/min minus 2.3°C to account for the 425 C-days FDD difference over a 180 day winter period.
Ice road operation procedures in the spring season for maintaining the ice road are similar at each site but are implemented at slightly different times of the year. Ice roads with spring operation procedures implemented can typically be employed until early April near Attawapiskat, until early May near Snap Lake and mid-May to late May and sometimes to early June near the Alaska man-made island (Table 1).
Table 1 Climate Data - Attawapiskat, Yellowknife/Snap Lake and Kuparuk
Figure 1. Location of Ice Road ProjectsMaintaining an Ice Road Insulating Layer
Maintaining and Ice Road Insulating Layer
Maintaining an insulating layer on an ice road can be accomplished with two basic options, as follows;
Using snow as a natural insulator typically involves grading and blowing snow onto the ice road as the spring season progresses. Snow windrows that accumulate along the ice road through the winter season can be employed for this purpose. Sometimes this procedure involves the use of more than one route option on the same ice road. As melting progresses in the spring season, one of the constructed routes or a portion of the width of a single road is covered with a layer of snow to preserve the surface and ice road profile temperature. At a key point when a load requires transfer on the ice road, this covered part of the ice road is uncovered and employed in a timely fashion. This procedure works well for a rig move where drilling will continue well into the spring season.
Constructed Ice Road Width and Spring Season Operation
Constructing and maintaining an ice road at a proper width is an important aspect for spring season extended operation. The ice road should be opened to full design width (approximately 60m) as shown in Figure 2 (CGU, 2003). The ice road should not be widened as the season progresses for safety concerns due to weakened and thinner ice under the snow banks at the margin of the ice road.
Figure 2 shows a typical ice road cross-section with 60 m road width and snow banks on road margin. The induced ice added thickness is formed due to exposure after the removal of the snow cover during the initial stages of building the ice road.
The use of a snow blower or high speed wing blade on a plow truck to avoid high snow banks at the road shoulder is a construction procedure that should be employed. High snow banks at a road edge cause high stresses and consequently will result in excessive deflection of the ice sheet. This will become a safety issue especially with spring season operation where the deflection pushes the ice road surface below water level and flooding occurs. In mid-winter this flooded water on the ice road will freeze and become part of the ice road. Often the flooded water under a deep snow bank will not freeze and, as such, is a safety hazard to equipment that ventures into the bank. In the spring season this flooding due to snow banks may not freeze. If the road is built too narrow, then the entire road can flood in the spring season and cause an early shut down of the ice road. Also with high snow banks when the ice road is insufficiently wide, a stress crack can form along the centre line of the ice road. This stress crack can cause flaking due to traffic which deteriorates the centre of the ice road.
Figure 2. Ice Road Cross-section
Access Ramps and Spring Season Operation
An access ramp to an ice road from shore should have properties that promote operation through the spring season, as follows:
Example 1 - Alaska Mainland to Man-made Island Ice Road
The sea ice road from the Alaska mainland to a man-made island in the Beaufort Sea, Prudhoe Bay, Alaska was operated during the 2010-/11 and 2011/12 winter and spring season.
Figures 3 and 4 show covering the man-made island ice road with a snow layer for protection against exposure to sunlight. Figure 3 shows this ice road with an approximately 10 cm protective snow layer on the left portion of the ice road on 2011 04 22. Figure 4 shows a similar 10 cm protective layer being placed with a snow blower on 2012 04 15. In both cases this snow layer was removed in early May to allow use of a portion of the ice road that was well protected from sunlight prior to use.
The snow blower in Figure 4 is also placing a 2 to 3 cm snow layer on the travelled surface on 2012.04.15 to produce a high albedo which reduces radiant heat absorption into the ice road. This 2 to 3 cm snow layer on the travelled portion of the roadway was packed by traffic to produce a good ice road traction surface as seen in Figure 3.
Daily inspection of the ice road must ensure that dirt and gravel is not dropped on the ice road compacted snow surface. The width of the man-made island ice road in Figures 3 and 4 was approximately 60 m (200 ft) snow bank to snow bank. No flooding of the travelled portion of the ice road due to excessive snow bank loads was noted during the 2010/2011 and 2011/2012 operation period.
On 2011 05 22 the ice temperature in this ice road at 60 cm depth was -6.5ºC (20.3ºF). This ice road was open to light traffic at this time but was closed on 2011.05.24 to all traffic due to surface degradation beyond accepted levels.
Borehole jack strengths on the man-made island ice road in the flooded build-up ice were measured at 14 MPa (2100 psi) on 2012.04.25. For comparison accepted borehole strength values for ice road operation are given by Spencer et al (2000) as 7.6 MPa (1100 psi). The man-made island ice road over the 2010/2011 and 2011/2012 spring season performed well with operation to the third week of May.
The snow and ice ramp of Figure 5 was east-facing and was constructed with ice chips with a fresh water cap. The fresh water was carried from an onshore lake. This east-facing ramp was employed to transfer loads in excess of 200 tonnes. This ramp was operational also to the third week of May with no bare surfaces exposed to sunlight. A freshwater ice cap was placed on the ice chip ramps at the man-made island and at the shoreline access, for improved surface strength during spring operation.
|Figure 3. Transporting a large crane on a grounded ice road to the man-made island with half the ice road surface under 10 cm snow cover 2011.04.22.||Figure 4. Blowing snow onto the made-made island ice road (2012.04.15)|
|Figure 5. Access ramp to the man-made island (2011.05.14)|
Example 2 - Snap Lake Ice Road
In May, 2011 De Beers Canada operated approximately 3 km of ice road at Snap Lake NWT during the spring season. Water depth under the ice road was typically 10 m or greater.
The drill rigs were removed from Snap Lake on 2011 05 09 by ice road which was approximately 35 days later then the closing of the main ice road to Snap Lake from Yellowknife. The Snap Lake ice road connects to the Tibbitt-Contwoyto Winter Road for the last leg to Yellowknife.
NWT Transportation (NWT, 2012) gives the average opening and closing dates for the Tibbitt-Contwoyto Winter Road over the last 10 years as January 30 and April 5 while the average for the last 5 years was January 29 and April 3 respectively. The ice road near Yellowknife typically deteriorates earliest and as such controls the closing date for the entire Tibbitt-Contwoyto Winter Road. (The NWT description of this ice road is "Winter Road" since part of the route is on floating ice and part is on tundra.)
Figures 6 and 7 show the ice ramp access from Snap Lake to shore. The snow ramp was built with rig mats under the snow at the shoreline and approximately 30 cm of packed snow on the entire 200 m ramp length. The ramp worked well on 2012.05.09 the date that the rig equipment was demobilized to shore. On May 8 the Snap Lake ice road to the drill locations flooded due to excessive snow banks along the narrow ice road as depicted in Figure 8. On the morning of May 9 a new ice road as shown in Figure 9 was constructed with a Komatsu WA450 loader and grader. The new ice road of Figure 9 was also below water level; however, with the rig move being done within a few hours of the road opening, flooding did not have time to occur. The rig move off the ice was completed on May 9 by 1500 hours.
|Figure 6. Snap Lake Spring Season Ice Road||Figure 7. Snap Lake Spring Season Ice Road|
|Figure 8. Snap Lake flooded ice road 20 m wide snow bank to snow bank 2011.05.11||Figure 9. Snap Lake ice road opened a few hours prior to photo 2011.05.09|
The Snap Lake ice road temperatures at 60 cm depth with no snow cover on May 8, 2011 were -2.0 to -2.3ºC. The ice road temperatures at 60 cm depth in the new ice road constructed on May 9 which was covered with approximately 25 cm of snow on May 8 was slightly colder than -2.0 to -2.3ºC which is colder than -1ºC for freshwater ice that Transport Canada (1975) required for safe operation.
Example 3 - Attawapiskat Ice Road
In the winter and spring of 2010, De Beers Canada operated approximately 5 km of ice road/tundra road on shallow fresh water ponds and muskeg near Attawapiskat in the James Bay Lowlands, Ontario, Canada. Extending the operation window into early April, 2010 was required to complete the exploration drilling program. Figures 10 and 11 show the Attawapiskat access ice road and the work site with ice pads on 2010.03.15 (March 15).
Construction of the ice road and pads was started in early January with use of tracked equipment during the early stages (snow machines and a D5 bulldozer). The D5 with a heavy metal drag attached behind it, was used to drive frost into the road in January to increase the build-up of frozen material. Flooding by water pumps was conducted through January and into February to complete construction of the ice road and pads. Use of heavy-wheeled vehicles such as the 32-tonne rubber-wheeled drill rig on the road was feasible from mid February until ice/tundra road closed on April 3.
Drilling from the ice pads was started near the end of February. Rig mats were placed under the drilling equipment for stability and also gave the ice pads a layer of thermal protection from the heat generated by the drilling operation. Traffic was blocked on the ice pads in areas to be used for drilling later in the season. This was done to maintain a high albedo (see Figure 11) and prevent accelerated pad surface deterioration from sunlight.
The use of the ice/tundra road and pads continued into March. Thickness profiling of both roads and pads was conducted on a regular basis. Profile ice temperatures were monitored on a daily bases.
Ice temperatures at 60 cm depth on April 3 were -2ºC (the date of equipment removal) in the fresh water ice pads and roads at Attawapiskat which was 1ºC colder than the -1ºC criterion for freshwater ice as required by Transport Canada (1975) (see Figure 12).
Restrictions on daytime use of the road by wheeled traffic were started on March 13. On this date ambient temperatures above freezing and increased sunlight started the melting of the top layer of the muskeg portions of the ice road. A lack of snow fall during the winter season and early spring melting of what little accumulation there was prevented the road from being covered by snow at this time. Heavy loads where restricted to early morning travel when the tundra and ice road surface was frozen.
Demobilizing of the drill site started on April 3. Movement of the drilling equipment was performed during the early morning hours. Rig mats were used as needed in areas of the road to allow for safe crossing. All major equipment, including the truck-mounted 32-tonne drilling rig was removed successfully by April 4 the date of road closure.
Practices that helped maximize the ice/tundra road operational season at the Attawapiskat location were as follows:
|Figure 10 Attawapiskat ice road constructed over muskeg and shallow fresh water ponds 2010.03.15.||Figure 11 Attawapiskat ice pads with areas blocked off to traffic 2010.03.15|
Figure 12 Attawapiskat ice temperatures at 66 cm from the top of ice on a 135 cm thick drilling pad 2010.2.25 to 2010.04.03
Summary for Extended Season Ice Road Operation
The three sites discussed in this report were successful in completing project requirements with extended spring season operation. At each site, engineering was employed to select and design procedures to allow safe operation into the spring season. Surface traction and overall ice road structural strength must be maintained for the entire extended spring season operation. Once one of these factors is sub standard in the spring season it is virtually impossible to correct. With careful design/monitoring and pre-emptive preparation (especially the access snow ramps) the regular ice road operation season can be extended into the spring melt season by approximately one to two weeks
Points from the three site cases that highlight procedures that can be employed to extend the spring season operation window are as follows:
In summary, it is emphasized that site engineering on a case-by-case bases with monitoring of conditions is key to a safe and successful extended spring season ice road operation. Performing engineering monitoring on site prior to any indication of melting or ice road deterioration is important for evaluating and understanding how weather patterns and forecasts will impact site condition as the spring season progresses.
AUTHORS (detail correct at time of original publication in 2012)
1.Allan G. Strandberg - Ausenco Sandwell - Calgary, Alberta, Canada - email@example.com
2.Paul A. Spencer – Ausenco Sandwell - Calgary, Alberta, Canada - firstname.lastname@example.org
3.George M. Strandberg – Ausenco Sandwell - Calgary, Alberta, Canada - email@example.com
4.Uwe Embacher – Consultant - Cochrane Alberta, Canada