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Alaska Region Modifies GPS Monument In Order to Install on Difficult Terrain at Critical Location


It is expected that for each of the 875 PBO GPS stations installed, small modifications will need to be made in order to accommodate the vastly different rock and terrain conditions. Most recently, the PBO Alaska field engineers proved to be masterminds by creating a hybrid of the short and deep drilled braced monument in order to install the instrument at a scientifically critical point in Alaska, important because it is located in close proximity to the Shumagin Gap, an area of the Aleutian subduction zone that has not experienced a large earthquake in over 100 years, quite possibly resulting in the accumulation of large strain that could result in a major earthquake.

Due to the lack of exposed bedrock at the surface and excessive mobilization costs to bring a drill rig to Sand Point, neither a standard short nor deep drilled monument would work at this specific location. Alaska Regional Engineer Ben Pauk directed his team to ‘work outside of the box’ to find a way to install station AB07. With help from PBO & UNAVCO Headquarters personnel, a permit was obtained from the land owner, Alaska Tsunami Warning Center (ATWC), and special equipment for the hybrid monument was ordered and shipped to the Alaska regional office.

Learning from the ATWC, who currently hosts a broad band seismometer at the location, the Alaska PBO crew was informed that when installing the seismometer back in 2003, bedrock was found at a depth of five feet below ground surface. With a plan and equipment in hand, crews located a suitable site 35 feet away from the ATWC seismometer and prepared for the installation with hopes that they too would hit bedrock at five feet ‘

On October 19th, the digging for this unique PBO station began with the help of a back hoe and operator provided by the city of Sand Point. In unusually dry, calm and pleasant weather for Sand Point, crews started digging, excavating a 20’ x15’ hole, just large enough to easily drill five holes once cleared. Crews became a bit nervous when the five foot mark was passed with no sign of bedrock, but with fingers crossed, they kept digging and finally hit bedrock at 6.5 feet.

Once the hole was dug and the bedrock cleared of dirt, the PBO crew immediately began the drilling holes for the monument. The holes drilled into the bedrock were the standard 1.25 inch diameter holes drilled for short braced monuments, however, the crew drilled five holes (one center hole and four angled holes) rather than the standard four holes (one central and three angled holes) specific to a short braced monument. Normally, each leg of a short drilled braced monument reaches about 11 feet in total length, with legs that extend below the ground surface six feet to secure the station into the ground, and above the surface 5.5 feet to ensure for good sky visibility for the antenna. However, because of the absence of bedrock at the ground surface, the crews needed to ensure that each leg was drilled 5.5 feet into the bedrock, extended up 6.5 feet from the bedrock to the ground surface, then an additional 5.5 feet above the surface to intersect with the top of the center leg. Thus each leg of the monument was about 15 to 17 feet long. In comparison, each leg of a deep drilled braced monument goes about 33 feet into ground and is about 40 feet long. Each leg of the modified monument consisted of 5.5 feet of one inch stainless steel rod welded into a custom made sleeve adaptor and a 12 to 15 foot section of 1.25 steel pipe plug welded into the top of the sleeve adaptor. The sleeve adaptor was specially made from 2 inch diameter stainless steel bar stock and consisted of a one inch slip fit drilled four inches deep on one end and a 1.25 slip fit drilled four inches on the other side.

After drilling all five holes 5.5 feet deep in to the bedrock, the Alaska team measured and cut the one inch diameter stainless steel to size so that each rod extended out of the top of the bedrock a little less than one foot. After cutting the one inch rods to size, they plug-welded on the new sleeve adaptors custom made for the installation on to the rods and epoxyed the rods in to each of the five holes. The crew then cut the 1.25 inch diameter pipe for the center leg to size and plug welded it into the top of the central adaptor sleeve in place. After the center leg was in place, they cut the four remaining legs to size and cut the top of the pipes off at the proper angle so they could be welded flush to the adaptor block on the top of the center leg and then plug welded the bottoms of the angled legs in to the sleeve adaptors. It was critical that the angled legs be the right length and intersect the center leg at the right angle, so the crew took great care in measuring the lengths of the pipes prior to welding the bottoms into the adaptor sleeves (there’s only one chance to get this right!). Prior to welding each of the four angled legs flush to the adaptor block on the center rod, they filled each leg with grout and rebar and then welded on an adaptor block to the center leg, similar to what is done for a deep drilled braced monument. After welding the angled legs into place on the adapter block, the crew then had the back hoe fill the hole to the original ground level. Once the hole was filled, the crew attached and a tack-welded the SCIGN mount, welded four gussets in to place and attached the antenna to the SCIGN mount. Final touches to the landscaping around the base of the monument shaped the dirt back to its original grade, leaving the area looking as nice as it had before the crews started the installation. The entire monument from start to finish, including digging and back filling the hole, took four very long days.

Leaving Sand Point after having completed such exhausting work, the PBO Alaska team squeezed into a very small, crowded, and bathroom-less puddle jumper to head back home. Despite the super bumpy and turbulent three hour ride to Anchorage, it wasn’t a surprise that all three of the worn-out crew members slept like rocks for the entire flight home.

It’s unclear as to whether or not Alaska will have to do another type of this hybrid monument; however, this experience certainly proves that the Alaska crew can adapt and develop very stout and sturdy GPS monuments to meet many of the challenging terrain conditions of the ‘Great Land’ while best utilizing the limited resources in many of the remote Alaska villages.

A special thanks go out to the staff of the Alaska Tsunami Warning Center who helped permit and locate a suitable site for the monument, and to Anthony Gunderson and Fred Purnell from the City of Sand Point for helping with logistics of getting the PBO gear off the barge at the City of Sand Point boat dock and to the site (in addition to doing some exceptional back hoe work on site!). Thank you to Edi Hodges, a wonderful host at the Hodges Bed and Breakfast, located in Sand Point just 2 miles south of the PBO site, for meeting the crew every morning bright and early with piping hot espresso and scrumptious cinnamon rolls. What a great way to start the day!


Last modified: 2019-12-24  00:13:19  America/Denver