PROJECT PROFILES
port of anchorage
Testing and Sampling Soil
Achorage, AK
Building a Stable Future from the Ground Up
The Cook Inlet near the Port of Anchorage, Alaska is attracting attention as it is now the center of an estimated $200 million dock expansion. The Port of Anchorage Intermodal Expansion Project (PIEP) is a major transportation infrastructure project that will include road and rail access, cruise ship, ferry, barge, and bus terminals, as well as an expansion to accommodate 1000-foot vessels. The project will greatly expand the current 100 acre port by an additional 83 acres, provide dock widening, 100-foot. cranes, and deepen the harbor by 10 feet to allow access to larger ships. This expansion will help the port deal with the projected 200% increase of activity over the next 20 years. The project, estimated to take six years to complete, has become the largest near-term marine transportation project in the United States.
Prospective Designs
A development proposal has been submitted by Peratrovich Nottingham & Drage Inc. outlining the construction of a dock based on their open-cell system technology. The model utilizes sheet pile membranes to create a bulkhead that would hold nine million tons of compacted gravel when completed. This would allow for a land based installation and cost savings of over 30% compared to traditional designs. This design has been successfully implemented at neighboring Port Mackenzie and other areas in Alaska.
Another option was provided by the engineering firm originally contracted, Tryck Nyman Hayes Inc. They have developed a design for a pile-supported dock similar to current West Coast ports. In this traditional design, the dock would be supported by piles driven deep into the ocean floor.
Troubled Past…
Stability issues are playing a pivotal role in this proposal because the region is prone to massive earthquakes. This area of Alaska experienced the second largest recorded earthquake in history in 1964, recording 9.2 on the Richter scale. Surprisingly, the earthquake itself caused less damage than the numerous tsunamis produced by the quake and resulting landslides.
It is essential that the new port incorporate design features to withstand a large earthquake. San Francisco, a city that’s had its share of earthquakes, must design its docks to withstand a magnitude 7.5 earthquake. In comparison, the new port in Anchorage will be designed to withstand a magnitude 9.0 earthquake, almost 800 times more intense. For this to be accomplished, the design must be based on a good understanding of the subsurface conditions and ground environment. To prepare a proper design, a $1.25 million geotechnical seismic study was conducted to collect more data on the strength and stiffness of the seabed near the Port of Anchorage.
A Call to California In the summer of 2003
The Federal Maritime Administration sub-contracted Gregg Drilling and Testing Inc., of Signal Hill, California to test and sample the soil near the Port of Anchorage. Because the Cook Inlet experiences tidal fluctuations of up to 36 feet, drilling boats or barges could become grounded. Instead, a jack-up drilling platform (see picture on right), similar to those used for near-shore oil drilling and capable of working in water up to 83 feet deep, was used.
The Skate III jack-up rig is also designed for rapid assembly and easy transportation since the pontoons double as containers for the jack-up legs and other components. In two weeks the rig was shipped from California to Alaska, assembled, and ready for use. It has a jacking capacity of up to 100 tons and is easily moved from one location to another by lowering the platform to water level and acting as a barge powered by a tug boat. Gregg Drilling mounted their equipment on this platform, and with crews of 5 to 6 people working 24/7, completed in-situ soil testing and sampling in little over a month.
Cone Penetration Tests (CPT) conducted in 39 locations provided a continuous soil behavior type profile of the expansion area. This was accomplished by pushing a cone penetrometer, attached to a data acquisition system, into the subsurface using a hydraulic ram. The cone penetrometer contains electronic sensors to measure tip resistance and sleeve friction, while a small filter behind the tip measures pore water pressure. The CPT provides a rapid, reliable, and economical means of determining soil stratigraphy, relative density, strength and hydro geologic information without generating soil cuttings. Many other sensors such as resistivity, ultra violet, and seismic geophones can be added to the cone penetrometer. For this application, it was important to analyze soil behavior and liquefaction potential in response to dynamic loading from earthquakes, ice, vibrating machine foundations, waves and wind. Therefore, a seismic module was added to the cone penetrometer to conduct seismic tests in a few of the locations. In addition to CPT, various sampling methods such as standard penetration tests (SPT), Shelby tube, and piston samplers were utilized to obtain 190 samples from 20 different locations. Many of these were taken from 100 to 200 feet. below the ocean floor. All samples were sent to Terracon Consulting for further evaluation by professional geologists and geotechnical engineers.
Further Investigations
The success of the Port of Anchorage seabed sampling program and the uniqueness of the drilling equipment quickly attracted the attention of the Alaska Department of Transportation. For nearly 30 years, the Department of Transportation has been contemplating a bridge across the Knik Arm linking the crowded Anchorage bowl with the largely undeveloped Matanuska-Susitna (Mat-su) side. The proposed bridge would eliminate the current two-hour drive circling the Knik Arm from the Port of Anchorage to Port MacKenzie. The state acted quickly to retain Gregg Drilling to mobilize the same equipment directly from the port so work could begin immediately.
In the middle of August, Gregg Drilling began site investigation in the two nautical miles that stretch between Carin Point and Port MacKenzie. Gregg Drilling was asked to drill a number of boreholes and collect samples in the often rough waters of the Knik Arm. Drilling conditions included clays, hard silts, loose flowing sands, gravels, cobbles, and a few boulders. Due to such variance in soil types, further work was required to gain a full understanding of the subsurface conditions. A CPT sounding was conducted to determine a continuous soil profile and a seismic CPT sounding was performed to a depth of 224 ft below the mud line. By the end of August, drilling became difficult as a mixture of bad weather and complicated drilling conditions plagued the crew. Work was postponed until the middle of September, allowing the rig to move to areas of deeper water, where more favorable tides were present. By the end of September, a total of seven boreholes had been drilled in the ocean floor spanning the Knik Arm. Samples and CPT data taken from the area will provide engineers with design parameters and allow planners to narrow cost estimates for the proposed bridge.
Heading Home
With the end of September approaching, it was time for Gregg’s drillers and engineers to head back home to warmer climes. Anticipation builds for port and bridge planners as Anchorage awaits test sample results and designs for some truly ambitious construction projects.
Example of an open cell dock design..
During the 1964 earthquake, a section of waterfront slid into the sea producing tsunamis that destroyed this dock.
The tsunami's power is displayed.
Gregg's jack-up rig used for testing and sampling soil under the waters of the Kink Arm near Anchorage, Alaska.