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Finding Solid Ground

Published by
World Cement,


Introduction

The majority of freight and supplies for the state of Alaska enters through the Port of Anchorage. One of these products is bulk powdered cement.

A number of years ago, Alaska Basic Industries, a joint venture between CalPortland and Alaska Sand & Gravel, the terminal owner and operator, looked at various options to increase the storage capacity at the Port of Anchorage. The existing terminal had silo storage that was inadequate to hold the entire cargo of a single cement ship, without either light loading the ship or using offsite storage. The import situation in Anchorage is further complicated in the winter months, due to the presence of ice on the Knik Arm (the location of the Port of Anchorage): the ships that are used for transporting cement are not built to travel through the ice present on the Knik Arm during the winter months.

 

As a result, the terminal needs to have sufficient storage of cement to meet customer demands from the end of October through to mid to late April, without additional ships. The terminal was struggling to meet the growing customer demand through the winter months with the storage capacity available on and offsite. Economic growth in the Alaskan market is resulting in increased demand for cement throughout the year, with some of the new customers requiring cement supply year-round. This was putting increased pressure on the terminal to store larger quantities of cement, before the start of the ice season, to meet customer demand.

Alaska Basic Industries did have the option to haul cement to an offsite facility, where a warehouse had been converted into a flat storage facility. This required precise coordination and planning for each ship, to ensure that both the terminal and flat storage were nearly empty and that bulk cement haulers were available. The offsite haulage of cement increased the cost of ship unloading. This presented a large operational security risk to the business, since any delay in the arrival of the ship potentially created a shortage of cement in the Alaskan market, due to the need to have both the terminal and flat storage facilities nearly empty.

Storage selection

The business identified the need to build additional cement storage on the terminal site at the Port of Anchorage as a priority. A number of storage options were considered, including flat storage, traditional silos, and storage domes (both dihedral and standard). In the case of the Port of Anchorage, the selection of the preferred storage solution was further complicated by the extremely poor quality of the soils, along with a water table that is only 5 ft to 7 ft below grade.

Flat storage is essentially a warehouse building with low to medium height load bearing walls, in which the dry powdered cement is unloaded into piles and stored. Material recovery generally uses a wheel loader to feed the material into a hopper for transport to the truck loadout system. The loadout is labour intensive, and very tough on personnel and mobile equipment, since the interior of the building gets very dusty during material recovery. Flat storage, while having the lowest ground pressure of all the options considered, requires the largest amount of real estate, due to the lower pile height. Available real estate at the Port of Anchorage, like most ports around the world, is at a premium. The availability of real estate, combined with the high operating costs, resulted in this option being rejected.

Silos have traditionally been used to store powdered cement. Silos come in many different forms, with either conical or flat bottoms, depending on the application and the method of material recovery. Conical bottoms tend to be gravity loadout, which has lower energy consumption, but they need greater silo height. Flat bottom silos tend to use pneumatic unloading (generally air pads) to aerate the material to get it to flow to the outlet. Silos were traditionally constructed of reinforced concrete, but with advances in the design of steel silos, either bolted or welded construction, steel is becoming more common. Bolted steel silos were considered for the Anchorage application. The terminal currently utilises steel silos; however, there was concern about unloading methods, and the complexity of integrating multiple silos with a single pneumatic transfer pump for material movement to the loadout system. The poor soils presented challenges with the foundation design for silos, with the seismic forces being the major component of the foundation design. Given these factors silos were not selected.

Domes have been used for a number of years for the storage of cement. Two types of domes were evaluated for this project, dihedral and standard, where the dome walls are load bearing. A standard dome design from Domtec International was selected due to the lower cost per cubic foot of storage capacity. An additional benefit was that the standard dome required less real estate for an equal storage capacity. The main disadvantage of the standard dome was the high possibility that a reclaim tunnel would be required under the dome. The dome design ultimately selected did require a reclaim tunnel due to the decision to use a mechanical reclaim system.

Cement withdrawal system selection

Once the type of dome had been chosen, the next step was to decide how the cement would be withdrawn from the dome. The dome suppliers offered two methods: fluidised floor and mechanical reclaim.

Fluidised floor systems have a number of advantages, including a lower installation cost and no moving parts inside the dome. The dome, when designed for a fluidised floor system, generally has some form of slope to the floor. This creates some added complexity with the dome construction and is a cost that must therefore be accounted for. Depending on the design of the fluidised floor, it is possible to eliminate the tunnel under the dome by using a side discharge port.

There were several concerns with the use of a fluidised floor in the Anchorage application. One was the ability to fluidise the cement and get it flowing after it had been in the dome through the winter months. The second was the power consumption required to operate the fluidised floor: the electrical utility in Anchorage is relatively small and, as a result, has a number of additional requirements relating to operation of motors larger than 30 hp. Additionally, the terminal owners had limited successful experience with fluidised floors.

The second option was a mechanical reclaim system, supplied by Cambelt International Corp. This was the reclaim system selected for the dome. The terminal owners had experience with mechanical reclaimers and own several of them in other locations. Cambelt’s mechanical reclaim system had several advantages in the Anchorage application. The use of a mechanical screw conveyor to transport the material to the reclaim hopper means that even cement that had been in the dome for a period of time could be broken up and reclaimed. The power consumption required to operate a mechanical reclaim system is less than that required for the fluidised floor. The big disadvantages of the mechanical reclaim system are the increased up-front capital cost and the need to install mechanical equipment inside the dome. The interior of the dome is a harsh environment, with the equipment being partially buried when the dome is filled, as well as always being exposed to high levels of cement dust.

Foundation design

The soils under the construction site at the port are exceptionally poor in terms of their ability to support the load of structures. The main contributor to the poor soils is a 400 ft layer of ‘Bootlegger Cove’ clay. The thickness of the clay layer prevents the use of piles and requires the use of a matt foundation to distribute the load of any structure. The solution was to surcharge the site with approximately 30 ft of material for a year before the commencement of construction. The intent of the surcharge was to minimise the settlement of the completed structure. The settlement of the surcharge was measured and recorded on a monthly basis, using carefully constructed and placed targets at key measurement points. Before the commencement of construction, the surcharge pile was removed from the site. There was concern that the precharged soil would rebound during the construction process; thus the initial construction activity for the dome foundations commenced immediately after the surcharge pile was removed. The amount of rebound was monitored and fortunately turned out to be within design limits.

The poor quality of the soils indicated that differential settlement could be expected between different areas of the foundations, depending on the loads imposed upon them. This meant that the foundation design had to either isolate sections, or provide a way for sections of the foundations to move independently, if the design loads were significantly different. For example the reclaim system selected had a large column that supported all the mechanical equipment associated with it. The column is located in the centre of the dome. The weight of the steel imposed a greater load on the foundations per square foot than the dome floor surrounding it. This required the central column foundation and the corresponding portion of the tunnel (walls and floor) to move independently, presenting design and waterproofing challenges.

A further complication to the design of the cement storage structure was the extremely rigorous seismic codes in place in Alaska. In fact, the dome that was finally built at the terminal experienced a 7.1 magnitude earthquake during the final stages of installation of the mechanical reclaim equipment. The post-earthquake inspection found no visible damage to the dome structure or mechanical reclaim equipment.

As previously mentioned, the soil conditions at the site prevented, the use of a piled foundation. The dome foundation was designed with a large ring beam installed on top of a significant layer of engineered fill to provide the load-bearing capacity required. The reclaim tunnel under the dome presented a number of challenges during construction, mainly due to preventing water ingress, while allowing for differential settlement between the different sections the foundation. The selection of a mechanical reclaim system required a sectional rather than a total matt foundation. This was due to the significantly higher loads seen by the central foundation under the reclaimer compared with the dome floor. The construction of the tunnel was successful in keeping the majority of the water out, despite being at the level of the water table; keeping the tunnel completely dry will be a continuing maintenance activity for the life of the dome.

Conclusion

The final decision was to install a 40 000 short t standard storage dome with a mechanical reclaim system. It was decided that this was the best decision for the terminal, despite construction and the anticipated continuing challenges associated with keeping the tunnel dry. The dome was constructed during the summer of 2015, and was commissioned in 2016. It has been problem free since startup and has alleviated the supply challenges that had previously faced Alaska Basic Industries, the terminal operator. During the first year of operation, the differential settlement between the foundation for the mechanical reclaimer and the dome floor was much less than anticipated. This has made the task of keeping the joint between the two foundations sealed to prevent water ingress easier. A fibreglass and epoxy resin coating has recently been applied, virtually eliminating water ingress.

 

This article was originally published in World Cement's BMHR supplement.

Read the article online at: https://www.worldcement.com/special-reports/28082017/finding-solid-ground/

 

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