Meadows a mountain of an engineering challenge
Long before Target opened and began moving merchandise last weekend, engineers worried about how to keep the store standing in place.Glenwood Meadows represents not only Glenwood Springs’ largest commercial development, but it is the most heavily engineered from a geotechnical and foundational perspective.That’s because it sits at the base of Red Mountain, on what geologists call an alluvial fan, or debris fan. The fan consists of debris that has washed off the mountain and spread out below.The chief challenges of the construction site are the threat of flooding, and the possibility of soil collapsing beneath foundations, especially when it becomes wet. Bedrock is more than 80 feet deep at Glenwood Meadows, and the soil above includes silt, sand, clay and gravel.Some of the measures to protect Glenwood Meadows include:– a million-dollar berm structure at the top of the site to shield it from rockfall and debris flows. A storm-water diversion system runs all the way to the Colorado River and can be tied into by subsequent developments.– underground site drains beneath the shopping center that are linked to the storm water system.– catch basins along Midland Avenue designed to help sediment settle so the water going into the river is cleaner.”Water is the enemy out there,” said Andrew McGregor, Glenwood Springs’ community development director. “The extraordinary measures are primarily designed to intercept that water.”To understand the problem that water can cause where soils can compact, one need look no farther than to the 104-unit Terraces condominiums, also on Red Mountain, but in south Glenwood Springs at the far end of Midland Avenue. There, condo buildings have been sinking and cracking, with poor grading and drainage contributing to the problem.The homeowners association sued developer Jay Harkins and others involved with the project and came away in April with a settlement of nearly $12 million.The money is being used to improve drainage and inject grout to shore up the foundation, among other repairs.Special techniqueIn a technique new to Glenwood Springs, ground-penetrating radar was used at Glenwood Meadows where public roads and utilities were being put in, McGregor said. “It’s a noninvasive way of looking at the soil profile and detecting any cavities or gaps or odd changes in the size of the materials, to make sure that things are properly compacted,” he said.Pipes with special joints were used for utilities so that if they leak, the erosion and settling caused by the leak won’t result in the pipes collapsing, he said.McGregor said the degree to which the measures being taken at Glenwood Meadows work will depend on the thoroughness of their design, the quality of their construction and the vigilance and ongoing maintenance that occur.The drainage system must be monitored to make sure it continues to work. Catch basins must be cleaned out regularly, a duty that falls to Glenwood Meadows’ metropolitan districts.Landscaping shouldn’t be overly irrigated, McGregor said. Also, some of the plants in parking lots are in contained planters, to keep irrigation water from seeping into the soil.McGregor said the foundation work at Glenwood Meadows was similar to what was used at the city’s Community Center and Municipal Operations Center, which flank the shopping center on either end. It consists of excavating some of the soil and recompacting it, as opposed to using steel pilings.The depth of the soil that is reworked varies from building to building, but McGregor said he believes it generally is about 10 to 12 feet.Opening on timeDuring the Target construction, rumors abounded of problems with the building’s foundations that threatened to delay its opening. But the store opened on time – and without foundation problems, McGregor said.”I’ve heard all the rumors too. We have no evidence that there are any building-related foundation issues at all.”He said all the building construction at Glenwood Meadows is proceeding as planned. As for the rumor mill, “I think a lot of it has to do with the amount of scrutiny that has gone into the design and construction,” he said.Robert Macgregor, the project’s Aspen-based developer, has heard the rumors as well. He blames them on “a lot of couch engineers. They sit and look at something and like to opine.”He said he has confidence in the engineers on the project. If anything, he thinks the debris mitigation structure is overengineered.”It looks like it’s capable of withstanding anything,” Macgregor said. “That’s a million dollars worth of ditch.”All of Glenwood Springs contains collapsible soils and steep hillsides, he said. “I think in a litigious world our engineers have gone to the nth degree to make sure everything is sound. I feel very good about that, other than what it’s cost.”City planner McGregor said the Meadows storm system is designed to hold up in a 25-year storm. Some recent storms on Red Mountain were probably of the kind expected to be seen every two to five years, and the system stood up to the storms.”We’ll definitely see worse,” he said. “Presuming the design is sound it can handle a lot more runoff.”Poor runoff waterWhile Glenwood Meadows’ drainage systems are holding up quantity-wise, the quality of some of the water reaching the river has been poorer than desired. Runoff bearing Red Mountain’s namesake color has bled into the Colorado River during storms, and red dust sometimes has filled the sky when the site has been dry and windstorms have kicked up.”As with any construction site of that size there are always some things that we don’t get right the first time and have to be revised,” McGregor said. “Obviously we’ve had some erosion issues during construction.”Dirt-moving has exposed soils to erosion, but as more of the project is paved, built on or revegetated, problems should be reduced, he said.As work goes on, the city and developers are looking at ways to better keep dust down, perhaps through more watering, and more use of temporary revegetation, including using more fast-germinating grasses, he said.Temporary detention ponds, silt fences and straw bales are among ways of intercepting sediment on the site and keeping it out of the river. The trick is trying to quickly get water off the site so it doesn’t lead to soil collapse, while not fouling the river, McGregor said.For now, “There’s more topsoil, more of that red stuff moving around than anybody would like,” he said.Contact Dennis Webb: 945-8515, ext. email@example.com
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