The dangers of landslides in Washington State is a given for construction companies and something that San Francisco-based Malcolm Drilling Company Inc. and Seattle-based Anderson Construction Company Inc., deal with on a daily basis.
On March 27, a huge landslide along the coastline of Whidbey Island (north of Seattle) caused a massive slice of land near Coupeville to collapse and with it one home. It also forced the evacuation of 34 others.
The homes were built on a fault line, which occasionally shifts due to seismic activity, and because of the topography and impact that rain and snow has on the soil, the land and homes were exposed to additional risks that could lead to landslides.
No injuries or fatalities have been reported from the event, which occurred before 4 a.m.
“We do heavy underground construction — typically drilled foundations, earth retention systems, and quite often we’ll do remedial work that is involved with landslides,” said John Starcevich, Malcolm Drilling project manager.
The firm, which employs highly trained crews and expensive and sophisticated equipment, takes safety seriously and before crews are sent to work on a job, thorough inspections of the site and its suitability for construction are researched by geologists and geotechnical engineers.
Before any project is approved, municipal officials in Washington State require thorough geotechnical investigations and reports, which include slope stability analysis and soil studies.
Starcevich said that following a landslide and before any work can be done, a careful evaluation of the site is made. Geotechnical engineers study the conditions at the site, including surface drainage, the types of soil and state of the soil layers, geological features and recent seismic activity as potential causes.
“Often marginally stable slopes are weakened by development if the surface drainage isn’t properly controlled,” he said. “It can saturate the soils, weaken them and exploit natural weaknesses in the soil structure itself.”
Geologists from the Washington State Department of Natural Resources were still examining the site to determine the causes behind the landslide as of press time.
ABC’s Seattle affiliate, KOMO, reported that the “landslide stretched across 400 to 500 yards and the earth dropped 600 to 700 yards down to the water, with trees and tons of dirt smashing into homes down below and wiping out a road.”
Central Whidbey Island Fire and Rescue Chief Ed Hartin told ABCNEWS.com “landslides are not a totally uncommon event on the island,” and that the geology of the island is “a constant slow moving active landslide.”
It is only after the governing municipality or organization and their consultants have determined the appropriate solution that remedial work can begin. Typically plans and specifications are prepared before the firm can begin operations such as stabilization, soil improvement, de-watering and other preparation work.
Remedial work following a landslide includes removing loose surficial soils to reach competent soils where benches are cut and drill rigs are brought in to install soil reinforcing elements, which can take many different forms.Vertical drills may be used to install vertical shafts or soldier piles and other vertical reinforcement and smaller crawler-mounted drills that articulate are used to install horizontal or inclined soil nails, ground anchors and tie-back anchors. Control of both surface and subsurface drainage also is very important to stabilize.
“They are quite versatile,” said Starcevich, whose firm uses German manufactured Bauer vertical drills, Davey Drills manufactured by Davey Kent Inc. in Kent, Ohio, and also from Germany, Klemm drills by KLEMM Bohrtechnik .
Starcevich had only seen pictures and video footage of the Whidbey landslide and like many, was awaiting more information on the causes that will be determined by examining the debris and the soil configuration.
“I could imagine that if the landslide is large enough, much of the landslide mass may stay where it is,” he said. “It’s come to rest and it’s worked itself into a more stable configuration in areas, but there are obviously some large vertical faces that pose some potential problems. Quite often those vertical faces are protected and anchored prior to placing fill in front of them . They’ll put tie-back anchors or some type of reinforcing elements in it and then potentially put a facing on it such as shotcrete the face. They may need something even more significant and have to combine it with vertical reinforcing elements too.”
Remedial operations can be expensive as earth retention systems in general “tend to be expensive because they involve a lot of time and effort to install under relatively difficult conditions,” he added. “It becomes a trade off. You ask where the other potential faults or failure surfaces are and it’s a question of economics — do you try to arrest further movement to save those homes and then, what is the cost of moving them or abandoning and demolishing them. When you try to arrest large scale slope instability, you may have to move a lot of earth.”
When working on landslide remediation, Malcolm Drilling often works closely with excavating firms with similar experience, with safety the prime concern which includes both corporate and local office safety inspectors, to ensure that Occupational Safety and Health Administration and Washington Industrial Safety and Health Act regulations are adhered to.
In addition to the eyes and ears of inspectors and crews, landslide remediation projects, require that the general contactor employ a monitoring program with some combination of surveyed monitoring points and/or installing slope inclinometers, specialized PVC casing installed in boreholes in the soil, and an inclinometer, monitoring instrument is used to map the inclinometer casing in the borehole from bottom-to-top to determine if any soil movement has occurred. These systems are installed prior to any remedial work.
“You have to come up with a viable monitoring plan that is reviewed and approved,” said Starcevich.
For new construction projects that are “green lighted’ by municipalities, Malcolm Drilling teams take nothing for granted.
“There is always the potential for unexpected things to occur and we try to look for potential problems and things that might occur,” said Starcevich. “Sometimes we’ll look at a job and say “we’re not comfortable with the potential risk there and we will not bid on a project.”
Anderson’s Approach
Based in Seattle, the Andersen Construction Company Inc. (ACC) was founded 62 years ago and specializes in commercial construction, and the construction and renovation of hospital and health care facilities. Steve Paul, the company’s general superintendent of the State of Washington, further explained that the process of approving a site for construction. A soils report needs to be prepared by a geotechnical engineering firm that is based on a survey of the property; use of existing topographical surveys; historical record searches to determine previous use of the site such as a gas station, location of a storage tank, or industrial activity; and if unconsolidated materials were placed on top of the existing native ground.
Based on that report, ACC “looks at the bearing capacities of the soil, how stable it is — there is a standard recommendation in a geotechnical report — and we’ll see what we need to do for slope stability and during wet weather,” said Paul. “If it is a deep excavation next to an adjacent building, we’ll engage a shoring engineer to design shoring system to support that building or adjacent hillside. You cannot disturb a neighbors’ property when you are digging your own so that nothing you do causes any settlement or vibration-related damage to buildings.”
Groundwater is a major consideration and factor that must be accounted for. A soils report will provide data on groundwater and the influence of moisture and rain. Creating de-watering wells is a common solution. Paul noted that many construction companies, due to the nature of their work, have considerable knowledge of soil types and conditions and how they react to wet weather.
In addition to a variety of mitigation measures, dealing with contaminated material (such as petroleum saturated material) is very much a constant and construction companies are required to have a plan that not only identifies and handles them, but properly disposes of them safely and legally at department of transportation and department of environment regulated disposal facilities.
“There is tons of homework that goes into an excavation project and one of the things that we plan for is not to put people in harms’ way,” said Paul, whose company subcontracts these scopes of work to excavation companies and before any crews begin work on a site, sends in safety and management teams to study a site based on the geotechnical report. “You plan the safety of your workers first and an evacuation plan. Large or small, most projects have some element of earthwork and the same principles come into play.”
The soils report is essential for ACC crews and subcontractors because it lets them develop a plan for how many vehicles and heavy pieces of equipment can be brought onto vulnerable portions of the site at any one time and this includes temporary surcharge loads (TSL) on existing slopes of shored excavations.
“It could be anything from the weight of a crane, excavator, concrete trucks or even the weight of a vehicle working next to an excavation,” said Paul. “A lot of times, if they are major loads, they are going to be engineered. If they are minor loads, the shoring engineer will provide general parameters and we will know whether or not for example we know they we can run trucks up to 10,000 to 20,000 pounds within 50 feet up to the edge of an excavation without a problem. If those loads exceed the general guidelines provided then we are going to need additional shoring.”
Extra shoring may also be required to support bringing heavy equipment up the edge to remove soil and the shoring design needs to take into unexpected loads that can occur in a concentrated area. The city of Seattle has identified critical areas where hillsides and slopes, if they are above 45 to 50 percent in grade, require in-depth soil studies and construction plans.
“Whatever building you are going to put up,” said Paul, “the recommendations from the geotechnical studies have to be incorporated into the plan.”
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