CASE STUDY – from the files of Integral…………
A landslide is a general term used to describe the downwards movement of organic materials, soils, and rocks. This movement is created by pressure caused by the weight of the upper portion of an escarpment. The pressure results in a gravitational flow towards the bottom of that slope.
British Columbia in general is susceptible to this type of activity because of glacial sediments that have occurred over the years. The natural elements that erode the land to cause this type of activity are very common in BC. The province has lots of complex geology in its steep mountainous terrain which unfortunately provides a variety of choice view lots for developing municipalities. The development of these lots eat away at the natural vegetation in urban areas like the Lower Mainland.Three distinct physical events occur during a landslide; the initial slope failure, the subsequent transport, and the final deposition of the materials. This activity can be caused by one or more of the following.
- Undercutting of a slope by water causing erosion
- Human activity too close to the slope
- Shock or Vibration caused by mining, construction or nature
- Loading of additional weight to the upper portion of a slope
Landslides may result directly or indirectly from the activities of people that undercut a slope or involve the construction of structures, or involve the placement of additional materials on top of a slope. People are one of the major causes of slope instability by changing the landscape through activities such as clear cutting or benching when they are not necessary. Vegetation that develops naturally on that hillside is there to absorb moisture in the soil and strengthen the integrity of that hill itself with a complex root system that binds the slope together.
Most escarpments have a natural creep to them. They move with a gravitation pull towards the bottom which is a slow process of a fraction of an inch per year. This can be sped up to a rate of feet per second caused by the eco system being disturbed by human hands or nature itself in the form of waves, streams, rivers, rain, ice, or snow.
Populated areas and transportation routes in British Columbia are exposed to a great variety of landslides. Landslides not only cause property damage, injury and death, they also affect water supplies, fisheries, sewage disposal systems, forests, dams, and roadways for years after an event occurs.
The sea to sky highway from Vancouver to Squamish has been affected by 14 major slides since 1906. 12 lives have been lost, 11 bridges destroyed, 4 homes demolished and a number of other structures totally lost.
Commonly referred to as the Berkley-Riverside Escarpment Landslide, the BC Coroner’s Office found in their report that this landslide in January of 2005 had been preventable. It cited the study conducted in November 1980 by Klohn Leonoff following three separate slides occurring in December of 1979.
The report made recommendations to the provincial government to develop safety plans. This report emphatically stated the following conditions were very much in evidence:
- At the top of the slope, fill had been placed to extend some of the building lots
- At the bottom of the slope, the slope had been cut back to extend some of the building lots
- Erosion caused by heavy rainfalls, and moisture had occurred to the slope
- Homes had been constructed near the crest at the top of the slope as well near the base of the slope well within the unstable zone
- Storm runoff was handled by open ditches at the base of the slope
- Minor groundwater or spring seepage zones were at various locations and elevations along the escarpment
- There was no major evidence of deep-seated movement to the hillside prior to the 1979 landslides
- The was no known activity to the slope prior to the upper and lower subdivisions being constructed
- Fill had been pushed over the crest of the natural slope to extend the lot area and was standing at a much steeper angle than the natural angle to the slope
- Development behind the crest of the slope had affected the natural run off routes
This report made the following recommendations to the slope
- Existing debris placed over the crest should be removed and no new debris be allowed to be placed there
- Vegetation should be controlled
- All storm drainage from residences should be connected to a storm sewer and not allowed to flow freely across the property to the slope
- Any existing abandoned septic tanks and systems should be removed and the cavity filled and compacted with non-organic material
- All homes should have their drainpipes checked twice a year and maintained in a good working order
- No structures fill or other activities should take place within a recommended distance to the crest or each property.
- The Golder & Associates report dated February 16th, 1978 should be adhered to. This report stated
- Existing ground streams should be controlled and redirect safely away from unstable areas
- Flood control channels should be developed for heavy rains
- All buildings should not encroach on the toe of the upper slope
- All retaining walls and other structures should have proper drainage systems installed at their base
After the 2005 landslide, BGC Engineering was retained to provide a risk assessment of the situation. This report confirmed that the conditions of the 2005 landslide matched those of prior landslides in the region and recommendations matched those for the prior landslides. The risk assessment also determined that the prior recommendations had not been addressed.
Now let me step outside the box for a minute. As most of you know I am only a few years younger than the Dead Sea. So let’s go back there. As a youngster just getting started in my adult world, I worked for a lumber company in Hamilton Ontario as a salesman. For these impressionable years, two contractors stand out in my mind, Jim Johnson and Paul Paquette. Jim was a new home builder and Paul a restoration contractor and renovator. They were my mentors. One of them taught me the rule for building at the top of a hillside: do not construct anything from the edge inwards unless the distance is no less than half the distance of the hillside. For example, if the hillside is 250’ vertically in height, stay away from the edge at least125’ and don’t disturb anything within the 125’. Simple – you bet. Does it work – I have no proof but I would have to say yes. There was another rule for building too close to the base of the hillside but I can’t remember what it was. Now back to the business at hand.
The BGC report stated “Where homes are constructed on or below sloping ground, landslide risks cannot be eliminated, only managed in an informed and proactive manner.”
So who is responsible to manage and act in this proactive manner? The homeowner? The municipality? Probably both even though the hillside may be on private property. Maybe there should be a right of way along the crest and base of these hillsides in the unstable zone as well as the hillside itself. This would allow trained professional inspectors at a municipal level to inspect these banks.
While the process of today is a relatively new method, benching really got its start back in the forties and fifties. At that time it was in the form of placing large steps of some degree into the embankment. Benching is a form of using the landscape as a retaining wall. The use of scrap tires brought this process to the forefront. Here’s how.
A retaining wall is a wall built to keep a bank of sand or soil in its place. Although retaining walls are typically made of concrete, whole scrap tires were used as an alternative building material for retaining walls.
Traditionally to construct a retaining wall with whole tires, the tires are stacked horizontally on top of each other. Adjacent tires are then clipped together horizontally and metal posts are driven vertically through the tire openings and anchored into the underlying earth as necessary to provide lateral support and prevent later displacement. Each layer of tires is filled with compacted earth backfill.
Today, these types of retaining walls are being replaced in North America with more traditional retaining walls because they can soon become an environmental hazard. They were, but should never have been, constructed where there is ground water runoff in the spring, or springs emanating up from the ground or rivers and streams nearby.
Also, the toxic gases released when the tires caught fire, caused a huge pollutant exposure and health hazard.
In urban areas, most municipal bylaw ordinances have burning bylaws regarding open fire pits. Typically the pit is required to be located away from any combustible material. Tires are combustible material. For example if the bylaw states 16 feet away, the pit must be 16 feet plus at least 7 feet additional for a total of 23 feet because the top row of tires will extend back into the bank at least 7 feet, thus making the fire pit only 9 feet away at maximum.
As a rule of thumb, tires that are stacked a certain number high (between 2 and 6) vertically should extend the same number horizontally into the bank and clipped together. This should be done on the first bottom row, then on the top row of each step.
For example, for a retaining wall 100 feet long and 10 feet tall (1,000 square feet), 600 tires could be used. The rate of placement would be 0.6 tires per square foot based upon a 4 foot tier step.
Older tire retaining walls – 30 to 40 years of age – can be a concern because at that time there were no regulations. Today most municipalities ban this type of retaining wall. They now are almost never recommended as an engineering solution.
The combination of any or all of these conditions is a recipe for disaster. The question is not “Will it happen?” the question is “When will it happen?” So let me ask you – Do you know what is in your backyard? …………….. and I don’t mean figuratively.