Landslides are frightening and an example of one more way nature has her way with us when conditions are right.  Such a large mass of soil and rock sweeping away everything in its path must be terrifying to see, and terrifying to be in.  I’ve seen the aftermath of large landslides and earthwork failures and investigated some.  They are a humbling experience.

The recent landslides in B.C. at Fairmont Hot Springs Resort and Johnsons Landing on Kootenay Lake certainly fall in that frightening category.  Others in B.C. and elsewhere in Canada, including the Atlantic provinces, are smaller but still serious in causing injury and financial loss.

News reports indicated that geotechnical engineers and geological speciallsts were on the disaster sites within hours of the landslide.  A good thing quickly getting knowledgeable technical people there.  Landslides are engineering failures, particularly when they affect people  The investigations they do and the data they collect are certain to assume the status of forensic engineering investigations.

(Geotechnical engineers are civil engineers who have specialised in the investigation and study of the physical properties of soil and rock as engineering materials)

These large landslides appear to have occurred after smaller landslides along streams – like Fairmont Creek, created dams causing water levels to rise.  Eventually the rising water would overtop the dam and wash it away and downstream.  The mixture of stream water and dam material would pick up other material along the stream bed to create the mass of soil that swept over the inhabited areas as a large landslide.

It’s certain more landslides are occurring in B.C. as I write, both large and small.  If not in or near inhabited areas then in remote areas for sure.

Landslides are natural geological events.  They occur when conditions are right – the strength of the soil on a sliding surface is not great enough to hold back the mass of soil.

The physics principle involved is the same as that underlying the reason we slip on ice in winter and fall and are able to ski on snow and have fun.

Sometimes the strength of the soil is just great enough that the mass of soil stays in place.  Until something comes along to reduce the strength that little bit so it’s no longer adequate.  Or increase the weight of the mass of soil.  That something can be water – rainwater.  The water is said to “trigger” the landslide, a term used by geotechnical engineers and sometimes the general public.

Once the mass of soil starts to move – the land starts to slide, it takes the easiest path like flowing water.  Simply downhill – down a slope, or down a natural channel in the terrain, for example, a water course or stream bed.  The mass of soil in a landslide can be quite “liquid”.

Geotechnical engineers can investigate, analyse, and predict with considerable accuracy that a landslide will occur.  They cannot really say when.  Except perhaps when it’s imminent if they are able to examine the terrain for the signs.  For example, signs like fissures in the ground surface – “tension” cracks to engineers, leaning trees, and muddy water like that seen at Fairmont Hot Springs Resort.

They can also advise with some confidence on the stability of a sliding mass of soil that has come to rest like the geotechnicians did at the Resort.  Their degree of confidence would likely be greater than that possible by structural engineers at the site of a collapsed building like the one at Elliot Lake.

Engineering Investigation

A geotechnical engineering investigation of a landslide – either before the event to predict the likelihood of its occurrence or afterwards to determine the cause – would have the fundamental components of an engineering failure investigation:

• Gathering data
• Analysing data
• Developing an opinion

Data would be gathered in two basic stages:

• Gather together existing data
• Gather new data

These basic stages are likely underway at present at the Fairmont Hot Springs Resort and Johnsons Landing.

Existing data is often concerned with conditions at the ground surface and new data with conditions below the surface.

Existing Data

Existing data might consist of:

• Air photos
• Infrared photography
• Topographic maps
• Geologic maps, particularly soil maps if the sliding mass is in soil
• Published soil physical properties
• Hydrogeologic maps
• Forest cover; vegetation in general
• Local weather and climate
• Walk-over surveys (several times during a study of the existing data)
• Local history and knowledge of past landslides

Reviewing and studying existing data like this, an experienced geotechnical engineer could offer a quite informed and fairly confident statement on the susceptibility of an existing hillside to landslides, or the cause of an existing landslide.

Statements like these have likely already been made about the landslides in British Columbia.

New Data

New data that would increase the confidence of the geotechnical engineer in his opinions would consist of:

• Surveying (topographic) the site to determine the size and dimensions of the landslide
• Estimating the volume of soil and rock that slid
• Augering and drilling boreholes to do field tests and get soil and rock samples for laboratory tests
• Determining the depth of the sliding surface
• Constructing groundwater monitoring wells to determine the depth to the watertable and the flow of the groundwater
• Installing instrumentation to monitor slope movement

Analysis

Investigative tasks like the above provide a lot of data to be analysed.  But the results of the analysis enable well-founded opinions to be formed on the cause of the landslide and remedial work to begin.  The analysis also enables areas prone to landslides to be avoided when building new structures or to be stabilized before building.