Urban geology

Tuesday, September 20, 2016

Paul Karrow, Department of Earth and Environmental Sciences, University of Waterloo

Some may feel the term “urban geology” is an oxymoron – in other words, something too ridiculous to think about. Certainly, many geologists don’t yearn to do field work in cities, with the ground covered by buildings and paved streets, and having to deal with private land and traffic everywhere. Field work there is commonly different, and with only the usual hand tools (soil probes, hand augers, and small shovels), access to the ground may be limited to school yards, cemeteries, parks, and rare empty lots. There may also be large natural exposures (lacking vegetation) along lakeshores or stream and riverbanks, but these can be dangerous without proper experience and great care for safety (see Figure 1).

Graphic showing the layers of sediment in the Toronto area exposed in the Don Valley brickyard and Scarborough Bluffs. Starting at 14 thousand years ago is the Iroquois Sand followed by the Halton Till, at 16.2 thousand years ago is Maple Beds follwed by the Newmarket Till ending at 30 thousand years ago. Then the Thorncliffe Beds, Sunnybrook Drift ends at 50 thousand years ago, followed by Pottery Road Beds mixed with Scarborough Beds until >80 thousand years ago, which is followed by the Don Beds, and finally the York Till ending at >125 thousand years ago.

Figure 1: Layers of sediment in the Toronto area exposed in the Don Valley brickyard and Scarborough Bluffs.

Considering that 80% of our population lives in urban areas, there is an abundance of reasons, both scientific and practical, to discover the geology of urban areas and make it available to the public. With a little searching, one can find a wealth of information about urban geology. Publications about urban geology vary from whole books to many articles on particular cities, as well as more specific articles about resources, construction problems and hazards, both geological and non-geological, in urban areas.

Most of the information on the geology of urban areas is held in the files of consulting companies, staffed by civil engineers, who drill holes to sample the rock and soil materials in order to design appropriate foundations for buildings or bridges or for water and waste pipes to serve urban areas. Access to such information is limited to those with proper purpose, either through these companies or through the clients or data owners (municipalities, government departments such as highways, architects, etc.) Other data collections include water well and oil and gas well records, which must be recorded with appropriate government agencies. These can total millions of records that extend tens to hundreds of metres deep for water wells or even thousands of metres for oil or gas wells.

Lastly, during construction of buildings, from houses to skyscrapers, or making trenches for pipelines, excavations provide exposure below the surface; these can be studied to reveal the materials and their properties and interrelationships to help construct a geological history. They may reveal fossils, some of which can be dated with radiocarbon or by other methods. Fossils also reveal the nature of the environment at the place and time they were deposited.

In all phases and at all levels, developing contacts with engineering consultants, other members of the engineering community, and representatives of municipal agencies (works and roads departments, water supply, waste disposal departments, and resources development sites) help to develop a network of data sources to aid progress in urban geology studies.

Sites of cities

There are a great variety of reasons for why cities are where they are. Crossing points of large rivers give access to combinations of travel routes by land and water, the latter being the dominant access for travel in the earliest years. Some, such as Sudbury, were by a major mineral deposit, while others were at a waterpower site for a gristmill or sawmill. Some grew around particular industries, such as Hamilton, Ontario, and its steel industry. Others chose a harbour site, like Halifax, Toronto, or Vancouver.

Mineral resources

Aggregates, such as sand, gravel, and crushed rock (commonly limestone) are essential for the construction of roads and buildings. While they may directly serve in construction, aggregates are a major constituent in concrete, a manufactured building material in most buildings from basements and sidewalks to large high-rises. Cement is manufactured in cement plants, where ground-up limestone with added clay is fired in large rotating kilns, then pulverized to create cement, a powder. Cement, mixed with aggregate and water, is poured into molds, which dries and hardens to form concrete. Sand and gravel form naturally from stream or shoreline erosion.

Clay and shale are used to manufacture brick, another common building material. Clay occurs as lake and sea bottom deposits, and is usually found under lower-lying flat areas.

Water supply, an essential need for human life and in many industrial processes, may come from lakes or streams, or more often, groundwater, which is derived from springs or drilled wells. Soil that is permeable, such as sand and gravel, forms aquifers, with its flow underground potentially impeded or guided by low permeability aquitards, such as clay. Water may be contaminated by natural mineral sources or by industrial processes. Waste must be treated in sewage plants and disposed of.

Construction problems

As most of Canada experienced the last Ice Age and was submerged under ice sheets up to thousands of metres thick, glacial deposits are widespread, with their characteristics leading to certain construction problems. The last ice sheet was at its maximum extent and thickness about 20 000 years ago, when the climate was colder than present; this ice slowly melted away as the climate warmed up to 7000 years ago. Flow of the glacier ice from three centres (one in central Quebec, another northwest of Hudson Bay, and a third in the Cordilleran ice sheet in British Columbia, see Figure 2) dragged along rock debris at the base of the ice. Large boulders are common and cause excavation problems. If they are distinctly different from local bedrock, they may be referred to as glacial erratics (Wat on Earth, WOE, S’93), commonly carried hundreds of kilometres from their source in the Precambrian Canadian Shield. Such boulders are used in residential landscaping because of their unusualness or attractiveness. More generally, glacial debris is called till, a sediment composed of a mixture of all sizes from boulders to clays. Because of its deposition under the ice, till is strong, stiff, and resistant to excavation (in other words, it is preconsolidated). Associated water-laid clay was deposited in glacial lakes and seas; such clay in the Ottawa-St. Lawrence valleys is called Leda clay, named after small marine fossil clams of the Champlain Sea (WOE, W’06) which spread over the glacier-downwarped land as the ice melted. This clay is soft, compressible, and “extrasensitive” because it liquefies when disturbed. Leda clay is found in Ottawa, Montreal, and Quebec City with associated building settlement and landslides.

Map showing the Greenand Ice sheet, Innuitian Ice Sheet, Laurentide Ice Sheet, and the Cordilleran Ice Sheet.

Figure 2: The last ice sheet.

Another distinctive clay was deposited in Canada’s largest glacial lake, Lake Agassiz, which extended from northwest Ontario, across Manitoba and Saskatchewan (see Figure 3). This clay contains swelling clay minerals which swell when wet and shrink when dry, and is found with landslides and floods in those areas.

Cyclic varve sediments from a glacial lake like Lake Agassiz; appears as a stripped rock formation.

Figure 3: Cyclic varve sediments from a glacial lake like Lake Agassiz.

Finally is the problem of frost action, which bedevils our cities each spring by damaging road pavement. Traffic on these affected roads causes potholes needing seasonal repairs.

Geological and other hazards

When you refer to geological hazards, the first thing people think of is earthquakes, which occur where large pieces of the Earth’s crust move relative to one another and severely shake the ground. They can cause the deaths of up to thousands of people in addition to large amounts of damage. Although I lived in California for a year and a half, and in British Columbia for a half year, I never felt an earthquake despite the high risk in those areas. The only quakes I have felt were right here in Waterloo. The last one felt here was in 2013, centred near Ottawa with a strength of over 5 (on the Richter Scale). Overall in Canada, the highest risk for earthquakes is near the ocean in British Columbia, with another lower risk area along the Ottawa and St Lawrence river valleys, and a likely strength in the west of 8, in the east of 7 (see Figure 4).

Seismic map of Canada, showing distribution of earthquakes greater than magnitude 2.5. High concentrations along the oceanic coasts, with the highest concentration and highest magnitude along the West coast.

Figure 4: Seismic map of Canada, showing distribution of earthquakes greater than magnitude 2.5.

Tsunamis, large ocean waves resulting from earthquakes in the sea floor, have occurred on both the east (Grand Banks, 1929, see Figure 5) and west (300 years ago) coasts of Canada. They can move long distances in the ocean; for example, Sumatra 2004 killed 230 000 people in the Indian Ocean. Japan 2011 killed 20 000, with $200 billion in damage and debris reaching British Columbia shores two years later.

Topographical map of the Grand Banks tsunami and the extent of the damage.

Figure 5: Map of the Grand Banks tsunami and the extent of damage.

Volcanic eruptions are another hazard. The chief risk is in Vancouver, which is in view of Mount Baker in the U.S. state of Washington.

Damaging landslides from the Leda clay of Quebec can cause tens of deaths. More common are rockslides on steep mountain slopes in British Columbia. They may start from earthquakes, heavy rain, or frost.

Flooding can occur in many places in Canada. Best known is their frequency in Winnipeg and the Red River in the springtime thaw. Best known in Ontario is Hurricane Hazel, which caused tens of deaths in 1954 in the Toronto area and led conservation authorities to clear dwellings from flood plains.

Sea level rise affects whole countries in some parts of the world, but is generally a local problem in Canada. Richmond, British Columbia, however, is situated a mere two metres above sea level, with seawater excluded by a system of dikes.

Geological hazards are of particular concern in cities because of the concentration of people in small areas and their common situation in or near tall buildings. If something goes wrong related to geology, the residents and their politicians have a particular responsibility to operate urban areas efficiently, minimizing the burden on the natural environment. This entails reducing energy demands and making cities more self-sustaining. Improved transit, local food production in urban gardens, and much increased solar energy with the abundance of roof areas can contribute to this self-sustenance, as can greater development of geothermal energy.

Making geology more public

At some geological meetings, geology is represented by tours displaying the varied uses of building stones, both decoratively inside as well as architecturally on the outsides of buildings. Commonly, that is the extent of urban geology on display. That effort can be greatly enhanced by visits to outstanding excavations such as subways or other projects. A partnership is needed to develop parks that display particular geological features (for example, landforms, water supply facilities, museums, drill cores) that reveal the nature, the stratigraphic record, and the geological history of the area. All would be buttressed by the availability of public courses on general geology, geological hazards, and role of mineral resources. Most larger cities have rock and mineral clubs that sponsor meetings with visiting speakers and field trips to sustain amateur geologists. Going even further, there should be more promotion of people taking geology courses or degrees without any career plans in geology. Much could be done by partnerships between universities and community colleges to develop a more educated citizenry, better able to knowledgeably influence community decision-making.