This EarthCache takes you to Spanish Banks, a popular beach in Vancouver with low-tide flats extending nearly 400 meters into the ocean. Here, you will learn about soil classification by comparing two types of soil along the beach.
1. Origin of Spanish Banks Soils
Spanish Banks Beach is a strip of flat land between the ocean on the north and steepland sediments on the south. While the steep hill on the south side of Northwest Marine Drive is made up of glacial till deposited in the last Ice Age, Spanish Banks Beach itself consists of modern sediments less than 10,000 years old. These sediments are loosely packed and easily erodible. Most of the material which makes up Spanish Banks Beach was originally part of the cliff face itself, but was eroded away by the wind and rain. Once broken into small parts, this material the became spread out at the base of the cliff.
2. Soil Classification
Soil Classification is the categorization of soils based on characteristics such as particle size and shape. Terms such as gravel, sand and silt are examples of soil classification. In Engineering, soil classification is important because it allows us to predict whether soils will experience settlement, and how much weight the soil at a given location can support. By completing this EarthCache, you will apply the basics of soil classification to the material at Spanish Banks Beach.
2.1 Particle Size
The three main groups of particle size are coarse-grained, fine-grained and organic.
"Coarse-grained" describes any soil with particles large enough to be seen by the naked eye. Coarse-grained soils are further divided into cobbles (larger than 75mm), gravel (4.75mm - 75mm) or sand (smaller than 4.75mm). When working in the field, these soils can be classified by estimating their size by eye or using a ruler.
Fine-grained soils have particles less than 0.075mm in size, so they cannot be seen without magnification. Fine-grained soils are divided into silts and clays. To determine whether a material is silt or clay, rub a small amount on your palm. If it feels gritty and washes off easily, it is silt. If it feels smooth and sticky, or a thin thread can be rolled from it, it is clay.
Organic soils are the third main sub-group, which aren't directly classified by their particle size. Organic soils contain a high proportion of decaying organic matter, and can be recognized by a dark brown colour, soft or crumbly texture, and organic smell. Sometimes, plant fibers may still be visible in the soil. Peat is an example of an organic soil.
A sample of peat soil.
2.2 Gradation
If particles are visible to the naked eye, a soil sample may be classified as either well-graded or poorly-graded based on how many different sizes of particles it is made up of. This is known as the soil's gradation.
Poorly-graded soil contains only one or two of the size categories described above. For example, this could be pure sand, gravel with some cobbles, clay with sand, or any other combination of one or two size categories.
A poorly-graded soil sample.
Well-graded soil, on the other hand, contains a mixture of more than two particle sizes. An example of a well-graded soil would be a mixture of cobbles, gravel and sand.
A well-graded soil sample.
2.3 Effects of Soil Characteristics on Construction
But why is the particle size and gradation of a soil important?
Fine-grained soils are less permeable than coarse-grained soils, meaning that water is slower to move through them. When a weight is placed on fine-grained soils, it can take weeks to decades for water to be squeezed out. As this happens, the soil changes shape; this is known as soil settlement. Organic soils are often soft and crumbly, and also easy to compress.
If soil settles unevenly underneath a building, the building can develop cracks, tilt sideways, or even collapse entirely. For example, uneven settlement is to blame for the Leaning Tower of Pisa's tilt.
Coarse-grained soils have larger gaps between particles, so water flows through them easily. When any weight is placed on them, water is squeezed out right away, so they are unlikely to change shape or settle in the months and years after a structure is built. However, they also fail to hold back water if the groundwater table rises.
Well-graded soils particles pack together well, since smaller particles can fill the spaces in between larger ones. When soil is packed together like this, there is a lot of friction between the individual particles, which makes the soil more stable. Poorly-graded soils don't pack together as neatly, so less surface area is in contact between the soil particles, and the soil is less cohesive.
Packing of well-graded vs. poorly-graded soils.
3. Questions to Answer
Visit both stages of the EarthCache and answer the following questions. Stage 1 is best visited at low tide and may be partially submerged if you are unlucky with your timing. However, you should still be able to answer the questions while keeping your feet dry.
1. Answer at both Stage 1 and Stage 2: Which of the following terms best describes the soil: cobbles, gravel, sand, silt, or clay? Do you see any evidence of organics?
2. Answer at both Stage 1 and Stage 2: Would you classify the soil at GZ as well-graded or poorly-graded?
3. Choose either Stage 1 or Stage 2 to answer: Describe one advantage and one disadvantage of constructing a building on this type of soil based on its characteristics.
In order to log this EarthCache, please send me the answer to these questions using the "Message this owner" link at the top of this page. Thank you for visiting!
Sources:
AG Classroom Store. Soil Sample (Silt loam) https://agclassroomstore.com/soil-sample-silt-loam/
Armstrong, J.E. (1990) Vancouver Geology. Geological Association of Canada - Cordilleran Section. Retrieved from https://www.gac-cs.ca/publications/JohnArmstrong_VancouverGeology.pdf
BCIT (2024) CIVL 2026: Construction Materials (Course materials). British Columbia Institute of Technology.
BCIT (2024) CIVL 3041: Soil Mechanics (Course materials). British Columbia Institute of Technology.
Burrard Bulletin. Spanish Banks Beach in Vancouver – Everything You Need To Know. https://burrardbulletin.ca/spanish-banks-vancouver/
City of Vancouver. Spanish Banks destination walk. https://vancouver.ca/parks-recreation-culture/spanish-banks.aspx
Geological Survey of Canada. GeoMap Vancouver. https://www.cgenarchive.org/vancouver-geomap.html
International Peatland Society. What is peat? https://peatlands.org/peat/peat/
NorthAbroad. How To Visit the Leaning Tower of Pisa & What To Expect. https://northabroad.com/leaning-tower-pisa-how-to-visit/
WM Trucking & Excavating. 9 Differences Between Silt and Clay You Did Not Know. https://www.wmtrucking.net/9-differences-between-silt-and-clay-you-did-not-know/