Friday, October 23, 2015

Why do I Need a Geologist in Order to Build my Deck?

Most of what the geoscience community does is profoundly practical. As you cross any bridge, enter any building, you have a professional geologist or engineering geologist  to thank for the fact that you are safe there. 

Q: I would like to extend (cantilever) my deck over my back slope. I am told by construction contractor that I will need a geologist to determine type of bedrock and/or soil and determine the depth required for installment of support piers (caissons) to support deck structure.

    Do I need to hire only a geologist for determining whether hillside slope will support a deck?
- Walter H

A: Laws and codes are different for different cities and in different states, and are different for flat or hilly ground, hurricane-, tornado-, or earthquake-prone terrains,  so I cannot directly answer this question. Some states require assessments by someone who has passed qualifying tests, and can designate "PG" (for professional geologist) after their name. Some states require an engineering geologist to do this sort of job. These people basically provide crucial experience and data to ensure conformance with local building codes. 

    I can indirectly answer your question by sharing my own experience, which may or may not be relevant. I chose a home with a great territorial view. The price I must pay for this view is that the home is built on a slope, of course. Any slope - especially something graded within the past 20-50 years and not already covered with semi-mature trees, is inherently unstable. For instance, after just ten years I had to pay for a rock retaining wall to be built at the bottom of my back yard/slope - because the soil was slowly creeping downward and had already buried my neighbor's fence 20 cm deep. In this area there are known/mapped slow or creeping landslides, also. On one public trail that I often walk, you can see hundreds of trees that are bent almost horizontal at the base, and then curve to vertical as they go up - a sure sign of a slow or creeping landslide.

    As part of the negotiation for my new house, the company selling it agreed to build a deck in the back. The distance from my bedroom door to the ground at that time was about 15 meters. That's a long first step if you are sleep-walking, so local building code had required the outside of the door to be boarded. I had no idea what the real costs of the final deck were, but an engineer came twice to my door and apologized. First, that he would have to make it wider than my realtor had suggested - to meet code. Second, he would have to connect each of three decks by stairs - to meet code. It had to serve as a fire escape suitable for children. Then an excavator came in and dug a 2-meter-deep trench, a meter wide and the width of my house, behind the house. They set up molds and brought in a monster machine that looked like a Snuffelupagus, and poured 5 concrete cylinders a meter wide and 2 meters tall each. These were even more deeper anchored with some kind of rebar to about 3 meters below ground surface. They brought in a small grader that covered/filled in the trench. To the imposing concrete pylons they bolt-anchored five pressure-treated beams (several of them 15 meters long!). THEN they began building the deck. When I asked the builder why so much precaution (it seemed like massive over-kill to me), he said that building ANY deck on ANY slope was fraught with problems, and from experience this was the MINIMUM precautions they must take. These precautions were built right into the building code.

    “Precautions against what?" I asked.

    "Against your deck joining your neighbor's party," was the reply.

     In the 12 years since the deck was built (it remains stable) I have seen several things including the bent trees and my own sliding lower backyard slope that convince me he was correct. 

     So much for my theory that I could get away with a couple of cinder blocks with some posts standing on them and do it myself.