There were a total of eight holes to be dug for my small cable suspension bridge. They would house four posts and four dead-men (more on dead-men later). The posts were going to be three feet into the ground, but the plan also called for concrete punch pads, so that meant another ten or so inches of hole to dig.
Why a punch pad? A two-foot wide pad of concrete would help to distribute the weight. The posts were bearing much of the weight of the loads. If I just set the post to the ground, the point load might exceed the bearing capacity of the native soil. If there were a big load on the deck, such as a tree that fell on it, the posts could sink into the ground and cause the bridge deck to be lower over the creek. The point load would be far less than if I distributed the weight in a concrete pad.
I figured 1,500 pounds of bearing load per square foot, so three square feet of concrete pad totaling twelve square feet would allow for a pretty hefty weight capacity. I also planned to make the pads about ten inches thick to give them more tensile strength in order to prevent cracking.
Meanwhile, the plan also called for backfill with 3/4 minus gravel, which would be tamped at regular intervals. The gravel would help to protect the posts from insects and water damage, plus it would help hold the posts more firmly in the ground.
To allow for the backfill, the holes needed to be about two feet in diameter. I set to work and soon discovered that digging a hole that was only two feet wide was awkward to work in. After digging the first 18 inches or so, I connected the two post holes, creating an oval around both of them. It was easier to work in, plus, there would be that much more gravel backfill to add more security to the posts.
I always use ready-mix concrete for small projects like these punch pads. The bags have the correct ratio of sand, gravel, and cement in them, so all one has to add is water and an energetic mixer. Some people don’t even bother to mix, but if you want a strong concrete base, you will want to mix it well.
I needed to mix about three cubic feet of concrete for each punch pad, which amounted to six bags of 60 lb. ready mix per hole. Rebar and drift pins were already set into place. The pins stuck up about two inches from the center once the pads were poured. When the punch pads were cured, it was time to set the posts.
Be sure to check out my new book, Building a Small Cable Suspension Bridge. There is a link to purchase it here: http://www.wildcatman.com. There is also a link there to contact me.
Images, diagrams, and text copyright 2013 by Marvin Denmark unless otherwise noted. Please do not copy and post my content anywhere without my permission. Thank you.