DIY Bulletproof Dock Anchor

DIY Bulletproof Concrete Anchor Mount

Your anchor mounts are crucial to the survival of your dock. They have to be strong enough to hold up to many tons of lateral force. After seeing many (mine included) fail, I came up with a better plan. This page is a little long and presented for a novice DIY guy. I've made several of these mounts and have yet to see one wiggle.

I figured that there were two ways to get a mount that would hold up to the forces our docks see. A big hole with a tons of concrete or 5 smaller holes arranged under a block of concrete that would have the same surface area engaging the soil as the big block of concrete would. Concrete isn’t cheap and most of us would have problems getting a concrete truck close enough to where we need the mount. A big block of concrete isn’t easily happening without a long day of hauling wet concrete in wheelbarrows from the street to where you need it in the back yard.

The design…
The typical dock anchor pole is a piece of pipe that is 4 or 5 inches in diameter with a means of attaching a cable at one end. It is placed in a hole dug with post hole diggers that might be 3 feet deep. 3 feet is about as far as you can get one with post hole diggers. The void around the pole is filled with 2 or 3 bags of concrete. The hole will taper from wide at the top to narrower at the bottom with an average diameter of about 10 inches. The 4inch pipe will occupy .01 cu. yds. and displace .25 of an 80 pound bag of concrete. The remaining hole will hold .05 cu yds. or about 1.75 80 pound bags of concrete. 2 bags will get this done but you are not going to have much of an anchor. With the weight of the pole, it might be 200 lbs. Holding a 5+ ton dock in place during 60+mph winds we see regularly is asking a lot from it. Its no wonder they fail all the time.

Anchor mount strength comes from its weight and surface tension with its engagement with the surrounding soil. Soil density also plays a role where noncompacted soil from a location that has been backfilled will not be as strong a connection as undisturbed soil. The single pole mount above has 1131.12 sq. in of surface area and if you got the whole bags in the hole still weighs less than 200 lbs.

An easy way to increase its strength is to add a form around the post and fill it with concrete. A 3 foot square box 1 ft. high holds 9 cu ft. of concrete. Concrete weighs 143.38 lbs. per cu ft. when mixed. 9 cu ft. would weigh 1290.42 pounds by itself. Added to your single pole above and the total is nearly 1500 lbs. Much better than the single pole with 2 bags. The 3 ft. x 3 ft. x1 ft. box also adds soil contact area which also helps with an additional 1728 sq. inches of surface area.

How can we make this even better? Let’s add 4 more 10 inch holes under the box. One hole in each corner adds 4528.48 more sq. in of soil grabbing surface area and 929.1 lbs. more weight making the entire mount weigh about 2430 lbs.

Let’s compare the above to a solid 3 ft. x 3 ft. x 3 ft. square block of concrete adequate for the job under most circumstances. 3x3x3 is a square yard or 27 sq. ft. It would weigh 3871.26 lbs., require 29 80 lb. or 46 50 lb. bags of concrete, and have 6480 sq. inches of surface area engaging the ground. My bulletproof anchor above has 6256 sq. inches locked into the ground and uses the soil as media to further add resistance to movement.

Some numbers for reference…
The concrete:
One 80 lb. bag of concrete = .6 cu ft. and weighs 85.8 lbs. mixed.
One 50 lb. bag of concrete = .375 cu ft. and weighs 53.6 lbs. mixed.
One cubic foot of concrete weighs 143.38 lbs. mixed.

The hole:
10 in wide hole has a circumference of 31.42 in. and 36 inches deep = 1131.12 sq. inch surface area.
Each 10 x 36 hole is .06 cu yards and 5 holes is .30 cu yards.
A 3x3x1 ft. box is 9 cu ft. or .33 cu yards and 1728 sq. inches of surface area.

The process…

Tools needed:

A wheelbarrow to carry the bags from your truck or trailer to the hole site and to mix the concrete in.
A water source, hose to the hole site. I have used a bucket and dipped water from the lake. Works but gets old fast hauling water.
Shovel for cleaning up the hole and mixing.
A hoe is good for mixing.
A small concrete mixer from Harbor Freight works but is slower than mixing by hand in the wheelbarrow.
Post hole diggers or a powered auger. I like the auger as it makes short work of the holes which can be the hardest part. Harbor Freight has one that works fine for not a lot of money. Be sure to get the 21 inch extension as it needs some more length to reach to 3 feet deep. Posthole diggers will get the job done as well. If using a powered auger, use the 8 inch bit. The hole will be a little bigger than 8 inches because of wobble. Not a big deal. You planned for 10 inch holes anyway.
Circular saw for cutting wood to make the form.
A drill or impact driver with a bit for deck screws for assembling and taking apart the form.
A concrete finishing trowel and concrete edge trowel. Makes the finished product look sharp.
An angle grinder with a thin cutting blade for cutting the rebar to length.
A pair of vise grips for twisting the mechanics safety wire around the rebar.
A pair of diagonal cutters for cutting the safety wire.
A bubble level for making sure the form is level before adding concrete. 3 foot long or longer is best. Shorter still works.
A hammer for tapping rebar where it needs to go and for getting the concrete to settle in the form.

Materials needed:

30 80 lb. bags of concrete or 48 50 lb. bags or concrete. Always get extra. Its better to end up with a little too much than to run out before you are done and need more. You can take back what you don’t open. 50 lb. bags were $2.04 less than the 80 lb. bags and your back will thank you.
90 linear feet of ½ inch #4 rebar. Rebar is a high carbon content high tensile strength solid steel bar with pronounced ridges all over its surface. The ridges lock it into its position in the concrete mix. It’s not expensive and Lowes sells it in 10 foot long pieces.
A spool of mechanic’s wire or zip ties to tie the rebar pieces together. Wire is the industry standard but zip ties are much easier to use and get the job done.
Half a sheet of ¾ inch or 23/32 inch pressure treated plywood. No OSB or particle board.
One pressure treated 8 foot 2x4.
Deck screws, 2 inch and 3 ½ inch.
A 5 foot long pole with a flange for attaching your dock spring and cable. The pole needs to be ¼ inch wall thickness. Square tubing and round tubing both work fine. Use 5 inch diameter. The flange should be from ½ inch thick plate with a hole large enough for the shackle to attach. To make it pretty, cut and weld a cap on it. Galvanized is nice but painted will be fine. A longer pole can be used, either make the center hole deeper or pound it into the soil at the bottom of the hole. The finished pole needs to stick up above the form 10 to 12 inches.

Location:
Pick a place that is as far to the side of the walkway as you can reach. The further apart the anchor poles are the better. Near trees are bad as roots can be tough to dig holes through. Solid soil is better. Try to avoid putting it in soil that has been moved, such the backfill area behind a sea wall. If you already have a post in the ground, use it for your center post. There is nothing wrong with including the existing concrete around it into your form. Its just that much less concrete you will have to buy and one less hole to dig.

Getting started:
Make the form. Cut four 12 inch wide strips from the end of the ¾ inch plywood. Cut two of these 36 inches long and two at 34 ½ inches. When assembled into the box shape for the form, it will be 36 inches square. Cut the 2x4 into 8 12 inch long pieces. Use the 2 inch deck screws to attach a 2x4 piece to the ends of the plywood from the inside of the form as illustrated. Assemble it on a flat surface like your garage floor using the longer 3 ½ inch deck screws. Now take it apart but don’t completely remove the outer screws. Back them out just enough to get it apart to make assembly easier later.

Clear out the spot you want to put it. Use a shovel to make a level surface. You may have to dig into the uphill side to accomplish this. Keep the dirt you dig out nearby as you will want to use it later after you are done to fill around the concrete on the high side. Assemble the form and place it where you want it to be. Mark your hole locations, one in the center for the post and one inside each corner of the form and then pick the form up and set it aside. Now using the diggers or auger, go to town making five 36 inch deep holes no wider than 10 inches. Any bigger and you will need more concrete. Clean as much as you can from the hole bottoms. I use post hole diggers to clean up the holes the auger makes.

Set up the form. Put the form over the group of holes and get it level. Use your shovel to remove dirt that may be holding it too high in any spots. Use your bubble level to make sure it is level.

Time for reinforcement.
The rebar adds shear strength to the form and keeps it from cracking later. Using the angle grinder with a thin cutting disc, I cut twelve 48 inch long pieces of rebar and drop 3 into each hole. I then cut 8 pieces 32 inches long to form a horizontal square in the box, twice. One just above the ground and one just below the top surface of the form. Use either mechanic’s safety wire or zip ties to first make the squares then to attach the 3 vertical 48 inch pieces in the corners. Make sure the 48’s are positioned in the holes so they are a few inches apart and parallel to each other. I then add a piece across the corners of the 2 horizontal squares and tie it all together. Make sure none of it protrudes above the top of the box or touches the sides of the form. When done, they should look like this…

Here is a cutting guide:
Use 9 10 foot long #4 rebar rods.
Cut 6 rebar rods 48,48, 24.
Cut 2 rebar rods 32, 32, 32.
Cut 1rebat rod 32, 32, 24, 24.
This will make 12 48 inch, 8 32 inch, and 8 24 inch pieces.

Get to mixing.
Put the post in the center hole but don’t worry yet about it being perfectly vertical. You can wiggle it where you need it as you add concrete. Set the wheelbarrow beside the hole so pouring concrete into the form is easy. Closer is better. Cut open and put 1 80 lb. or 2 50 lb. bags of dry mix in the wheelbarrow and add about a quart of water. Start mixing. I use a turning it over with a shovel method. Working it with a hoe is good too. Add water as you mix to get a thick wet pancake batter consistency. Pour it into the form and repeat… a lot.

As it is added to the form, use your shovel to move it where it you need it. Eventually, the form will be full. Double check the post for being vertical with your bubble level. Use the finishing trowel to smooth out the top. Use your hammer to tap lightly on the sides to settle the concrete mix against the form. This will work out any air voids and result in a smoother finish on the sides. Go wash off everything (tools) with concrete on it before it has a chance to dry good. Give it an hour or so then use the corner trowel to put a nice, rounded edge on the form top. Now leave it alone. It’s time for a beverage or two. Next day, back the screws on the form out and check out your handy work. Looks good doesn’t it.

Now, do it again on the other side… next weekend.

What does it cost???

Just about everything you will need can be picked up at your local hardware store. The exception will be the post you use. Using today's prices (and they do change often...), here is a breakdown of what I would have to spend to make one Bulletproof Dock Anchor.

50 lb. bag of concrete mix is $4.32 x 48 bags is $207.36
80 lb. bag of concrete mix is $6.98 x 30 bags is $209.40
#4 .50 inch rebar is $7.48 x 9 is $67.32
2 x 4 x 8 is $5.68
4 x 8 x 23/32 pressure treated pine plywood is $51.18
2 inch deck screws are $10.98
3.5 inch deck screws are also $10.98
Mechanic's safety wire - 25 foot roll from Harbor Freight is $.98
8 inch zip ties 100 pack from Harbor Freight is $2.29
5 inch x 60 inch x .25 inch is $225.56 for galvanized and $160.69 for bare steel from the Metal Supermarket online.
5 inch x 5 inch x 60 inch x .25 inch square tubing is $175.53 from the same source.
or, 5 inch x 5 inch x 60 inch galvanized I beam from guardrail replacement can be had on marketplace for $25 each and will work great. The I beam has a cap on top that can be easily drilled to mount your Dock Spring / cable without any welding.

So the materials will run $355.79 plus tax if I use the 50 lb. bags and $2.04 more if 80's. The center post is up to you but I would shop marketplace on Facebook for scrap pipe or I beam.

Concrete work doesn't require any special tools. You can use a 12 inch piece of 2x4 as a trowel to smooth out the top if you want but the right concrete tools make the job easier. Harbor Freight has a concrete tool kit for only $16.99 and if you get them from Lowes or Home Depot, expect to spend about $50. Wheelbarrow, shovel, hoe, drill/driver, hammer and a level are things most of us already have or have a neighbor we can borrow them from. Around here, we all pitch in and help neighbors with projects like this.

Any capable person can make an awesome Bulletproof Dock Anchor Mount by themselves but sharing the work with buddies makes it go a lot easier and faster. You will be whooped after digging, forming, cutting / tying rebar, hauling bags to the hole, mixing 48 bags or 2400 lbs. of concrete and cleaning up but it feels good to know you can do it. You made that.