Drainage under piers

Started by youngins, April 30, 2007, 07:25:46 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

youngins

We have clay soil. For our education, what the primary purpose is of putting, lets say, 8" inches of gravel in the pier hole before setting in the pier? Also, what the risk is of NOT putting any gravel under a pier?

Thanks
"A spoonfull of sugar helps the medicine go down.."

glenn kangiser

#1
A few things, but in my opinion holes with gravel in clay soil could be made to be better by having a way to drain them.

It gives you a way to equalize out irregularities between the pier and the soil - the gravel will fill irregularities then, especially crushed gravel will lock together like a solid when loaded equally distributing the load on the pier to the entire area below it- even if odd shaped.

Any hole in clay soil can have a bathtub effect - holding water in there and just keeping it there until heat and capillary action causes the water to be drawn from the hole.  Wet clay especially if expansive can heave causing footings, foundations and slabs to crack and move.  This is where I feel that connecting the holes with gravel trenches then draining them to lower elevation if possible would be beneficial.  Some places don't have a place to drain to.  Some places don't have very expansive clay.

In a case like yours it would be good to try to raise the areas near the building a bit to divert most of the water around the footings - not letting them fill with water and then the gravel would work as a dry well taking care of small amounts that happened to get by.  

Gravel around a pressure treated post would allow it to dry and give a break between the post and the soil so fungal growth may not get started as easily. Fungi in wood can send tendrils into the surrounding soil pulling in moisture with which to grow and decompose the wood.  Even treated wood can vary in its resistance to decomposition.  There are also better grades of treatment that are less likely to have problems as soon.

Not putting gravel - maybe less chance for adjustment in case of clay movement - unequal weight bearing between soil and pier causing possibly more chance of uneven support or settling when it gets wet.

Maybe others can add to this.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.


youngins

Thats some good info--

Is sinking a possibility/concern when using gravel?

For instance, would:

1. Gravel spread outward due to the compression from the pier/footer

2. Soil shifts underneath gravel
"A spoonfull of sugar helps the medicine go down.."

glenn kangiser

#3
All depends on your soil conditions.  The best way to tell how it would affect you is to think about what happens when you have an example of a similar condition.  If your ground gets real soft along way around a hole full of water, then maybe filling with soil cement mix would be a better way to go around a concrete pier.  The soil cement is fairly waterproof and wouldn't have the spaces to allow water to soak into the ground around the footingm as gravel would have.  The gravel on the bottom could provide leveling and a bit of drainage then soil cement could keep the liquid water from getting into the hole as bad.  If your yard is fairly stable and well drained, then the area around the footings should be also if well drained.

Rocks tend to keep clay soil more stable and well drained at their contact surface. Each rock is like a little roof or hat keeping the soil under it dry.  On the surface this provides more drainage keeping the clay dry.  In a hole a bunch of rocks serve as a storage area prolonging contact between the water and the clay - this could tend to soften the area around the pier more.

Keeping rain runoff on top and drained away would be more beneficial in this case.  Grading so water never gets under the building is also a code requirement in most places - something like 2% slope away for 5 feet  if I remember right.  See below.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

Amanda_931

Expansive clay (the kind in the simplest forms of kitty litter) expands when wet.  Good for sealing leaks, not what you want to build on.  

Any clay in an area where it can be both freezing and wet can extrude ice, break pavements, pick up badly designed foundations, etc.

Drainage is good.  


glenn kangiser

#5
OK -- change that slope info -- Two places recommended 6" in the first 10 feet or 5% minimum -more is better - around here by banks - hills etc they let us go 5 feet minimum then bring it around the side of the house from there.  More requirements too, of course -- can't remember them all. :-/  Note that it could go up after  that point.
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.