Below Grade Tanks

An in ground tank has several advantages.

• The ground will tend to stabilize the temperature, 
• The system can be built lower and still rely on gravity
• Less exposure to sun

There are many ways to build an in ground tank.  Each method comes with it's own pros and cons.
Here is the list of cons
 
     1.  Set a molded tank into the earth.
           a.   Cost per gallon is high
           b.   Shape and size is limited
           c.   Susceptible to buoyancy if the tank is drained.
     2.   Build a concrete or block tank below grade.
           a.   Very costly
     3.  Build a wood box with a liner and set it into the earth
           a.   Costly in both time and materials.
           b.   Wood will eventually fail to insects and rot
           c.   Attention must be paid to the inward forces of the water table and or the earth.
     4.  Set an IBC into the earth
           a.   Prone to UV deterioration
           b.   Attention must be paid to the inward forces of the water table and or the earth.
           c.   Susceptible to buoyancy if the tank is drained. 
     5.  Dig a hole and line it with pond liner.
           a.    Uneven sides
           b.   Attention must be paid to the inward forces of the water table and or the earth. 
           c.   Susceptible to buoyancy if the tank is drained. 

Building a concrete tank is costly and labor intense, but the longevity is unparalleled. If you are dealing with a high water table it may be your best choice. Here is a link to a discussion I was recently involved in which addresses water proofing concrete tanks.

A molded tank set into the earth has the greatest appeal to many.   It is by far the easiest method and molded tanks can be found at reasonable prices.  An old hot tub can be used if you are feeling thrifty.

My least favorite is #3 because wood to earth contact is never a good idea.
I can not think of any advantage that would compel me to choose this method.  

Setting an IBC into the earth will provide a uniform smooth non toxic tank.  Protecting it from UV is important, but manageable.   To provide side support and prevent caving when the tank is empty, the back fill must be tamped down and compacted or cement should be added to the back fill to ensure stability.

Using a pond liner in a carefully excavated hole has some interesting advantages.   Pond liner is not susceptible to UV and it can be made any shape and size you wish.   This is the method I would like to describe here.

For the cost of the liner, some concrete and the form boards this method is simple and affordable.

Dig the hole to your specifications.
Drop the liner in and fill with water.  
Let the excess liner lay on top of the earth.
Then build a form around the edge and on top the the excess liner.  Include any conduit and rebar you wish and pour a high strength concrete into the form. 
Next day remove the forms and enjoy.

If you are pouring a low edge of 6" or less, the forms can be built very simply by drilling holes through the wood and connecting them with a piece of baling wire inserted inside of a spacing tube as shown above.  The wire can be tied off with a nail.  The spacer will remain inside the concrete after the forms are removed. Other materials such as a piece of paving block or even small piece of wood can be laid at the bottom. 
Lay a piece of rebar on the wires and secure the top with a piece of wood.   Be gentle but work the air bubbles out by running a rod up and down along the inside of the form.  Knock on the form with a hammer to settle the mix.  If this is not done there will be voids along the edges.

The concrete edge can be made any height, color and width.   You can make it  a work of art or as simple as you wish.

Here is a link to concrete information.

The simple rule is
1 part Portland cement
2 parts water
3 parts clean sand
4 parts gravel