Construction status (slabs, cistern)

Pandeck over basement

October through December were full of form building, in-slab plumbing and electrical, steel (rebar, red iron, pandeck), grading, underground cistern, and a few rain delays.  The Fawall block walls starting going in today (1/3/11) so I have some things to catch up on before reporting on the external wall phase.

My favorite part of the project thus far has been the design and construction of the storm water cistern. During the design phase, rain water storage kept getting put off until finally laid to rest as a rectangular shaped dotted line on the landscape plan. Since this was an essential component of our sustainability objectives and had to be built early in construction, I researched various storage methods including material and construction costs with the following requirements:

• 5000 gal minimum
• Underground
• Pump suitable for drip/micro irrigation (minimum 10 gpm @ 20psi)
• $5000 budget ($1/gal)

I found several suppliers of plastic underground tanks maxing out at 2500 gal and larger fiberglass tanks. These were the most intriguing underground plastic tanks:

• Conservation Technology 2500 Gallon Double Torus www.conservationtechnology.com/rainwater_storage.htm

• Norwesco 2500 Gallon Below Ground Holding Tank www.tank-depot.com/productdetails.aspx?part=TN2500WT

Fiberglass tanks in 5000 gal size cost approximately $7000 and the 2500 plastic tanks are in the range of $2300-3500 each plus shipping ($671 for two Norwesco tanks). We also looked into precast concrete vaults which cost around $8000. The Norwesco tank appeared to be the most economical "off the shelf" solution at $2300 per tank. Other costs to consider for water storage are excavation, backfill and piping between tanks if needed.

My contractor convinced me he could build a concrete tank for less than the cost of the plastic tanks. The plastic tanks have limitations in how deep they can or must be buried and their dimensions may not work efficiently in the space available.

Purely looking at cost per gal of the storage container (w/o shipping), it breaks down approximately like this:

• Norwesco 2500 gal - $1/gal
• Double Torus 2500 gal - $1.50/gal
• Fiberglass 5000 gal - $1.50/gal

I was preparing to order Norwesco tanks when my contractor suggested building a steel reinforced concrete cistern. The idea was to build a tank using 4'x8' plywood forms on the inside and earth as the outside form. This also meant the tank could be any rectilinear shape desired, a big plus in our situation due to space constraints.  The area we were considering for the tank lies between the garage and root zone of a large Valley Oak tree. We decided to construct the tank in an "L" shape around the end of the garage and about 6" to 18" underground. This will conform around the garage and not encroach on the root zone or very far into the backyard planting area. The finished inside tank dimensions worked out to be 8'W x 4'H x 14'L on each leg of the "L" or effectively 8x4x36=1152cf=8,559gal.







Add a fiberglass 24" manway for $300 delivered from (www.frpsupply.com), PVC sleeves for the inlet and outlet and you have a nice sturdy underground water storage solution. The 2x4 and plywood forms were removed 10 days later through this manway.  I will report on the total cost breakdown once all of the numbers are in. So far, it is not looking as good as originally expected but likely still lower $/gal and more efficient site utilization.

Rain can be collected at a theoretical maximum rate of 620gal per 1000sf of horizontal roof per inch of rain. However, there are losses in the collection process (splashing, leaks, filtration losses) so I will derate by 20%.

An appropriate pump for this application is a "Shallow well jet pump" which consists of a jet pump and a pressure tank. This type of pump is capable of lifting water 0-25 ft and filling a pressure tank. Water is "demanded" from the pressure tank for use and refilled by the pump as required. Pumps and tanks for our specifications run about $300-$400. I will likely go with a packaged pump/tank system like a Sears Craftsman Professional 1/2 hp Shallow Well Pre-Plumbed System (8 gmp @ 40 psi).

The  roof collection area is approximately 1500-2000sf. Water will flow into rain gutters, through downspouts and exhaust in two or more vessels. The vessels are intended to be both decorative and functional as sand filters. Pipes under the vessels will carry the water through another secondary filter(s) and into the cistern. There are a few items yet to consider including venting, biological water treatment, overflow and water level monitoring.

At the time the top of the cistern was poured, the remaining sub slabs of the house and remaining CMU block walls were also being poured. The finish slabs which contain the plastic PEX tubing and form the finished floor will be poured much later in the project after the heavy construction traffic subsides.

Justin vibrating CMU wall



 

  

living room slab pour / entry coat closet opening





Around the slabs the plans called for 2in of rigid foam insulation. The contractor and city inspector were uncomfortable with the slab not tying into the foundation perimeter. A compromise was agreed to such that the insulation began directly above the connections of the steel anchors from the slab to the perimeter concrete. The first 7 or so inches of slab is insulated which should substantially reduce the thermal path from the topping slab (hydronic heat) to the perimeter foundation to earth.

Faswall was qualified as an alternate source for the exterior block walls (www.faswall.com) and was delivered this week. More on the Durisol to Faswall substitution in the next update….