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Geomembrane and nonwoven textile help build eco-friendly Monterey House

February 1st, 2009 / By: / Environmental, Feature, Geomembranes

Introduction

The Monterey Bay Idea House (Monterey House), the latest in the line of select West Coast homes built by Sunset magazine to showcase new and innovative products, presents a multitude of eco-friendly features in response to the need for energy efficiency, function, and sustainability.

Located in Monterra, an exclusive residential community in the mountains overlooking the Pacific Ocean in west-central California, the house was designed working closely with Monterey County officials to illustrate how “green building” can be done. “The county wants to encourage going green by expediting permits and offering other incentives,” explains Tom Messenger, who served as project manager. “The Monterey Bay House needed to be at least 70% green through environmentally friendly features such as the use of sustainable products and reclaiming water.”

California, and especially the Monterey Peninsula, has always had issues with a lack of water, and there have been recent concerns that volunteer water rationing might become mandatory. In addition, California regulation states that the footprint of a building dictates the number of bathrooms allowed in a house based on water availability. Harvesting rainwater offers homeowners the ability to have an unlimited number of bathrooms.

Comprised of 3 connected “farm buildings” enclosed with native stone walls surrounding a traffic-rated courtyard (“motor court”), the home presented tight site access and made it difficult to store water in a limited footprint. The need for more water on less space was the driving issue when selecting a stormwater solution.

The allotted space for water collection also needed to be put under the motor court. “Whenever you have a structural component with a traffic area over it, the product needs to be rated very high,” explains Messenger. “In California, a residential area has to be rated for a fire truck so the design has to be very structurally strong.”

Water detention and management

A water detention system was installed that allows for the harvesting of rain and stormwater to be re-used as a “gray water” source for toilet flushing, car washing, and irrigation. The 49-ft-long x 34-ft-wide excavation contains 65 chambers that hold 20,000 gallons — enough to water the entire half-acre landscape area for one year.

First, the hole was dug and then a base layer of rock was installed and compacted according to the soil engineer’s requirements. Next, the bottom and walls of the trench were lined with a class 2 nonwoven geofabric. Then a custom-made geomembrane liner was installed followed by another layer of class 2 nonwoven geofabric to help provide protection from rocks puncturing the liner during the backfill process.

The chambers, manifold, and standpipe connection were then installed and the backfill process of the embedment stone began. Once this backfill process achieved a consistent cover of 6in. of stone above the chamber crowns, then the geofabric was folded over the embedment stone and the finish-grade fill was installed. Two dump-truck loads of fill-stone were dumped onto the chamber bed and leveled with a backhoe maintaining 8in. of cover between the wheels and the top of the embedment stone. The finished stone was then compacted per the specification and was ready to accept up to HS-30 (30 tons) of traffic loading.

Geosynthetics details

The Monterey area is one of the most water-stressed regions in California. Because this installation was a cistern-and-store rainwater for reuse in landscape irrigation, a geosynthetic liner was used to provide a watertight envelope.

Approximately 2,400ft2 of reinforced geomembrane liner material was formed into a rectilinear box, 34ft wide x 44ft long x 5ft high, and with a 4in. pipe-penetration boot fitting welded into the bottom of the impervious liner at the end corner to accommodate the equalization inlet/outlet from the adjacent pump basin.

Once the base was in place and tested to 95% compaction, and the geotextile layer placed over the base and positioned up the sides of the excavation, the liner bundle was positioned in the excavation and unfolded, creating a seamless watertight liner around the sides and bottom of the entire cistern containment volume.

The fabrication produced a reasonably stiff and easy to deploy impervious liner that made for quick installation. After preparation and compaction of a base layer, a layer of 4oz. nonwoven geotextile was put down to provide an additional buffer for the liner, followed by the placement of the prefabricated liner. After the liner was in place, an additional buffer layer of nonwoven geotextile was placed over the liner to act as a screen for the clean drain rock that would be placed around the stormwater chambers and used to fill out the liner envelope and increase the storage capacity.

After the chambers were in place and the storage envelope was backfilled with clean drain rock with a void space of 40%, the overlapping edges were folded over the compacted drain rock, and a final top cover of nonwoven geotextile was placed over the entire profile to seal it off from infiltrating fines but still allow for seepage into the storage volume by direct rainfall from the motor court. The final top layer of road base and a finished coating of decomposed granite completed the construction of the dual-purpose motor court.

To fill the resulting cistern, all runoff from the roofs and surrounding site is collected in a network of laterals. Runoff from the upper portion of the site flows directly through a filter into the pump basin adjacent to the buried cistern, and runoff from the lower portion of the site flows into a filtered pump basin below the buildings and is pumped back up to the cistern pump basin. From the pump basin, water equalizes into the cistern until it is full, and then overflows to a regional runoff dissipation system by gravity.

The filters for the stormwater pump basins are also distinct, constructed from prefabricated bags. Originally intended to be filled with gravel and placed around drop inlets to prevent sediment and debris from entering a municipal stormwater collection system, the zippered bags are constructed from 2 layers of green woven geotextile with an inner layer of sorbent material.

Ron Bygness is the editor of Geosynthetics magazine; rwbygness@ifai.com. Bill Wilson is an associate with Carlile Macy Engineering; www.carlilemacy.com.

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