Other Underground Movements

There are other things moving about underground at the Visitor Center. Within the assembly of the building, there are several other systems moving about under the floor slabs.

• Storm Drain Lines
• Sanitary Sewer Lines
• Grey Water Lines
• Conduits for Power
• Conduits for Communications/Data

Throughout design and the development of construction drawings there is a lot of planning that goes into systems integration and coordination and how all these building systems are woven together in a tidy manner. Within the construction sequence, the building must be layered from the ground up and there are certain systems that must go into the ground before being covered up. This post focuses on the systems that are placed underneath the concrete slab on grade.

The following structural section is a vertical cut through the wall that is just behind the restrooms in the lower office area. It shows the contractor the various layers from the sub grade up to the lower concrete slab on grade as well has how they integrate with the footing, vertical retaining wall and the upper slab on grade. I have noted where the underground plumbing and electrical systems are typically integrated into the layers.

The plumbing drawings are pretty specific about which lines go where in plan. The storm, sewer and grey water lines are all drain lines and rely on gravity to make the fluids flow downhill with the piping.  Lines greater or equal to 4” must be sloped a minimum of 1/8” per foot. Lines equal to or less than 3” must be sloped a minimum of 1/4” per foot.

The electrical drawings are not as specific and mainly tell the subcontractor where the panels are located and where the outlets are located. The electrical sub is left to his own means as to where the best route is for conduits carrying power and data wires so long as he connects the points.

The waste water lines from the toilets are being connected directly into the sanitary sewer which leaves the building and flows into the city sewer system. The sinks and shower are being connected to the grey water system which treats the water creating non-potable water which in turn used as a water source to flush the toilets. The grey water system is part of the sustainable design to achieve LEED gold certification.

The following series of photos shows the sequential stages of construction where the underground plumbing and electrical is being installed. The first photos show the sub grade being prepared and where the lower office area will be located on the site.

As construction progresses the perimeter of the lower office area becomes more defined and begins to give the plumber and electrician a sense of the floor layout.

If you recall, following the rough excavation of the site, the construction documents call for existing grade to be cut 2 feet below the floor areas and replaced with select fill. The under floor plumbing is trenched and placed within this layer of fill. The plumbing lines are then stubbed up through the concrete slab in the proper locations prior to the concrete slab being poured. Within the lower floor slab where the lower office space is located there are some areas of concentrated plumbing. There are waste lines serving two bathrooms and a shower room. Each restroom has a toilet and a sink. The shower room has a sink and a shower. The plumbing subcontractor must accurately locate the fixtures so that the plumbing lines stub up in just the right location. This is a bit of a challenge considering there won’t be any walls in place for a long time; I’m not sure how they pinpoint things so accurately at this stage. In order to meet code, each toilet must be within inches of being in the right location in relation to the finish walls for proper ADA design.

The electrical and data conduits are placed in the 6” layer of granular fill. They typically are not concerned about slope and are just finding the most direct route to get from A to B.

Once all the plumbing and electrical systems are in place below the slab the preparation for the slab begins. The granular fill is all leveled out and this is where one of the underground systems discussed in a previous post is put into place. The bentonite waterproofing system is being laid horizontally in this application just beneath the slab with attention given to sealing around piping and conduits that will turn up through the slab. The concrete will actually be poured directly over this layer.

The final step is to set in place the reinforcing steel which is #4 rebar (that’s a ½” diameter steel bar) at 18 inches on center each way (both directions). The concrete is then put in place. In this situation, the concrete is being pumped from a pumper truck which is out of the picture. The concrete trucks back up to the pumper and empty their concrete into a hopper. The extended hose from the pumper truck allows easier movement and placement of the concrete across the slab area. The concrete slab is then troweled and finished and left to cure.