PPP Notes - Civil and Mechanical Utilities

SITE CIVIL/MECHANICAL UTILITIES

 (These notes are compiled from AGS)

SUBSURFACE DRAINAGE SYSTEMS

 

• Subsurface drainage systems are very different engineering designs from surface drainage systems. 
• Surface drainage systems intercept and collect stormwater runoff and convey it away from building and site with the use of large inlets and storm drains.
• Subsurface drainage systems typically are smaller in size and capacity and designed to intercept the slower underground flows of a natural groundwater table, underground stream, or infiltration of soils from surface sources. 
• Surface and subsurface systems typically require discharge either through a pumping station or by gravity drainage to an adequate outfall.

 

STORM DRAINAGE UTILITIES

 

• Storm drainage utilities are designed to collect and dispose of rainfall runoff to prevent the flow of water from damaging building structures (through foundation leakage), site structures, and the surface grade (through erosion).
• The two basic types of surface drainage are the open system and the closed system.
• The OPEN SYSTEM, which utilizes a ditch/swale and culvert, is used in less densely populated, more open areas where the flow of water above grade can be accommodated fairly easily.
• The CLOSED SYSTEM, which utilizes pipes, an inlet/catch basin, and manholes, is used in more urban, populated areas, where land must be used efficiently and water brought below the surface quickly to avoid interference with human activity.
• The two systems are commonly combined where terrain, human density and land uses dictate.
• A pervious or porous paving is often used for parking and other hard site surfaces.
• This drainage system allows water to percolate through the paved surface into the soil; similar to the way the land would naturally absorb water.

 

DESIGN CONSIDERATIONS FOR SURFACE DRAINAGE SYSTEMS

 

• When designing surface drainage systems, follow these guidelines:
• Lay out all slopes, grates, swales, and other drainage features according to the ADA, without restricting accessible routes for persons with disabilities.  Refer to applicable codes, standards, and regulations for accessibility requirements.
• Lay out grades so runoff can safely flow away from buildings.  If drains become blocked, do not allow backed-up water to accumulate around the foundation. 
• Keep in mind that an open system, or one in which water is kept on top of the surface as long as possible, is generally more economical than a closed system.
• Keep in mind that an open system, or one in which water is kept on top of the surface as long as possible, is generally more economical than a closed system.
• Consider the effect of ice forming on the surface when determining slopes for vehicles and pedestrians.
• Consult local codes on such criteria as intensity and duration of rainstorms and allowable runoff for the locality.
• Note that formulas given in this discussion are meant for approximation only.  Consult a qualified engineer or landscape architect to design a site-specific system.

 

POROUS PAVING SYSTEMS

 

• Porous paving materials, methods for sizing channels, and design considerations for porous paving systems are all vital for quality landscape design.

 

POROUS PAVING MATERIALS

 

• The two principal types of porous paving are monolithic surfacing material and unit pavers.
• MONOLITHIC POROUS PAVING is stone aggregate bound with asphalt or Portland cement.  The aggregate must be sorted to exclude the “fines” or sand-sized particles that normally fill the voids between larger pieces.  Without the fines, water is able to run through the paving material.  Generally, porous asphalt and concrete are both strong enough for parking and roadway surfaces and pedestrian uses.
• PRECAST CONCRETE UNIT PAVERS, with shapes that allow water to flow through them, can also give surface stability for parking or driveways.  Paver types are available for exposed placement, or for burial just below the surface.  In the latter case, the soil-pea gravel or vegetation in the pavers is exposed and can help percolate precipitation into the ground.
• To reduce runoff and increase water absorption, porous paving must be underlaid with a bed of unbound aggregate. 
• The unbound aggregate acts as a structural support and forms a reservoir to hold precipitation until it can percolate into the soil. 
• Use of porous paving may permit use of a significantly smaller and simpler storm drainage system.

 

METHOD FOR SIZING CHANNELS

 

• Channels and pipes for handling water runoff may be sized by determining the flow of water (Q) with the formula Q = Va. 
• “V” is the velocity of the runoff water in ft./sec. as determined by the Manning formula, and “a” is the cross-sectional area of water given in square feet. 
• For a given “Q,” adjust the channel or pipe shape, size, and/or slope to obtain the desired velocity (one that will not erode earth, grass ditches, or other features).
• The Manning formula is V = (1.486/n) x (r x 0.67) x (S x 0.5), in which:
• n = values relating to surface characteristics of channels
• r = hydraulic radius
• S = slope drop in ft./length
• Use the formula for calculating runoff (Q = C x I x A) to determine the flow required for a site; compare it to the capacity of a channel sized according to the Manning formula to determine whether the channel design is satisfactory.
• Q = ratio of runoff in cubic feet per second
• C = ratio of runoff to rainfall
• I = rainfall intensity in inches per hour
• A = area of the watershed in acres

 

DESIGN CONSIDERATIONS FOR POROUS PAVING SYSTEMS

 

• Design considerations for working with porous paving systems include the following:
• Soils around porous paving installations must have a minimum percolation rate of about ½ in / hr., and should not be more than about 30% clay.  On sites where the slope is greater than 3%, terracing the paved areas allows the bottom of each reservoir to remain level.
• Proper specification and supervision are important in the installation of porous paving materials.  Soil under the reservoir must not be unduly compacted during construction.
• Porous concrete can withstand heavier loads than porous asphalt.  Because it does not soften in hot weather and may be more susceptible to freeze/thaw damage, it is better suited to warmer climates.  Additives may be introduced to improve cold climate performance.
• Porous asphalt has good freeze/thaw resistance, but is best suited for areas in which traffic is limited, such as employee parking.
• While clogging of monolithic porous paving is generally not a problem, recommended maintenance may include use of a hydrovac once or twice a year, as well as the prompt removal of leaves and windblown sand.
• The reservoir below porous paving has no fixed depth but is designed according to the slope of the site, the soil percolation rate, and the size of the design storm.  Consult a civil engineer or landscape architect.