How to avoid potentially costly pitfalls when considering the use of plastic sub-base replacement tanks?
TRL have recently published a report, commissioned by the Highways Agency, on a pilot scale trial of reservoir pavements (Chaddock and Nunn, 2010). One part of the trial included the use of Permavoid sub-base replacement attenuation tanks within a trial section of road pavement. EPG have helped to design the trial system and interpret the results to verify the design procedures that are used for sub-base replacement.
TRL trial with HGV traffic (photograph courtesy of TRL/Aggregate Industries)
Sub-base replacement for sustainable drainage systems is a method of constructing a macro pervious pavement. Water storage is provided in geocellular plastic tanks within the road pavement construction. The Permavoid system has a proven track record for use in sub-base replacement, which is underpinned by extensive laboratory testing, independent full scale trials and rigorous analysis.
EPG have been employed as a consultant by Permavoid to provide independent design advice for the system; having extensive experience of this and numerous other types of geocellular tank. Steve Wilson, Technical Director of EPG, having investigated the failure of geocellular tanks for other companies understands the critical issues that should be considered by those contemplating the use of plastic tanks below areas such as car parks or more heavily loaded areas
Plastic tanks used as sub-base are subject to very high loads (more so than tanks installed below the pavement layers) and, as such, specific properties are necessary for them to behave as part of the pavement.
‘Standard’ plastic tank modules that do not have the specific properties for use within the pavement zone are prone to collapse, which may occur in the long or short term; especially during pavement construction when tanks are subject to the highest loads and the cover layer is at its minimum e.g. during placement of asphaltic layers. The TRL trial has shown that Permavoid is capable of supporting heavy traffic loads over a sustained period of time.
As an aid to assessing the suitability of a geocellular tank for use as sub-base replacement, EPG has produced the eight step check below to ensure that systems have been tested and designed in accordance with CIRIA Report C680 and other best practice guidance.
8 steps to check compliance with latest guidance and standards for geocellular sub-base.
| 1. Is the design checked and approved by a Chartered Engineer with extensive geotechnical experience (a Geotechnical Advisor as defined by the AGS)? |
| 2. Does the laboratory testing follow the loading rates recommended in CIRIA C680 |
| 3. Is the design load taken at the yield point and NOT the point of maximum load from the test results (as defined in CIRIA C680) – see below |
| 4. Does the design consider deflection of the tank and the effect of this on the overlying pavement materials – see below |
| 5. Does the system have a proven track record in sub-base replacement applications including independent monitored full scale trials, cyclic load tests, interlock testing etc. |
| 6. Do the individual modular units tie together or interlock using robust load bearing connections that prevent movement between individual units and resist bending forces thus providing a raft structure rather than individual plates. |
| 7. Does the system incorporate a validated structural anti-shear component between multiple layers of tank? |
| 8. Does the geotextile or membrane encapsulation have sufficient strength and durability to permanently span over gaps in the plastic structure and has it been tested for this application? |
Yield point
All current and developing UK and international guidance on the design of geocellular tank systems requires the quoted strength of the units to be taken at the yield point of the load – deflection curve. The quoted design strength should NOT be the point of maximum load on the curve.
Deflections
The deflection of the tank units is a vital consideration in design. Design calculations should provide an estimate of the likely deflection of the units and compared to the serviceability deflection limits of the proposed pavement surfacing materials. The calculations should incorporate a partial factor of safety on material properties of 1.5 for the deflection analysis. Creep under semi-permanent loading from vehicle traffic should also be considered (e.g. in supermarket car parks where bays can be occupied for a significant time).
Ties
In sub-base replacement applications the interlocking mechanism between structural units is extremely important because the units are subject to shear and rotational forces; therefore, ties between units should not simply be for locating purposes during installation, but rather a structural component with sufficient validated bending and lateral strength to resist the forces imparted in sub-base replacement applications.