By Bob and George Koerner
To construct and examine a scaled-down version of a final design to substantiate its appropriateness is often a worthwhile pursuit.
One of the earliest such efforts with geosynthetics was done by Karl Terzaghi in 1958 at the Mission Dam in British Columbia, Canada. The task was to hydraulically connect an earth dam’s steel sheet pile cutoff wall with a multicurved concreted abutment. To do so, a prototype design using a tapered woven nylon bag of 6.1m (20ft) in length was wrapped around a grout pipe. When placed between the two surfaces to be sealed, the bag was filled with a cement-bentonite grout and when hardened was found to perform as anticipated. The full scale 19.8m- (65ft-) high cutoff was constructed accordingly and with confidence that the scheme would work properly.*
*Terzaghi, K. and Lacroux, Y., “Mission Dam: An Earth and Rockfill Dam on a Highly Compressible Foundation,” Géotechnique, March 1964 pp. 13-50.
Such prototype efforts might be considered passé, but perhaps in certain areas of geosynthetics they still should be considered,
especially for test pad investigations. Two applications come to mind: (i) strength reduction damage assessment of geotextiles and geogrids, and (ii) puncture damage to geomembranes.
Regarding strength reduction assessment of geotextiles and geogrids to the subgrade beneath and/or the soil placed above, the appropriate procedure is given in ASTM D5818. Its primary utilization is to compare tensile strength before and after the test pad simulation takes place. For example, the ratio of wide-width tensile strengths before and after load placement leads directly to a numeric reduction factor for installation damage (RFID). This is then used to obtain or substantiate the products’ allowable strength for design purposes.
Regarding puncture damage to geomembranes, a new guide (GRI-GS11) is now available in which the test pad configuration is an important initial consideration. Table 1 presents several of the various options.
The four test pad configurations shown in the table are described as follows (see Figure 1):
- Above ground with uniform stone thickness. This cross section is meant to confirm a given design including the intended geomembrane, geotextile, stone type, and thickness. Lateral containment of the stone may be problematic for thicknesses greater than 300mm (12in.).
- Above ground with variable stone thickness. This investigative type cross section is meant to discover what minimum stone thickness is necessary to avoid puncture to the underlying geomembrane. The geomembrane and geotextile are held constant along with the stone type, but not its thickness. Again, stone confinement for thickness greater than 300mm (12in.) will be difficult.
- Below ground with uniform stone thickness. Unlike the above ground configuration, the given design stone thickness cross section will be contained from lateral movement by the surrounding soil. It is meant to confirm the adequacy of a given set of materials (i.e., geomembrane, geotextile, stone type, and thickness). It can be used for stone thicknesses much greater than 300mm (12in.).
- Below ground with variable stone thickness. This investigative type cross section is meant to discover what minimum stone thickness is necessary to avoid puncture to the underlying geomembrane. Unlike the above ground configuration, however, the stone will be laterally contained which is an advantage for thick layers of stone. It can be used for stone thicknesses much greater than 300mm (12in.).
Other significant aspects of the guide include:
1. The type of soil and/or rock of the foundation material on which the geomembrane is placed is a critical item. If stones (or rock) are present at the proposed site, the geomembrane will possibly be punctured from below. It might be necessary to place an additional protection geotextile beneath the geomembrane. If this is not the intent, a proper foundation material must be agreed upon by the parties involved.
2. The type(s) of traffic loading/repetitions is another critical item and must be agreed upon by the various parties involved.
3. The width of the test pad should be at least 100% greater than the agreed-upon placement and compaction equipment for the two above ground options, and 50% greater for the two below ground options. In this latter regard, the confinement by natural soil is an advantage. Other advantages of the below ground options are that equipment will not have to ramp-up or ramp-down from an elevated test pad placed on the original ground surface as well as the associated safety considerations.
4. The length of the test pad should be three times greater than the agreed-upon placement and compaction equipment for the constant thickness option and from five to ten times greater for the variable thickness options.
5. Placement and compaction equipment must be decided upon by the parties involved. Dump trucks will generally deliver the stone and a bulldozer will generally spread it. Compaction will be achieved by both tire-pressure from trucks and caterpillar pressure from the bulldozer. A separate roller may also be involved for additional compaction. Some important considerations that must be agreed upon are as follows:
- weight and tire pressure of trucks.
- weight and caterpillar size of bulldozer.
- method of dumping stone on the protection geotextile.
- number of truck and/or bulldozer passes along the length of the test pad.
- urning of the bulldozer on the test pad—e.g., where and how often.
- dwell-time of trucks and/or bulldozer staying in a fixed position.
- details of an additional compactor if used.
6. Exhuming of the geomembrane will generally begin as soon as the test pad trafficking concludes. Most of the stone can be removed using a backhoe but its bucket should be fitted with rubber teeth. Within about 100mm (4in.) of the protection geotextile, however, stone removal should be by hand to avoid unintentionally damaging the geomembrane. Note that this entire process is a short-term installation damage assessment. If long-term creep puncture is a concern, the test pad experiment can be continued as long as necessary.
7. Once the geomembrane is removed, examination for holes should proceed. If more than a few holes are present a hole density assessment should be performed. Not only the number of holes, but also their size, are important. This is to be done visually with ample photographs and/or video taken. If indentations, and not holes per se, are present, wide-width or axi-symmetric tension tests (compared to the as-received geomembrane) may be performed for a more complete assessment.
8. Site reclamation should be conducted in a proper manner, which depends on the future use of the site, its location, and other local issues. Admittedly, test pads are quasi-research tasks that cost time and money. If, however, the design situation pushes the limits of typical applications, and the results are acceptable, the confidence gained in the short term is assured. The long-term behavior is, of course, a related but different issue that is often assessed on the basis of laboratory testing and/or case histories that are regularly reported in the literature.