TN 2.01 is a technical note providing information about the minimum and maximum cover depths using corrugated HDPE pipe for traffic applications.

Minimum cover in traffic applications for pipe diameters from 4”- 48” (100-1200 mm) installed in traffic areas (AASHTO H-20, H-25, or HL-93 loads) must have at least 12" (300 mm) of cover over the pipe crown, while 54”- and 60” (1350 and 1500 mm) pipes must have at least 24” (600 mm) of cover. The backfill envelope must be constructed in accordance with the Installation section (Section 5) of the Drainage Handbook and the requirements of ASTM D2321. The backfill envelope must be of the type and compaction listed in Appendix A-5, Table A-5-2 of the Drainage Handbook.

Table 1 covers minimum cover for ADS N-12, N-12 ST, and N-12 WT (per AASHTO) with AASHTO H-20, H-25, or HL-93 Load. This condition is represented by a Class III material compacted to 95% standard Proctor density or a Class II material compacted to 90% standard proctor density, although other materials can provide similar strength at slightly lower levels of compaction. Structural backfill material should extend to the crown of the pipe; the remaining cover should be appropriate for the installation and as specified by the design engineer. If settlement or rutting is a concern, it may be appropriate to extend the structural backfill to grade. Where pavement is involved, sub-base material can be considered in the minimum burial depth. While rigid pavements can be included in the minimum cover, the thickness of flexible pavements should not be included in the minimum cover.

The maximum burial depth is highly influenced by the type of backfill and level of compaction around the pipe. General maximum cover limits for ADS N-12, N-12 ST, N-12 WT pipe, (ASTM F2306 and AASHTO M252/M294 Type S pipes) are shown in Table 3 for a variety of backfill conditions. Table 3 was developed assuming the pipe is installed in accordance with ASTM D2321 and the Installation section (Section 5) of the Drainage Handbook.

TN 2.01 provides additional information including tables, diagrams and notes for reference.