# Assessment of Simplified Rating Factor Methodology for Shallow Cover Metal Culverts
## 2 Definition and Origin of Minimum Cover Requirements  
The word \"cover\" is widely and broadly used in section 12 of AASHTO LRFD design specifications, yet is not explicitly defined in \"12.2--DEFINITIONS\". Section C12.6.6.3 (commentary for \"12.6.6.3--Minimum Cover\") states that, \"Depending on the pipe material and the pavement type above it, the minimum cover may include the pavement thickness and base course, along with the sub-base\". Figure 1 from section C12.6.6.3 indicates that flexible pavement thickness is not included in the minimum cover definition for flexible pavement over flexible pipes but is included for rigid pavement over flexible pipes. The opposite definition apparently applies to buried rigid pipes. Furthermore, observe the note at the bottom of Figure 1: \"The minimum cover dimension is not to be confused with the fill height used for calculation purposes, which shall be from the top of the pipe to the top of the surface, regardless of the pipe type or pavement type\". Therefore it is important to recognize that there is no single meaning of \"minimum cover\", but rather its definition depends on how the dimension is applied to the engineering or construction use. \(H_{\text{min}}=\text{minimum allowable cover dimension}\). Note: The minimum cover dimension is not to be confused with the fill height used for calculation purposes, which shall be from the top of the pipe to the top of the surface, regardless of the pipe type or pavement type. Figure 1: “Figure 12.6.6.3–1—minimum cover orientation” of AASHTO LRFD section C12.6.6.3 (adapted from [14]). The minimum cover requirements for the design of culverts under normal traffic load are defined in Section 12.6.6.3, \"Minimum Cover\" of AASHTO LRFD and Section 12 of AASHTO Standard Specifications. Table 1 summarizes these requirements as Table 12.6.6.3-1 of the AASHTO _LRFD Bridge Design Specifications_[14]. The requirements for minimum cover established in Table 1 aim to avoid damage to the pipe during construction before the asphalt layer is placed [15] or to prevent \"thermal expansion in thermoplastic pipe\" that \"can affect pavement performance under shallow fills\" (Section C12.6.6.3 in AASHTO LRFD). However, the temporary cover construction scenario or thermal expansion is not pertinent to rating factor analyses since ratings are done after, and often long after, the structure has been in service. Hence, meeting the minimum cover requirement in Table 1 is not equivalent to safety in load rating analyses. According to _The Corrugated Metal Conduit as a Compression Ring_[16], once a pipe has been installed in compacted backfill with sufficient fill height, its strength can be evaluated as a thin ring in compression. Under such conditions, the flexural resistance of the pipe wall is not considered to support a significant portion of the vertical loads; the loads are resisted through axial compression. To accommodate this resistance mechanism in practical applications, [PERSON] [16] stipulated that the minimum cover depth should be at least \(1/8\) the diameter of the culvert, which was based upon field test experience and has been presented in minimum heights-of-cover tables associated with culvert construction. In research by [PERSON] [17], experimental evidence and the results of finite element analyses indicated that metal culvert structures are effectively restrained from excessive deformation if they are surrounded by good quality backfill and under a depth of cover greater than one-fourth of the span, which is defined as the maximum inside width of a pipe or the distance between footings of an arch culvert. The stresses induced by vehicle loads diminish in magnitude and become more uniform with increasing depth. Under these circumstances, even without consideration of the flexural stiffness or moment capacity, the culverts are able to resist all imposed loads entirely through ring compression action.