Our Roads and Hardpan
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Hi All, Napoleon called mud the fifth element, after water, air, earth and fire, because his army, like many others before and since, frequently got bogged down in it. In most parts of the country unpaved roads commonly turn into knee-deep mud during periods of heavy rain, and in these areas, developers must pave roads in new housing developments. Most areas of southeastern Arizona don¡¯t have deep mud even after monsoon downpours, and developers can get away with leaving roads unpaved as happened here in Fairfield Estates.? Why is this? It¡¯s because in the desert southwest there is a layer of cemented soil just below the surface called caliche hardpan. As long as the hardpan is intact, it is almost like pavement. However, if it¡¯s breached, then the soil beneath the hardpan is susceptible to relatively rapid erosion. The attached Figure 1 shows an excellent example of breached hardpan that can be seen in the cow pasture at the east end of Paseo Venado. The breached track can be followed about 800 feet north from the gate along the fence line. The photo shows that once the hardpan is breached, the underlying soil can be gullied. I don¡¯t know how thick the original hardpan was in the cow pasture, but Figure 2 shows that the remaining hardpan is less than 5 inches thick. Figure 3 was taken on Sexton Pl near the entrance to our driveway. Last time the road was graded, it was ¡°crowned¡± with the edges cut several inches lower than the center. The material shown in the photo between the road surface and the top of the hardpan is gravel and dirt that was pushed there in the grading. At least 10 inches of hardpan has been removed between the top of the hardpan and the present road surface on Sexton Pl near our home. I don¡¯t know how much hardpan remains below the present road surface at the edges, but my guess is not much.? I don¡¯t know how common the problem is of ¡°thin
remaining hardpan¡±
on other unpaved roads in Fairfield Estates, but it should be
considered in any
decision regarding future grading. We don¡¯t want our roads gullied
like the
breached track in the cow pasture. Ken
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