Geotechnical requirements providing guidance for new streets

Much has been written and said about Kenton County’s now two-year-old subdivision regulations and the specifications it includes for new street construction. One of the main reasons for the increased focus is the application of geotechnical engineering testing and reporting standards included in the regulations.

“Staff recommended increasing the geotechnical engineering standards in the subdivision development process from the beginning of the effort to write the new regulations,” said Scott Hiles, CPC, Director of Infrastructure Engineering. “We knew that getting the geotechnical engineer involved to evaluate the suitability of the soils that supported the pavements was an important part of providing pavements that didn’t break down prematurely.”

The results of the new standards are becoming apparent just now.

“The Kenton County Planning Commission was concerned about subdivisions that had already been approved at the time the new regulations were adopted,” said Hiles. “For those developments, changing the rules that governed them mid-stream didn’t seem fair.”

Members of the planning commission approved a grace period for most subdivisions that were approved at the time the new regulations were adopted. Those developments were allowed to continue using the old regulations under certain conditions.

“That meant those previously-approved developments didn’t have to provide the additional engineering to evaluate the suitability of the soils,” said Hiles. This grace period initially limited the number of subdivision developments that were required to have the additional geotechnical engineering.

Even with the grace period, there have been 13 subdivisions since the increased standards were adopted that were required to get a geotechnical engineer’s report that evaluated the soils.

“Staff had long suspected that a major contributing factor to premature street failure was the presence of poor soils incapable of supporting the standard pavement design,” said Hiles. “But without the geotechnical assessment and oversight there was no way to require that the poor soils supporting the new streets be improved.”

Of the 13 subdivisions required to get a geotechnical engineer’s report, eight were identified as containing problem soils that would not produce the strengths necessary to adequately support the street pavements.

“That means that without the additional geotechnical engineering required by the new regulations, some of the new streets would have been constructed on soils incapable of providing proper support. Those streets could have failed prematurely because of those poor soils.”

Hiles said that of the eight subdivisions where poor soils were identified, the recommendations to remedy the issue ranged between a combination of undercutting the poor soils and replacing them with new soil capable of producing adequate strengths, to chemical stabilization of the poor soils.