by Lizzie Wratten
Take a quick stroll around any Pacific Northwest neighborhood and see if you can locate the petty annoyances left behind by some contractor who didn’t fully compact the soils. In order to understand the consequences of these improperly compacted soils, we first need to understand what is going on underground. Underneath our feet, there are various utilities and services that our society depends upon. (See previous blog post, Know What’s Below for more information on that subject).
Once a trench has been dug and the utility or underground service has been placed or repaired, the soil that was removed needs to be put back correctly; or, a structurally competent soil needs to take its place if the native soils are unusable due to silt/clay content or weather. These replacement soils are compacted in lifts, meaning one layer at a time, with either a jumping jack, plate compactor, vibratory roller, or hoe-pac. Our job as geologists/engineers working on projects where underlying structures soil needs to be replaced, including roadway surfaces and sidewalks, is to test these soils as they go in and make sure they are suitable for use as backfill and adequately compacted (generally to 95% of the maximum dry density, a number attained through laboratory testing). If the soils are not adequately compacted at the time they are placed, they will slowly settle over time due to water, gravity and overriding forces. This means anything placed over the soils will also settle, and not necessarily in a regular or projected manner.
The irregular settling of unsuitable, uncompacted or inadequate soil leads to the frustrations we often see on our roads and sidewalks. One of the more popular locations left softer than recommended is the area around manholes and storm drains within the roadways. These concrete structures can be fragile and expensive, making operators nervous about running compacting machinery nearby. When the road surface is finally placed over the top, the soils underneath slowly compact leading to subsidence and potholes. The manhole itself won’t move because it is on an undisturbed base but the soils around it will subside, leading to unnecessary bumps and jarring while driving your vehicle. Other aggravations occur when trenches across roadways are not properly compacted, which leads to the whole stretch slowly settling and creating the perfect place for water to pond.
Improperly compacted surfaces lead to concrete cracking, fractured pavements, and as previously mentioned potholes. While these may be petty annoyances at first, they can and will lead to severe damage over time not only to the roadway surfaces but to your vehicle’s wheels and tires, not to mention your spine and disposition.
However, with a little bit of care and attention, these compacted problems can be completely avoided all together. The Riley Group, Inc. (RGI) is here to provide that attention by observing the methods of backfilling and compaction to ensure that proper standards are maintained, then testing the soils as they are compacted to warrant adequate compaction is being achieved and prove that the soils were properly replaced. Using nuclear gauges, T-probes, and proof rolls to identify loose soils before they are covered in concrete and asphalt, we at RGI can have that loose material removed and use suitable soils to replace it. Prevention is possible and RGI is available to answer any questions and/or provide a consultation on any PNW project.
