Submitted by Geoffrey Brooks, Somersett Homeowner
Double Rockery Wall Failure at Trail Ridge Ct.
The SOA had a report prepared by CME (Construction Materials Engineers, Inc) of Reno to look into the reasons behind the failure of the rockery walls. This report may be accessed via the following link:
Trail Ridge Rockery Wall Failures Report
The report is thorough, makes interesting reading, has plenty of documentation concerning the building of the walls and whether they were built to normal standards.
CME dealt with how the wall was engineered – pretty standard. They also talk about the location of the walls, built in a part of the Peavine foothills that were terraformed to allow for the building of housing and Golf Course fairways and greens. These rockery walls were placed in areas where fill had been placed to enable pads and houses to be built. Analysis of the soils showed that a mixture of many different soil types, including clay soils, were used to backstop the walls.
Nothing unusual here, other than if one was to buy a building pad in Somersett – soil analysis would be required, and if clays were found (apparently quite prevalent in the foothills), they would have to be removed. Clay does not make a suitable foundation material in a building pad on which a house is to be built!
CME’s report indicates that we had unusually heavy rainfall a year ago, causing the soil behind and under the wall to become super-saturated. The lower wall (on the SGCC), was found to be built to 14’ high, outside the normal 10’ height. Those boulders are mighty heavy, and extra 30% in height adds immensely to the load on the wall.
So, the weight of the rocks caused the soil under the wall to move, and the wall came tumbling down. The slumping soil de-stabilized the upper wall which secured the building pads for the houses on the edge of the golf course – and that came down as well.
Why do soils flow when saturated? In the Santa Barbara hillsides, the sandy soils, with houses, gardens, rockery walls have been washed away by these “mud flows”. Once mud gets going it is difficult to stop! Clays form thixotropic systems – very thick when static – but under sheer they will flow very nicely.
Digressing slightly, In cosmetics they aid the even spreading of foundations on the skin… their ability to flow when rubbed in is responsible for the great soft feel of the product.
Trail Ridge (in Mountain Crest) has suffered from failing pads (prior to the 2017 rains) causing Toll Brothers to spend upwards of $500K on fixing up 5 houses, securing them on the pad. This was blamed on improper pad construction. Terra forming gone awry? I suspect that Toll Brothers in Village 6 – the layout so dramatically changed, requiring walls to be removed and new ones built – have made sure that all the 160 + pads are a great place for a house to be built.
Based on what has happened, it seems that the Trail Ridge area in Somersett is especially vulnerable. Other areas in Somersett where there are extensive rockery walls separating folks from golf, include Laurel Ridge… these walls did not fail. We have 71,000’ of rockery walls only 760’ have failed. We have an understanding of what went wrong. As sad and expensive as this event was, it can be fixed. Walls over 10’ high, built on, and backstopped by fill can be inspected… the chances of further failures are slim
Somersett is well, life goes on. By understanding the failures, we can manage the risk! Now, we have nothing to worry about.