By Expert No. 42777, R.G., C.E.G.
Although this winter has been relatively mild, last year’s devastating landslides such as Bluebird Canyon and the La Conchita resulted in millions of dollars of property damage and several deaths. These failures, and thousands of other smaller failures, have resulted in insurance claims and legal actions that will be supported by attorneys and experts for years to come.
This article will serve as a primer on the general types and causes of slope failures and the role that geologist’s provide as investigative consultants.
Types Of Slopes
Slopes vary in shape (mainly steepness), origin of the underlying soil (fill, cut, natural), and type of geology or soil (clay, sand, bedrock). As a general statement, the steeper and higher the slope and the weaker the soil or bedrock, the more prone the slope is to failure.
Slopes vary in their steepness. Most current fill and cut slopes (manufactured slopes) are graded to have a 2:1 (horizontal to vertical) gradient; that means for every one foot up, the slope extends 2 feet out. Steeper slopes were either constructed before about 1985 or are natural slopes that have not been graded. A natural slope can be near vertical. Many older graded slopes were built to a gradient of 1.5:1.
Soil and Geology
The material that underlies the slope varies from soft, man-made fill to hard natural bedrock and the type of material governs the type of failure that will occur. Man made fill slopes, created during development of an area, tend to have smaller failures than slopes that are underlain by bedrock. Bedrock slopes tend to be more resistant to failure but can have preferential planes of weakness or “daylighted” bedding that can promote failure in certain conditions and these failures tend to be very large.
Types Of Failures
Although slope failures vary drastically in size and depth, water, either flowing over the surface or seeping deep into the ground, acts as the triggering culprit. Water acts to lubricate slide planes, soften clay and silt material by reducing cohesion of particles, and physically washes away sandier soil.
Shallow or surficial failures are the most common type and are commonly called mudflow or erosion failures.
Shallow failures occur when the upper few feet of fill or “colluvial” soil on a slope becomes saturated; that is, the internal pore spaces within the soil are filled with water. This weakens the internal strength of the soil and may cause the soil to behave more like a liquid than a solid causing the soil to “flow”. Most commonly, these types of failures occur on steep slopes, slopes that have sparse vegetation, or in areas that have a concentrated flow of water.
Downhill homeowners tend to bear the brunt of these failures when the soil and debris flow onto their properties and often into their homes resulting in substantial property loss and potential injury. These failures can be caused by triggering events such as broken pipes, poor drainage from upper properties, and from construction defects such as slopes that were built too steeply or inoperative or poorly maintained drain lines.
Large-scale landslides like the Crown Cove/Niguel Summit and Vista Royale Landslides in 1998 and the recent Blue Bird Canyon Landslide this past year are less common but tend to be more devastating than surficial failures.
These failures occur when the “driving” forces overcome the “resisting” forces of the slope. The balance between the two forces is very much like a teeter-totter. During development if fill is placed at the top of a slope its weight can “drive” the slope down. Conversely, if during grading, soil is removed from the base of a slope, the disappearance of the weight of the soil reduces the resisting forces and allows failure to happen. A combination of this grading caused the Crown Cove and Vista Royale Landslides.
Deep failures tend to be caused by “weak” geology. That is, the bedrock is oriented out of slope, has numerous clay layers, or is fractured from faulting and past landsliding. In the case of the Crown Cove and Vista Royale Landslides, the pre-development investigation failed to properly disclose the presence of the weak rock and development placed extra dirt at the top of the slope to create lots and cut into the base of the slope to make room for other lots. In the case of Blue Bird Canyon, the area has long been known to be unstable and will continue to move unless a large-scale repair costing tens of millions of dollars is performed.
The geologist’s role in investigating slope movement generally relates to assessing the scope and size of the failure, analyzing the causes or contributing factors that led to failure, and determining the type of repair necessary to rehabilitate the slope and any damaged structures.
Documents related to the slope development such as soil and geology development reports and grading plans are reviewed to determine the pre-development condition of the area and what changes were done to the ground surface. Aerial photographs and geologic and topographic maps of the area often supplement the document review.
As discussed above, the type of material that underlies the slope is very important. Therefore the geologist will perform a subsurface investigation consisting of excavating exploratory test pits and/or drilling borings to view the condition of the underlying material and take samples of the soil and bedrock materials. Often, a two-foot diameter boring will be drilled and the geologist will be lowered into the hole to physically look, feel, and touch, the soil and bedrock in order to actually observe failure planes or geologic weakness; this information becomes invaluable when presented during legal proceedings. Instrumentation to monitor the amount and depth of ground movement may also be employed to further analyze the slope failure.
The samples obtained during the investigation are then taken to a laboratory so that tests can be performed to determine various soil parameters such as strength or grain size.
Cross-sections, drawings that show what the subsurface looks like, and maps are prepared by the geologist to further the understanding of the extent of the failure and what factors may have contributed to its movement. Information such as what the pre-failure and pre-development geometry of the ground was, and current and historical groundwater levels are added to the cross-sections.
Once all of this information has been gathered the geologist will work in concert with a soil engineer to perform slope stability analysis in order to help determine the cause(s) of the failure, the liability of relevant parties, and scope and magnitude of repair.
Slope failures, both big and small, can be devastating events that cause extensive property damage and loss of life. While many of these failures occur in natural slopes during times of heavy rainfall and could be attributed to “acts of god”, many failures are the consequence of contribution from man.
It is important for a geologist to properly investigate the landslide in order to develop relevant information that can be useful to all affected parties. A geologist who can easily and simply explain complex scientific information is invaluable in support of litigation.