Earthquakes are an uncommon, but serious, risk for California structures and landscapes. During a site inventory, landscape architects may uncover underlying seismic risk factors on their client’s project site. After consulting geologists and registered civil engineers investigate the site and document hazardous conditions, they produce a mitigation plan for architects, engineers, and landscape architects to follow during the design, construction documentation, and implementation.
Three Ways to Mitigate Seismic Risks
Like most things in life, seismic risks can be mitigated in several ways.
Avoid the Risk
Sometimes, it’s just not worth the risk to human life and structures to build on parts of a site. When catastrophic seismic failure is likely and cannot be reasonably and safely mitigated to an acceptable level, not developing part of site is the best option. Be sure to consider off-site threats that may negatively impact a site.
Reduce the Seismic Risk to an “Acceptable” Level
Engineering methods can reduce liquefaction and slope failure risks to an acceptable level that does not likely threaten human life or structures.
Accommodate The Hazard
Engineers can design foundations and structures to withstand seismic risks to an acceptable level. Some techniques that reduce the hazard require ongoing maintenance that must be maintained by the end users of the site in order to maintain adequate safety.
Before discussing the various methods of mitigating seismic risks to buildings, sites, and human life, lets look at some basic seismic stability concepts.
“Only removal or densification of all potentially unstable soils, or permanently lowering the groundwater can fully eliminate landslide and liquefaction hazards.”
“Where the structural load is light in weight, such as in typical single-family residential houses, a post- tensioned slab foundation system may be used to accommodate differential settlements up to one inch. The reinforced slab gives the slab sufficient rigidity to span voids that may develop due to differential settlements.”
“For heavier buildings with heavier loads and a relatively uniform mass distribution, a thicker mat foundation is feasible. Buildings supported on continuous spread footings with isolated footings can be interconnected with grade beams to improve support.”
“Slope stability depends upon the slope geometry, driving forces, distribution of earth materials, ground water conditions, material densities, and material strengths.”
Many projects require several different mitigation methods to successfully reduce seismic risk from earthquake-caused landslides and liquefaction.
Mitigating Landslide Factors
Grading is a common method used to mitigate landslide risk on slopes and flat sites.
“Grading techniques are used to prepare level building lots, to provide a means of removing and replacing poor soils, to stabilize landslides, to assure proper drainage, minimize liquefaction and reduce differential settlement.”
“Rainfall-induced slope failures occur more often and are more widespread than earthquake-induced liquefaction failures.”
Four Types of Landslides
Landslides are classified into four different categories: rotational landslides, fill displacements, soil flows and rock falls.
“Rotational slides are characterized by a somewhat cohesive slide mass that rotates along a relatively deep failure plane. The rotational failure occurs at the base of the landslide along one discrete, or several relatively thin zones, of weakness. The principal failure mechanism is the loss of shear strength at depth along a rupture surface that results in sliding of the rock or soil mass within the slope.”
“If naturally occurring landslides are not sufficiently removed before engineered fill is placed, movement below a fill can be reactivated along the pre-existing failure plane and transmitted to the surface, causing settlement of building pads (Rogers, 1992). Strong earthquake shaking can cause this type of slope failure even in properly engineered fill that has been placed above a graded surface. Material strength changes, water content and settlement that occur through time in the fill can add to the failure potential.”
To appropriately mitigate rotational landslide risks, you need to identify the problem areas first. “The standard practice for ensuring stability against earthquake-induced rotational slides is to adequately explore all potential instabilities and treat them during the rough grading phase before construction begins. The overall grading goal is to reduce the driving forces in the upper parts of the slide mass and to increase the resisting forces in the toe area of the slope by providing shear keys or buttresses in the subsurface. Most deep fills need to have water diverted from the fill to enhance stability. Sub-drain galleries are used to prevent pore water pressure build-up in constructed engineered fill.”
“Grading can totally remove the landslide or flatten the surface slope angle by ‘laying back’ the slope face to a stable angle. Grading is also used to reduce the weight of the slide mass and direct water away from the surface to prevent infiltration. In some cases, lightweight fill materials will be used to lighten the weight at the head of the slide and layered geofabrics used during recompaction will be used to increase shear strength in the body of the slide. A buttress fill constructed at the toe of the slide will help support the upslope portion of the mass. Buttress fills have a wide base, typically ranging from one third to almost the full height of the slope being buttressed. Fill should be compacted to a minimum of 90 percent of the maximum density….”
“Smaller scale slope failures can occur on graded benches. This type of failure is often a function of improper erosion control measures and lack of drain upkeep. The common mitigation for this type of failure is to prepare adequately compacted fill and stable cut slopes, and ensure an adequate setback.”
“The general geometry for setbacks has been a part of the Uniform Building Code for years.”
“The general heights and distances for compacted fill are provided by code and considered typical regulatory minimums for graded lots. However as development has moved into steeper slopes, the geometry is an inadequate parameter measure of slope safety. Stability on difficult lots depends upon material strength, compaction, and aspect of bedding and other discontinuities rather than geometry.”
“Fill displacement failures are displacements that commonly occur at depth beneath a deep fill or between the natural ground at the edge of a cut slope and the engineered fill of the bench or pad. This type of failure is caused by static gravity force, and results in gradual settlement over time or accelerated settlement in response to dynamic earthquake forces.”
“The most common fill displacement hazard is differential settlement, which can severely damage building foundations, roads and lifelines….This hazard impacts side hill benches that have been cut for house pads and roads built on fill. Excessive settlement and fissures can also occur in deep canyons that have been filled in with imported material.”
“The standard practice for stabilizing settlement failures at cut-fill transitions is to over-excavate during construction and grade the bedrock surface in multiple steps to provide a gradual slope transition. Fill should be compacted to a minimum of 90 percent of the maximum density as per ASTM D1557. Scarification provides a bond between the fill material and the underlying native rock. The overall grading goal is to minimize the difference in bearing capacity across the cut-fill boundary.”
The long-term stability of cut and fill slopes and deep canyon fills requires drainage and erosion control measures and ongoing maintenance.
- Establish vegetation to help reduce erosion and hold the slope.
- Maintain irrigation systems to avoid saturating slopes.
- Maintain sub-drains to reduce pore pressure at the base of the slope.
- Make sure surface drains are functioning and free of debris.
- Repair superficial erosion damage.
- Ensure building pads drain properly.
“Soil flows/slips are generic terms for shallow disrupted slides composed of loose combinations of soil, surficial deposits, rock fragments, weathered rock and vegetation. The principal failure mechanism in this type of flow is fluidization of the soil mass, caused by a reduction in shear strength due to increased pore water pressure during rain.”
“Soil flows can be subdivided by grain size into debris flows where the material is coarse-grained and earth flows where fine-grained. The geomorphic character, speed and travel distance of a soil flow is dependent on the particle size, slope and water content within the slide mass. Debris flows form steep, unvegetated scars in the head region and irregular, hummocky deposits at the toe. They most commonly occur on slopes greater than 65 percent. The 1994 Northridge Earthquake in southern California triggered more than 11,000 shallow disrupted slides.”
“Earth flows are characteristically slow moving and may continue to move for a period of days to weeks after initiating. The main hazard from flows occurs where they impact projects in the downslope or runout area. They are typically triggered by periods of prolonged rainfall following a period of less intense precipitation. They can move very rapidly and travel relatively long distances, making them a significant threat to life and property. Flows can also occur on the outer slope of engineered fill faces where saturated surface soils lay above more highly compacted engineered fill.”
“The main mitigation is to either avoid the hazardous zone or deflect the flow material. The most common solution is to provide an adequate setback from the runout zone. Grading solutions include removing excess material from the upslope swales, reshape the gully profile to reduce the driving forces, lowering the slope gradient and restricting water inflow into the soil mass. Offsite flows can be mitigated using catchment basins, protective structures such as embankments, diversion or barrier walls and by requiring setback distances. The most effective measure to protect structures against earth flows and debris flows is to accurately define the potential failure area and require a setback from the runout path.”
“Rock falls and topples consist of weakly cemented, loose or intensely fractured and weathered material on slopes.”
“Natural fracturing patterns and incipient failure planes determine block sizes. The principal failure mechanism in this type of failure is loss of tensile strength on very steep slopes. This loss of tensile strength is commonly accentuated or triggered by infiltration of water, freeze-thaw cycles or strong earthquake ground shaking.”
“Rock falls commonly occur on high angle cut slopes, ledges, steep slopes, and in particular, highway and railroad cuts where slopes have been undercut either during construction or over steepened by progressive removal of small slope failures and ongoing maintenance. They pose a substantial hazard to vehicles along roadways and to structures downslope at the base of canyons.”
“The most common mitigation on steep slopes with large blocks and well-defined discontinuities is to increase the resisting force by pinning individual blocks and slide masses with rock bolts and anchors. More highly fractured rock masses can be contained by installing reinforced caissons, covering the slope with wire mesh or by scaling overhanging rock from the slope face. Another common solution is to separate the structure from the hazard with an adequate setback, build a graded berm to divert or adequately contain the material. In more homogeneous, fine-grained material, grading can be used to decrease the steepness of the slope and reduce the driving forces. These soil/weathered rock mixtures can also be pinned using soil nail or sprayed with gunite to stabilize the slope faces. In some cases, the unstable material must be removed from the slope face using mechanical means or hand labor.”
For More Information
The quoted parts of this article are from the California Geological Survey Special Publication 117A.
Other Articles About Investigating Seismic Risks
- Street Eats Car…Ground Subsidence in Action
- Little Known Reasons for Ground Subsidence in California
- Understanding Geology To The Core: Three Site Inventory Investigation Methods
- Watch How Core Drilling Works
- Warning: California Is Landslide Territory
- Watch California Landslides in Action
- How Does the Seismic Hazards Mapping Act Affect Site Development?
- Two Ways To Investigate Seismic Hazards In California
- Critical Site Investigation Items For A Geotechnical Report
- Liquefaction: When Solid Ground Turns To Mush