Liquefaction and the Risk to Washington State’s Transportation Bridges
Washington State Department of Transportation
Liquefaction and the Risk to Washington State’s Bridges MATS Lab January 2008
Minimizing or Preventing Liquefaction
Costs to Address Liquefaction
With each new earthquake, engineering science has an opportunity to apply its expanding knowledge of liquefaction and use it to refine liquefaction design methods. In light of this, we now know that liquefaction can occur over a wider range of soils and conditions. Hence some existing structures, and even some previously assessed structures proposed for future construction, that were thought to be in soils that do not liquefy are now recognized as being founded in soils that do liquefy.
New developments in liquefaction science may cause many existing bridges and structures to not meet current design standards for liquefaction. Current Nickel and TPA project estimates do not adequately account for what we now know about liquefaction. Methods to mitigate (prevent or minimize) liquefaction typically increase the cost of a new or replacement bridge by 20 to 40 percent, and bridge widening by 50 to 75 percent. These liquefaction cost impacts will affect budget needs for some currently funded Nickel and TPA projects. It will also affect our long-term funding strategy to preserve public safety by preventing liquefaction damage to highway structures located on key transportation corridors.
We’ve learned that liquefaction can be minimized or prevented by improving the ground. The primary ground improvements to reduce liquefaction fall into three general categories: • making the soil more dense • changing the soil composition • reinforcing the soil to strengthen it WSDOT selects liquefaction mitigation techniques after considering the site conditions, environmental constraints, and cost effectiveness.
Contact: Tony Allen, WSDOT Geotechnical Division State Materials Laboratory 360-709-5450
The Problem
What is Liquefaction?
The Washington State Department of Transportation (WSDOT) manages more than 7,000 miles of highway that cross widely different soil types. These roadways include approximately 3,000 bridges. The majority of these bridges are located in areas known to have earthquakes. Liquefaction occurs in some soils during earthquakes and can significantly damage or cause collapse of bridges and walls located in these soils. Liquefaction is a concern for WSDOT due to the risk to public safety resulting from potential bridge or retaining wall collapse, and limitations it could place on the region’s mobility. It can have a negative affect our state’s regional commerce and quality of life, especially if our bridges are damaged or are out of service for long periods of time.
Liquefaction is a physical process that reduces the strength of a soil due to ground shaking during earthquakes. As a consequence of liquefaction, watersaturated soils behave as thick fluids rather than solids, similar to quicksand.
Public Disclosure Request 10-0292 for Elizabeth Campbell
Liquefaction takes place when seismic waves pass through a saturated soil layer. In saturated soils, the space between individual soil grains is completely filled with water. Water pushes on the soil grains, and that influences how tightly the soil grains are pressed together. Before an earthquake, the soil grains are touching each other. As the earthquake shakes the ground, the water pushes harder on the soil grains and separates them. Because the grains are no longer touching, there is
Left: Showa River Brideg- Niigata, Japan, 1964 Right: Nishinomiya Bridge- Kobe, Japan, 1995
little friction between the grains and the soil moves like a thick fluid. Figure 1 illustrates how soil grains separate and lose strength during liquefaction. If pore space water pressure increases to the point where the soil’s shear strength can no longer support the weight of the overlying soil, buildings, roads, houses, etc., then the soil will flow like a liquid and cause extensive surface damage.
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Liquefaction and the Risk to Washington State’s Transportation Bridges
What Triggers Liquefaction?
A number of factors must be present for liquefaction to take place. Liquefaction happens most easily in loose sandy soil, though some gravels and silts may also liquefy. The soil must be below the groundwater table so that the soil is saturated. In addition, the length of time that the ground shakes during an earthquake, and the intensity of the shaking, must be great enough to cause these loose soils to liquefy. The earthquakes experienced in recent history (Nisqually, 2001) were about a third of the size of what could occur. Figure 2 shows the relative scale of the recent earthquakes to the earthquakes that we need to consider for design of transportation structures such as bridges.
Left: Struve Slough- Watsonville, CA. Loma Prieta Quake, 1989 Right: Mingju Bridge, Taiwan Quake, 1999
Energy Released from Typical Earthquakes 160 140 Subduction zone earthquake (e.g., 1964 Alaska, 1960 Chile)
120 100 9 8 6 4 2
1,000 yr earthquake (e.g., Seattle fault) 1995 Kobe Japan earthquake 2001 Nisqually earthquake Hiroshima atomic bomb Average tornado
0 3
4
5
6
7
8
9
10
Earthquake Magnitude
Fig. 3
70 60
Typically, liquefaction of these susceptible soils does not become widespread until the peak bedrock acceleration is approximately 0.15 or more. The greater the intensity and duration of shaking, the more widespread and severe the liquefaction will be. The Nisqually earthquake in 2001 was large enough to cause limited
liquefaction to occur. The design level 1,000 year event could cause much more widespread liquefaction. Figure 3 illustrates the concept that as earthquake size and shaking intensity increase, the number of occurrences within a given time period decrease.
Fig. 2
50 40 30 20
Increasing Earthquake size and duration
The locations of WSDOT owned bridges relative to contours of potential seismic acceleration (i.e., shaking intensity) for a 1,000 year event and potential liquefiable soils across the state are shown on Figure 4. Table 1 summarizes the specific number of bridges that may be affected by liquefaction. Figure 4 shows that most of the liquefaction problems are located in the western half of our state.
1,000 yrs Nisqually Earthquake 500 yrs 2001 500 yr Design Earthquake 1,000 yr Design Earthquake
Washington State Department of Transportation
How Can Liquefaction Affect WSDOT Bridges?
How is WSDOT Addressing Liquefaction?
World wide, liquefaction has been one of the most significant causes of damage to bridges during past earthquakes. Liquefaction can damage bridges in many ways including: • Causing tipping of foundations in and above liquefied soil • Causing ground and foundation settlement • Causing soil to flow downhill, moving bridge foundations laterally
Bridges constructed prior to 1985 were not designed to resist liquefaction. After 1985, we started to design bridges to prevent collapse during liquefaction.
Fig. 4
0
Liquefaction Water fills in the pore space between grains. Friction between grains holds sediment together.
to be widened rather than replaced as corridor improvements are made and additional highway lanes are needed. The design for widening these bridges must include resistance to liquefaction in order to meet current design standards and to insure the public’s safety. Since the existing bridges were not designed for liquefaction, they also may need to be stabilized to avoid collapse during an earthquake. If stabilizing the existing bridge is too costly or technically not feasible, complete replacement of the existing bridge with a new wider structure may be necessary.
Table 1
19% (572) of 1940-1985 era bridges are located in areas where the combination of soil type and seismic hazard level necessary to cause liquefaction are present.
Number of earthquakes versus shaking intensity for given time periods in Seattle
Water saturated soil
Right: Tumwater, WA- Nisqually Quake, 2001
28% (827) of all bridges are located in areas where the combination of soil type and seismic hazard level necessary to cause liquefaction are present.
10 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Earthquake Peak Ground Acceleration
Fig. 1
Bridges constructed before 1940 are generally too old to be considered for widening to meet capacity needs, and are instead likely candidates for replacement. These bridge replacements are being built according to current design standards. However, bridges built between 1940 and about 1985 were not designed for liquefaction, but have significant life remaining. These bridges are more likely
Left: US101, Olympia, WA- Nisqually Quake, 2001
Water completely surrounds all grains and elminates all grain to grain contact. Soil flows like fluid. Public Disclosure Request 10-0292 for Elizabeth Campbell
23% (674) bridges constructed earlier than 1985 are located in areas where the combination of soil type and seismic hazard level necessary to cause liquefaction are present.
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