As an index for the assessment of liquefaction potential, the liquefaction index (PL value) is adopted in earthquake damage assessment of many local governments in Japan. A liquefaction index is calculated from the safety rate to liquefaction (FL value) for every depth derived from drilling data, geology sections and conditions of geomorphological unit. The possibility of liquefaction and the safety rate to liquefaction (FL value) or a liquefaction index (PL value) are generally connected as follows.

FL > 1.0 -- There is little possibility of liquefaction in the depth.

FL <= 1.0 -- There is the possibility of liquefaction in the depth.

PL= 0 -- Liquefaction potential is quite low.

0<PL<= 5 -- Liquefaction potential is low.

5<PL<=15 -- Liquefaction potential is high.

15<PL -- Liquefaction potential is very high.

There are total upper load pressure and effective upper load pressure as an input parameter used by many of FL methods. They are calculated using the values of soil properties measured by drilling etc. Although detailed explanation of total upper load pressure and effective upper load pressure is omitted in each outline of method, it is generally expressed as follows.

Total upper pressure is expressed with the sum of effective upper total pressure and pore water pressure, and is given by the following formula.

Effective upper load pressure means the vertical pressure to the stratum of a certain depth, and is given by the following formula.

2.1.FL Method
2.1.1.Architectural Institute of Japan (1988)
(1)Input/Output
1. Pre-condition
1. Preparation of drilling data (depth distribution of soil, groundwater level, N-value, granule part content, clay part content, plastic index, the unit weight of a stratum, and the unit weight of groundwater)
2. Input Data
1. JMA Magnitude of the earthquake
2. Peak ground acceleration
3. Depth from the ground
4. N-value
5. Granule part content
6. (Clay part content, Plastic index)
7. Groundwater level
8. Total upper load pressure (calculated from unit weight of stratum)
9. Effective upper load pressure (calculated from the unit weight of a stratum, the unit weight of groundwater, and groundwater level)
3. Output Data
1. Rate of safety to liquefaction (FL value)
(2)Outline of the Method

With the " Recommendations for design of building foundations" by Architectural Institute of Japan (1988), liquefaction potential is judged by the following methods.

The stratum made into the object of a liquefaction potential assessment is shown below.

1. Saturated soil shallower than 20m
2. The granule part content Fc is 35% or less of stratum.
3. Even if Fc is 35% or more, the clay part content P is 10% or less or the plastic index Ip is a 15% or less of silt layer with low plasticity.
4. The stratum in which clay part content exceeds 20% can be excepted from the object for an assessment.
First, the ratio of equivalent cyclic shear stress generated for every depth in the ground of an examination point is calculated.

　





The amended N-value () in a certain depth is calculated.



Using the prepared 5% shear strain amplitude curve, the liquefaction resistance ratio  of saturated soil corresponding to amended N-value is calculated.

Where τ₁ is liquefaction resistance in the horizontal cross section diagram.

At the last, the rate FL of safety to liquefaction generating in every depth is calculated as follows.



(3)Reference

Architectural Institute of Japan (1988) Recommendations for design of building foundations (in Japanese).

　

　

2.1.2.Japan Road Association (1996)

(1)Input/Output
1. Pre-condition
1. Preparation of drilling data (depth distribution of soil, groundwater level, N-value, average particle diameter, 10% particle diameter, granule part content, a plastic index, the unit weight of a stratum, and the unit weight of groundwater)
2. Setting the earthquake type (inter-plate earthquake / inland earthquake)
　
2. Input Data
1. Peak ground acceleration
2. Depth from the ground
3. N-value
4. Mean grain diameter
5. Granule part content
6. Groundwater level
7. (Plastic index)
8. Total upper load pressure (calculated from unit weight of stratum)
9. Effective upper load pressure (calculated from the unit weight of a stratum, the unit weight of groundwater, and groundwater level)
　
3. Output Data
1. Safety rate to liquefaction (FL value)
(2)Outline of the Method

Japan Road Association (1996) revised “Specifications for Highway Bridges” issued in 1990 based on the generating situation of liquefaction by the 1995 Southern Hyogo Prefecture Earthquake as follows.

1. The range for object soil of liquefaction assessment was expanded from only alluvial sandy soil to diluvial or gravelly soil.
2. The underestimate of the intensity in a portion with high N-value was resolved.


Liquefaction potential is assessed for following strata.

3. Alluvial sandy soil (other layers are included if required) in principle
4. Saturated soil shallower than 20m, with groundwater level shallower than 10m
5. Soil with Fc<=35%
6. Soil with Fc>35% and Ip<=15, here, Fc: granule part content [%], Ip: plastic index
7. Soil with mean grain diameter 10mm or less and 10% grain diameter 1mm or less
　The rate of safety to liquefaction (FL) in every depth of the ground is defined with the dynamic shear strength ratio  of the stratum and the seismic shear stress ratio  acting on the stratum as follows.

　



The seismic shear stress ratio  is derived from following formula with peak ground acceleration.



　The dynamic shear strength ratio  of the stratum is calculated with cyclic triaxial strength ratio  from following correction formula.



The coefficient  is defined according to the property of seismic motion as follows.

a) Type 1: Seismic motion by great inter-plate earthquake with low occurrence frequency

Large amplitude acts for a long time repeatedly.



b) Type 2: Seismic motion by large inland earthquake with very low occurrence frequency



Cyclic triaxial strength ratio  is defined experientially from following formula



where  is amended N-value in consideration of the influence of a grain size. In the case of the reclaimed soil, 0.05 is subtracted from this formula.

Amended N-value of sandy soil is,



and that of gravelly soil is as follows.



In addition, normalized N-value () for effective upper load pressure of 1kgf/cm² is given below.



(3)Reference

Japan Road Association (1996) Specifications for highway bridges: Part V seismic design.

　

　

　

2.2.PL Method

2.2.1.Iwasaki et al. (1980)
(1)Input/Output
1. Input Data
1. The distribution of FL value to a depth of 20m
2. Output Data
1. Liquefaction index (PL value)
(2)Outline of the Method

The liquefaction potential of the ground is not assessed with FL method although it assesses the generating possibility of the liquefaction in a certain depth. Iwasaki et al. (1980) defined the value (a liquefaction index, PL value) acquired from the weighted integration of FL value for depth, and made it the index about liquefaction potential of the ground.



where  is a weight function to the depth, and has given bigger weight to the shallow portion.



Fig.2.2.1 Example of FL value and weight function 

(3)Note

The result depends on the method that derives FL value.

(4)Reference

Iwasaki, T., F. Tatsuoka, K. Tokida, and S. Yasuda (1980) Estimation of degree of soil liquefaction during earthquakes, Soil Mechanics and Foundation Engineering, 28, 23-29 (in Japanese).

　

　

　

2.3.Other Methods

2.3.1.Tokyo Metropolitan Disaster Conference (1991)
(1)Input/Output
1. Pre-condition
1. Preparation of drilling data (depth distribution of soil and N-value) with a depth of 6m or more
2. Input Data
1. Soil Type
2. N-value of sand bed
3. Thickness of surface and sand bed
3. Output Data
1. Liquefaction potential (high / low / no possibility)
(2)Outline of the Method

Liquefaction potential is assessed according to the flow shown in Fig.2.3.1.

In the “Assessment by soil type”, the soil is assessed as liquefiable when the sandy soil shown in Table 2.3.1 is included in the soil bed in the drilling data. In the “Assessment based on N-value”, liquefaction potential is assessed as shown in Fig.2.3.1. The sand bed with bigger N-value than the critical N-value shown in the figure does not liquefy. The sand bed with smaller N-value than the critical N-value has liquefaction potential. In the “Assessment by soil structure”, the thickness of sand bed with smaller N-value than critical N-value (H2) and that of surface course (H1) are compared, and liquefaction potential is assessed according to the criteria shown in Fig.2.3.3.

　

Fig.2.3.1 Liquefaction potential assessment method
(Tokyo Metropolitan Disaster Conference,1991)

　

Table 2.3.1 The method of Liquefaction Potential Assessment
(Tokyo Metropolitan Disaster Conference,1991)

Liquefaction Potential	Soil Type
High	Sand, fine sand, medium sand, silty sand, sand with clay, sand with shell, sand with humus, sand with gravel
Low	Embankment*, landfill*, silt, loam, humus, sandy gravel, others

*In the case that the soil is clearly loose sand bed and under the groundwater level, it was regarded as being easy to liquefy.


 

Fig.2.3.2 Criteria of Liquefaction Potential Assessment (Tokyo Metropolitan Disaster Conference, 1985)	Fig.2.3.3 The relationship between the thickness of surface course  and that of sand bed (Tokyo Metropolitan Disaster Conference, 1985)

(3)Note

The target earthquake motion of this method is that of JMA (Japan Meteorological Agency) seismic intensity 6 and 250 gal of peak acceleration. Therefore, the case where the earthquake motion except JMA seismic intensity 6 is not taken into consideration.

For this reason, it should be noted that the liquefaction potential of every place point is assessed for the earthquake motion not of arbitrary earthquakes, but of JMA seismic intensity 6.

(4)Reference

Tokyo Metropolitan Disaster Conference (1985) Tama chiiki ni okeru zisin higai no soutei ni kansuru houkokusho (In Japanese. Webmaster translates the title “Report of earthquake damage assessment in Tama region”).

Tokyo Metropolitan Disaster Conference (1991) Tokyo ni okeru chokka zisin no higai soutei ni kansuru chousa houkokusho (In Japanese. Webmaster translates the title “Study on assessment of earthquake damage in Tokyo”).

Tokyo Metropolitan Disaster Conference (1991) Tokyo ni okeru chokka zisin no higai soutei ni kansuru chousa houkokusho, higai soutei shuhou hen (In Japanese. Webmaster translates the title “Study on assessment of earthquake damage in Tokyo, Volume for method description and countermeasure suggestion”).

　

2.3.2.Matsuoka et al. (1993)

(1)Input/Output
1. Pre-condition
1. Preparation of a geomorphological classification map
2. Input Data
1. Peak ground velocity
2. Geomorphological classification
3. Output Data
1. Liquefaction potential (very high / high / relatively low)
(2)Outline of the Method

In the past earthquake case analysis, Matsuoka et al. (1993) found out that the liquefaction potential differed by each geomorphological unit. Furthermore, they searched for the standard peak ground velocity with which each geomorphological unit liquefies (Fig.2.3.4). The liquefaction potential is assessed easily from Fig.2.3.4 and Table 2.3.2 considering both of geomorphological classification map and the distribution of peak ground velocity of arbitrary earthquakes.

This method was verified according to the liquefaction in case of the 1987 Chiba-ken toho-oki earthquake.



　

Fig.2.3.4 Geomorphological Unit of the Digital National Land Information of Japan and Geomorphological Unit for Liquefaction Assessment (Matsuoka et al., 1993)

　

Table 2.3.2 Liquefaction Potential and Peak Ground Velocity (Matsuoka et al., 1993)

Liquefaction Potential	Ratio of Peak Ground Velocity to Critical Peak Ground Velocity
Very High	1.25 -
High	1.00 - 1.25
Relatively Low	0.75 - 1.00

(3)Note

The relation between the liquefaction potential acquired by this method and PL value etc. is not clear.

The Digital National Information of Japan is the dataset including various digitized data about national land of Japan, e.g. land use, geomorphology, etc.

(4)Reference

Matsuoka, M., S. Midorikawa, and K. Wakamatsu (1993) Liquefaction potential mapping for large area using the digital national land information, Journal of Struct. Constr. Engng. AIJ., 452, 39-45 (in Japanese with English abstract).