2.4 Reviews of previous researchers on liquefaction susceptibility
2.4.1 Evaluation and propitiation of the Results of Previous Researches15
Through an experimental study, Seed et al. (1966) showed that application of uniform cyclic shear stresses on undrained medium to dense sand specimens generated high excess pore water pressures that led to liquefaction. They reported deformation in terms of shear strains in the range of 6% to 35% for specimens with relative densities between 50% and 90%. Seed (1979) called this type of liquefaction (i.e. development of excess pore pressures due to the applied cyclic shear
1976).
Kramer (1996) described liquefaction under two main categories: flow liquefaction and cyclic
when the shear stress required for static equilibrium of a soil mass (static shear stress) is greater
cases in which the static shear stress of the soil is less than the shear strength of the soil in liquefied state.
Lee and Albaisa (1974) also investigated the influence of confining pressure and grain size on the volumetric strain. The influence of confining pressure was found to be significant only for developed excess pore pressure ratios greater than ru=0.6. In general, larger volumetric strains were observed with increasing confining pressures ( =0.8% and 1.0% under 206kPa and 413kPa confinement, respectively, at ru=0.9). Soil type and size of the grains were reported to have relatively significant importance on the amount of volumetric strains. Coarse grained soils led to larger volumetric strains compared to the fine grained sands at all excess pore pressures. Lee and Albaisa pointed out that grain shape may be a more fundamental characteristic than the grain size.
Tatsuoka et al. (1984) studied the influence of various parameters on volumetric strains after initial liquefaction (ru=1) through cyclic stress-controlled, undrained simple shear tests. They found that the amount of settlement significantly depends on the induced maximum shear strain and density of soil. The settlement was found to be relatively insensitive to the overburden pressure. Tokimatsu et al. (1987) compiled previous data (Tatsuoka et al., 1984; Lee and Albaisa, 1974; Yoshimi and Hiroshi, 1975) and reported that the maximum shear strain is an important factor influencing the settlement after liquefaction. Tokimatsu et al. (1987) presented correlations between relative density, maximum shear strain and the volumetric strain, as shown in Fig. 2-5.
The data show consistent trends despite the fact that different sands were used in each of the investigations. The volumetric strain decreases significantly with increasing relative density. It is also evident that larger induced shear strains result in larger volumetric strains at a constant relative density.
Figure 2-5Relationship between volumetric strain, induced strain and relative density for sands (after Tokimatsu et al. 1987)
Ueng et al. (2009) conducted a study on settlement of saturated clean sand using a large biaxial laminar shear box. Various one and multi-directional sinusoidal input motions were imposed by a shaking table at different frequencies and accelerations. Loading accelerations, varying from 0.03g to 0.15g, and durations, from 5s to 30s, caused both liquefied and non-liquefied results. It was found that the settlement of a sand deposit without liquefaction during shaking was generally very small Significant volume changes up to 8% occurred only when there was liquefaction of sand. They reported that post-liquefaction volumetric strain of the sand decreased with increasing relative density regardless of shaking amplitude, frequency and direction (1-D or 2-D shaking), but increased with shaking duration (i.e. number of loading cycles).
There are many liquefaction mitigation techniques used by engineers in practice to decrease the risk of liquefaction and consequent hazards. Most common methods to improve the engineering properties of the soils can be classified as densification, reinforcement, grouting/mixing and gravel drainage (Mitchell et al., 1995; Adalier and Elgamal, 2004; Yegian et al., 2007). Mixing geofibers in liquefiable materials may decrease the potential for liquefaction by increasing the dynamic shear modulus of the soil deposit and reducing the development of excess pore pressure (Maher and Woods, 1990; Noorany and Uzdavines, 1989). Use of wooden piles may be one of the most practical reinforcement methods due to technical feasibility and reducing green house gases
(Numata et al., 2009; Yoshida et al., 2012). Increased dynamic shear modulus can potentially limit the excess pore pressure generation. Main advantages of maintaining low excess pore pressure (ru<0.5) are: (1) Major soil strength and stiffness is preserved, enabling the stratum to continue providing the necessary vertical and lateral support to the overlying structure (Adalier and Elgamal, 2004) (2) Large soil settlements are prevented since dissipation of higher excess pore pressures (ru>0.6) was shown to be the main cause for the large post loading settlements (Lee and Albaisa, 1974; Tokimatsu et al., 1987).
Although liquefaction resistance can be determined from laboratory tests, it has been shown that several parameters can significantly affect the results of the reconstituted soil specimens. Early studies by (Mulilis et al., 1975; Park and Silver, 1975)showed that differences in the structure of the soil produced by different soil preparation methods could significantly influence the liquefaction resistance.
In Japan, some studies on the countermeasure against liquefaction-induced flow have already started few years ago before Kobe earthquake as mentioned above. However, many studies on the countermeasure were initiated after the Kobe earthquake by model tests and analyses. Shaking table research has provided valuable insight with respect to liquefaction, post-earthquake settlement, foundation response and lateral earth pressure problems. Extensive research works have been carried out in order to study and understand the failure mechanisms and the behavior of earth structure under seismic excitations using shaking table tests (Koga et al., 1990; Orense et al., 2012).
The recent advances in experimental procedures in the field of earthquake engineering research are presented. The extensive damages caused by the earthquakes during the last decade led to a gradual change in the general public awareness of this type of natural disaster. The scientific community intensified its efforts to better understand the way different types of structures behave under seismic excitations in order to increase their safety (Toma and Atanasiu, 2010).
Developments in earthquake geotechnical engineering, including understanding the ground behavior during seismic shaking, effects of the earthquake on the geotechnical facilities, thorough studies on the site amplification, have also shown tremendous progress. Shaking table tests have the advantage of well-controlled large amplitude, multi-axis input motions and easier experimental measurements. Their use is justified if the purpose of the test was to validate the numerical model or to understand the basic failure mechanisms of either a structure or a structural element (Prasad et al., 2004).
Field studies by Tokimatsu and Yoshimi (1983) demonstrates that sands with more than 20 percent clay will not liquefy. Seed et al.(1983) arrived at the same cessation. Ishihara (1993) delineates two Japanese studies that speculate that soils with greater than 10 percent clay are
non-liquefiable. Moreover, laboratory studies by Ishihara and Koseki (1989) and Yasuda et al. (1994) found a strong correlation between increased plasticity of the fines and increased cyclic resistance.
Lee and Fitton (1968) deduced that the inclusion of clayey fines might improve the cyclicstrength of a soil significantly.
Table 2-1Summary of literature review on the liquefaction resistance of sandy soils and different countermeasures against liquefaction
YEAR INVESTIGATOR FINDINGS
1968 Lee.K.L. and Fitton.J.A. Lee and Fitton concluded that:
1) Fine silty sands have the lowest cyclic strength at constant confining stress and relative density
2) With increasing grain size, peak pore pressure response decreases
3) Grain size has more effect on cyclic strength than grain shape or grain size distribution
4) Cyclic strength decreases with decreasing grain size for granular soils
5) Cyclic strength increases with decreasing grain size in silt and clay
6) Clayey fines may increase cyclic strength appreciably, while silty fines may result in decreasing the cyclic strength
1970 Ohashi.Y. Ohashi deduced that based upon the data obtained from the 1964 Niigata Earthquake, soil is likely to liquefy if:
1) Uniformity coefficient Cu < 5 2) Fines fraction< 10 %
1971 Seed.H.B. and Idriss.I.M.
Seed and Idriss inferred that for very fine sands, with D50
approximately equal to0.08 mm, are most susceptible to liquefaction
1971 Silver.M.L. and Seed.H.B.
They studied the behavior of dry uniform silica sand under seismic loading conditions and found that the shear
strain, rather than shearing stress, controls the rearrangement of soil particles and consequently the settlement of sand deposits. They found that the important parameters influencing the settlement were: (1) relative density, (2) magnitude of the cyclic shear strain, and (3)number of strain cycles
1974 Lee.K.L. and Albaisa.A. Lee and Albaisa summarized that the relationship between the ratio of the cycle to the number of cycles required for 100 percent pore pressure ratio and pore pressure ratio form small band for a given sand over a large range of densities and confining stresses
1977 Ishihara.K., Sodekawa.M.and Tanaka.Y.
They concluded that for soil with 0 to 100 %t fines, fines with PI of 20
1) Maximum pore pressure ratio increases with increasing D50
2) Cyclic strength increases as percentage of fines increases
3) For a given fines content, cyclic strength enhances as OCR increases. Difference become greater with increasing fines content
4) Cyclic strength enhances as OCR increases. Difference gets larger with decreasing D50. for a given D50
1979 Wang.W. Wang drawn the following conclusions:
Soil is susceptible to remarkably strength loss or liquefy if:
1) Percentage of particles smaller than0.005 mm is less than 15 percent
2) Liquid limitless than or equal to35 percent
3) Natural water content is greater than or equal to0.9LL
4) Liquidity Index is greater than or equal to 0.75
1980 Jennings.P.C. Jennings summarized the criteria for liquefiable soils in Chinese building codes, soils meeting certain criteria are considered non-liquefiable. These include soils with:
1) PI 10
2) Clay contents 10%
3) Relative densities 75%
4) void ratios 0.80.
5) Other parameters are intensity, epicentral distance, grain size and gradation, the depth of the sand layer, and the depth of the water table
1982 Dobry, Ladd, Yokel, Chung, Powell
1) They concluded that for a given number of cycles, the relationship between shear strain and pore pressure ratio is identical over a wide range of densities
2) Pore pressures do not begin to establish until the threshold strain is reached
1982 Chang.N.Y., Yeh.S.T.and Kaufman.L.P.
1) They concluded that the case studies reveal that most liquefaction events have occurred in silty sands and sandy silts
2)The effect of mean grain size is more than the effect of gradation
3) Cyclic strength of a silty sand decreases from 0 to 10 percent silt content then increases to a silt content of 60 percent where it levels off
4) Sand grain to grain contact still prevails at 10 percent silt content
5) Sand grains are merely floating in the silt matrix above 60 percent silt content
6) The effects of D50 and Cu become less important to
cyclic resistance as the number of cycles to failure increases
7) Cyclic strength increases with increasing D50for clean sands
8) Differences in permeability due to differences in silt content lead to differences in pore pressure development 9) Fine sands more susceptible to liquefaction than coarse sands
1982 Finn.W.L. Finn inferred the Chinese criteria for liquefaction, soil is liquefiable if the
1) PI and the clay content are less than 10 percent
A new seismic code proposed for soils withD50>0.05 mm and granular soil > 40 %
1983 Tokimatsu.K. and Yoshimi.M.
1) They revealed that in 1923 Kanto Earthquake, the Old Arakama bridge foundation settled more on clean sands than on silty sands despite clean sands having higher N values and in the same earthquake, sands with less than 8 percent fines settled more than sands with greater than 20% fines
2) Following 17 world-wide earthquakes and based upon field investigation:
A) 50% of liquefied soil had less than 5% fines B) No liquefied soil had more than 20% clay
4) Sands with fines content more than 10% have greater liquefaction resistance at same SPT blow count
1985 Robertson.P.K. and Campanella.R.G.
1) Robertson and Campanella revealed that cyclic stress ratio to generate liquefaction versus cone tip penetration is a function of grain size. They draw separate curves for D50< 0.15 mm and D50> 0.25 mm.
2) Liquefaction resistance enhanced by reducing D50
below a D50less than approximately 0.25 mm
1986 Hamada et al. 1) Liquefaction of saturated sands during earthquakes 2) Tilting and sinking of the buildings founded on saturated sandy soils with significant soil liquefaction potential
1994 Yasuda.S.,
Wakamatsu.K.and Nagase.H.
1) They concluded that cyclic strength increases slightly as the fines percentage increases
2) Cyclic strength increases as PI increases
3) Undisturbed samples of silty sand are stronger than remolded samples
4) If soil contains more than 40 percent fines, then increase in SPT blow counts from 1 year and 50 years after fill placement is most significant
5) Strength gain with time is more rapid for silty sands than for clean sands
1994 Singh.S. 1) sands with 10, 20 or 30 percent silt with 50 percent relative density have slightly lessliquefaction resistances than clean sand at the same relative density.
2) Cyclic strength increases with increasing silt content at constant void ratio. This may be due to increasing relative density as more silt is added at a constant void ratio.
1994 Chang.N.Y., Yeh.S.T.and Kaufman.L.P.
1) Sand exhibits increasing dilatancy with increasing silt content in compression
2) Sand exhibits only slightly increasing contractiveness with increasing silt content in extension
3) Resistance increases as void ratio decreases at constant silt content
4) Resistance increases as silt content decreases at
constant void ratio
5) Resistance increases slightly as silt content increases at constant sand skeleton void ratio
6) Silty sands are liquefiable only in extension not in compression
1996 Hamada et al. 1) Liquefaction is the most important geotechnical factor which has caused damages to buildings and coastal structures during earthquakes
2) Tilting and sinking of the buildings founded on saturated sandy soils with significant soil liquefaction potential
2004 Adalier and Elgamal 1) Explore methods to improve the engineering properties of the soils
2) Discussed densification, reinforcement, grouting/mixing and gravel drainage to improve liquefaction resistance
2007 Yegian et al. densification, reinforcement, grouting/mixing and gravel drainage methods to improve soil strength and decrease liquefaction potential
2009 Ueng et al. 1) It was found that the settlement of a sand deposit without liquefaction during shaking was generally very small (
2) Significant volume changes ( up to 8%) occurred only when there was liquefaction of sand.
It was reported that post-liquefaction volumetric strain of the sand decreased with increasing relative density regardless of shaking amplitude, frequency and direction (1-D or 2-D shaking), but increased with shaking duration (i.e. number of loading cycles).
2009 Numata et al. Use of wooden piles is one of the most practical
reinforcement methods due to technical feasibility and reducing green house gases
Practical example about wooden piles used as countermeasure against liquefaction (Niigata station building) was discussed and its effect on carbon stock.
2011 R.G. Cole 1) Timber piles provided a cost effective means to mitigate the effects of liquefaction and lateral spreading 2) Timber piles increase the stiffness of the soil mass reducing the shear strains during the design earthquake.
3) By embedding the timber piles into the underlying non liquefied soil layer, the timber piles provided restraint against lateral spreading.
2012 Yoshida et al. (1) The logs installed in the liquefiable soil layer could increase the resistance of ground against liquefaction.
This effect was caused by the following four effects; 1) replacing the loose sand with logs, 2) densifying the loose sand by installing the log, 3) restraining the shear deformation by fixing the top of logs into gravel layer 4) dissipating the water pressure along the periphery of logs.
(2) The bearing capacity of ground where logs were installed would be expected due to the skin friction of logs and above effects even if the duration time of shaking might increase.
(3) The log pilling around the foundation of house was effective to reduce the settlement of house. The most effective way was to install logs with the inclination because the deformed area below the house could be smaller.
2013 Yoshida et al. Field surveys were conducted for the structures which were supported by wooden piles after the 2011 Great Ease Japan earthquake and damage to structures and soundness of wooden piles was investigated.
Effectiveness of log piling into a liquefiable sandy soil against the settlement of structures was studied by shaking table tests and following conclusions were drawn:
There were no damage due to soil liquefaction such as settlement and tilt of structure supported by wooden piles in the Hebita purification plant. This is the evidence that wood pile is applicable to one of the liquefaction countermeasure technique.
The level of decay of wood from the foundation of a purification plant was extremely low, even though they were buried in the soil under the water level for 22 years.
Because the logs installed in the liquefiable sand layer could increase the resistance of ground against liquefaction, the settlement of structure which was supported by logs would be reduced during earthquake even if the duration time of shaking might increase.
Therefore, the liquefaction countermeasure technique by log piling could be applicable to the water purification plants and sewage treatment plants.
2.5 Summary
The previous results of geotechnical studies on liquefaction studies were scrutinized in order to discover the effects of different improvement methods on the liquefaction resistance and excess pore pressure generation characteristics of sandy soils.
The previous studies show that one of the useful techniques in achieving increased soil strength and confining pressure is to reduce liquefaction potential by using wooden piles. Many Laboratory and field studies have been conducted to find the effectiveness of wooden piles against liquefaction. Inclusions of logs have improved effect on dynamic properties of soils and contribute to increased dynamic shear modulus, damping and liquefaction resistance of sands.
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