Settlements hazard of soil due to liquefaction along Tabriz Metro line 2 Tabriz Metro 2 hattı boyunca sıvılaşmaya bağlı olarak meydana gelen

Öz After the occurrence of liquefaction due to earthquake, the settlement of soil layers damage to structures located on the ground or the underground. In the last two decades, different experimental methods were used to determine the rate of volumetric strain (settlement) and maximum shear strain based on field and laboratory test data. The main purpose of the present study is the evaluation of the rate of settlement after the occurrence of liquefaction in soils and study relationship between liquefaction potential index (LPI) and settlement. The results of the standard resistance penetration test along Tabriz Metro Line 2 used to estimate the liquefaction potential of soil layers in 54 boreholes. Then, the value of settlement in soil layers due to liquefaction in both dry and saturated soil layers were evaluated. In continue, LPI was calculated. The results of this study showed that the rate of settlement in saturated soil layers was remarkably higher than the layers above the underground water level and with an increase in the density of the soil layers, the rate of settlement and soil volumetric strain decreased. Also, there is a good adoption between LPI and settlement values in soil layers. Deprem nedeniyle sıvılaşma meydana geldignde, zemin katmanlarının çozulmesi içinde bulunan veya yeraltı yapılarına hasar verebilir. Son yirmi yılda, farklı deneysel yöntemler alan ve laboratuar test verilerine dayandirilarak hacimsel gerilme (zemin oturmasi) ve maksimum kayma gerginliğini belirlemek için kullanıldı. Bu çalışmanın temel amacı, zemin sıvılaşmasından sonra zemin oturma oranı oranının değerlendirilmesi ve sıvılaşma potansiyel endeksi (LPI) ile oturma arasındaki çalışma ilişkisinin değerlendirilmesidir. Tabriz Metro 2 Hattı boyunca Standard Penetrasyon testi (SPT) 54 sondaj deliğindeki sonuçlarindan zemin katmanlarının sıvılaşma potansiyelini tahmin etmek için kullanıldı. Daha sonra hem kuru hem de doymuş zemin katmanlarında sıvılaştırmaya bağlı olarak zemin katmanlarında oturma oranı hesaplanmıştır. Devamında LPI hesaplanmıştır. Çalışmadan elde edilen sonuçlar gösteriyor ki, doymuş zemin katmanlarindaki oturma oranının, yeraltı su seviyesinin üzerindeki zemin katmanlardan belirgin derecede yüksek olduğunu ve zemin katmanlrinin yoğunluğunun arttığında oturma oranının ve zemin hacimsel gerilimin azaldığını gösterdi. Ayrıca, zemin katmanlarında LPI ve zemin oturma değerleri arasında iyi bir uyum var.


Introduction
When saturated sand deposits are subjected to shaking during an earthquake, pore water pressure is known to build up leading to liquefaction or loss of strength. The pore water pressure then starts to dissipate mainly towards the ground surface, accompanied by some volume change of the sand deposits which is manifested on the ground surface as settlement (or volumetric strain). Volumetric strain after liquefaction is influenced not only by the density but more importantly by the maximum shear strain which the sand has undergone during seismic loading. The sandy soil layer is saturated and drainage is limited the condition is prepared of fixed volume situation and the major effect of the seismic shocks is generation of exceed pore water pressure. Therefore, the deposit settlement of saturated sand requires a longer time, varying from a few minutes to a few days, depending on the permeability and compressibility of the soil and the length of the drainage path [1]. The main purpose of this study is to evaluate the rate of settlement in the soil layers along Line 2 of Tabriz Metro and correlation with liquefaction potential index (LPI). In continue, in the following paragraphs discussed and described.

Liquefaction and settlement
If saturated loose sandy soil layer is subjected to seismic loading it tends to compression and volumetric reduction in the lack of drainage an increase in pore water pressure is probable. If the pore water pressure in the sand deposit increases due to a continuous seismic loading, its quantity be equal to the total stress. Based on the concept of the effective stress, it can be written: Where the effective stress is σ΄ and the total stress is σ and the pore water pressure is u, and, if σ is equal to u, σ ˊ is equal to zero. In this condition, the loose sandy layer can lost shear strength. Such a condition is called liquefaction. Liquefaction of saturated loose sand during an earthquake is a damaging factor for buildings, earth dams, retaining walls and etc. The magnitude of an earthquake and its duration, void ratio, relative density, fines content and soil types, over consolidation ratio and the range of shear stress imposed on the soil are important factors for happening of liquefaction. In recent years, various field methods have been employed to assess this phenomenon. The standard penetrations method (SPT) [2]- [5], Cone penetration method (CPT) [6] and geo-seismic tests by measuring the velocity of the shear wave can be mentioned among field methods [7]- [9]. The tendency of sand to become compressed while under earthquake vibration has been studied and analysed. The soil layer compression appears as settlement on the ground surface. The dry sand above ground water level compression occurs rapidly; typically, the settlement of a sand layer is completed after an earthquake, but the settlement of saturated sand requires a longer time. The settlement occurs when the pore pressure caused by earthquake is dissipated. The required time for settlement depends on the permeability and density of the soil and the length of the drainage course, with the time varying from a few minutes to several days. It is difficult to determine the settlement caused by an earthquake. The errors between 25% and 50% are common in the static settlement prediction and these errors increase in the case of more complicated loading of the earthquake [10]. The rate of the settlement in the sand layers, based on the field test in the two states of the dried layers [11]- [12] and the saturated layers [13]- [19] are evaluated.
In continue, general conditions and the soil layering in the study area and summary of the standard penetration method used for assessing the liquefaction potential is described. Also, the evaluation method of the probable settlement in soil layer above and under ground water level states is mentioned. Finally, the results of the study are explained.

Ground water level
In order to assess the liquefaction potential of the soil layer and the rate of settlement (volumetric strain) value, the geotechnical information of 54 boreholes along of the Tabriz Metro Line 2 collected. The Line 2 of the Tabriz city Metro, having an approximate length of 22 km, starts from the vicinity of the railway in the western part of the city and passes through Qaramalek and Qara-aqaj to the Bazar area in the central parts.
The line passes the Daneshsara square, goes under the Mehranroud River, proceeding to the Abbasi Street and Shahid Fahmide Square. It continues from the Shahaid Fahmide Square towards the Baghmishe town and by changing its path, goes to the south east and finishing finally in front of the international Exhibition in Tabriz. This route is on even ground from the beginning to Baghmishe, but encounters ups and downs as it proceeds towards a hilly topography in the east. In the eastern part, the difference between the highest and lowest points along the route is about 140 metres. The position of the route is shown in Figure 1.
The level of the ground water can be considered as one of the main factors in the assessment liquefaction potential of the soils. Along the route level of the ground water level changes, so that in one of the drilled bore holes the water in the Artesian condition had overflowed the surface of the bore hole, while in some of the bore holes waters was not found before a considerable depth. The results of the study show that ground water level changes were not much after being static and the higher level of the ground water related to the spring season.
Overall, the depth of the ground water was found to vary from 2 to 30 metres. The balance of the ground water decreased from east to west, showing that the water flow was from east to west, corresponding to the slope of the Tabriz plain. Ground water depth variations in the city of Tabriz are seen in Figure 2. Also, ground water level in the bore holes along path is proposed in Figure 3.

General geology of study area
The city of Tabriz is surrounded by the Eynali (Oon-Ebne-Ali) mountain range in the east-west, and not-so-high consolidated alluvial deposits and conglomerates in the south. The general slope of the plain is towards the west and, as a result, the direction of the general drainage of the surface and underground water is also westward. The surface of the plain is generally covered by alluvial deposits. The average height of the city of Tabriz is 1340 metres above sea level. The difference between the highest and the lowest points of Line 2 of the Metro route is 285 metres (Figure 4), [22].

Soil stratification in study area
Azerbaijan, with respect to stratigraphy, has a long period of expansion and the surroundings of the Tabriz plain also have extensive Cambrian outcrops, but the stones and the alluvium in the area of Tabriz do not date back to such a time period with their formation components being related to the Cenozoic and Quaternary periods. The Cainozoic component in the Tabriz plain started from the Miocene Age and lasted up to the Quaternary era. There is no indication of Palaeocene, Eocene and Oligocene-sediments to indicate pre-Miocene formations, proving that the area is not stratigraphic in nature.

Tabriz North Fault
The area under study is inside the city of Tabriz, in the southern part of the mountain range of Eynali that passes through the red Continental classic sediments (mid Miocene) and young alluvial sediment. The red sediments and have gypsum and salt. This formation is mostly composed of sand stone, marl, siltstone and conglomerates along with gypsum and salt. In this area of Tabriz, the continental sediments of Pons in have traces of coal. The mentioned coal is not pure and has been found to be in the form of short and inapplicable shape present in the hills of Baghmishe and Sari Dagh inside the yellow and fossiliferous marl. The marly-hilly formation of Baghmishe that contains coal in the south-west of Tabriz has an outcrop of high thickness, and fish sediment has been located on them at Sinitic, Lapilli and Diatomite. The alluvial of the fourth period including, soft to hard conglomerates, is located on this sediment. The Line 2 of the Tabriz city subway, from its starting point in the west to the Baghmishe town, is covered with alluvial sediment, which , moving west, develops layers of marl and clay stones and siltstone or outcrop comprising a thin covering near the surface of the land. Under the alucia depostes of the Abbasi street towards the east, there are marn and sandstone conglomerate layers at a dept of less than 10 metres. The geological sequances and formations of Tabriz is shown in Table 1.

Structural geology
The city of Tabriz is located in the west Alborz zone and follows the tectonic regimes ruling it. The forming of the Tabriz plain sediment in it and the formation of tectonic structures that often emerge as fractures or faults follow this system. The Tabriz plain is surrounded in the north by the mountains of Eynali and on the south by the volcanic altitudes of Sahand and its pyroclastic sediments. The reverse function of the north Tabriz Fault with the slope to the north had caused the collapse of its southern part. As a result, parallel to the northern part fractures with normal displacement, the southern plains have been created, resulting in a gardenlike collapse of the east-west continuation. The current formation on which Tabriz is located is the result of such a collapse. As a result of this collapse, the rest of the Miocene and pyroclastic sediment of the east and the south of the city are observable in lower height balances. Furthermore, the erosive function due to the entrance of the big rivers caused the deposit of alluvial material with high thickness in the plain. Regarding the headwaters of the river from the south and the east of the Tabriz plain and its elongation in an east-west state by moving towards the west, particle reduction is expected. According to the fault system activity and the occurred earthquakes in the region and observation of fractures in younger sediments, the area is tectonically active. The Alpine-Himalayan belt is one of the world's most important seismic belts, in which Iran is located. Azerbaijan is also located in this belt and had experienced destructive earthquakes in the past. There are many large and small faults in the region that may cause destructive tremors.

Assessment of liquefaction potential
An assessment of the peak ground acceleration (PGA) of study area should be performed to analyze the bore holes and identify the liquefaction potential to determine the rate of settlement in the layers of the soil. The length of Tabriz North fault from Bostan abad to Sofian cities is at least 90 km but it seems to continue towards the south-east and the north-west. Therefore, according to the Iranian Code of Practice for Seismic Resistant Design of Buildings [24] the PGA equal to 0.35g (475 years is the return period and a useful life 50 years) and Mw equal 7.5 are considered. Assessment of the liquefaction potential of the soils in the study area based on the simplified method proposed by Idriss and Bolanger [4] is carried out. In this method, first, the value of cyclic stress ratio (CSR) is estimated expressing the rate of the severity of the earquake load in a Mw=7.5. That is evaluated using the equation bellow: Where amax is the peak ground acceleration, g is acceleration of gravity, σV total stress in the depth in the question, σ΄V effective stress in the same depth, rd coefficient of shear stress reduction using the form Figure 5 is estimated and MSF (Magnitude Scale Factor) is earthquake magnitude scale factor that is calculated based on Andrus and stoke researches in 1997 using equation 3. Mw is earthquake magnitude: Figure 5: Variations of stress reduction coefficient with depth and earthquake magnitudes [23].
Second, in order to determine to cyclic resistance ratio (CRR) of the soils simplified and modified method proposed by Seed et al. [3] are used. In this step, the results obtained from the standard penetration test are modified based on the following equation proposed by Skempton [25]. Value of parameters can be observed in Table 2.
Where, NSPT, the number of standard penetration resistance test, CN coefficient of the over burden stress, CE the coefficient of the hammer energy, CS the coefficient of the sampling method, CB the coefficient of the bore hole diameter, CR the coefficient of the rod length and (N1)60 is the modified number of the standard penetration test. After that, according to the presented proposal by Idriss and Boulanger [4], the overburden tension correction factor (CN) is determined using the following equation:  ( 1 ) 60 = ( 1 ) 60 + ∆( 1 ) 60 Where, FC is equal fines content in soil layer.
In the calculation of the CRR, if the amount of effective vertical stress at the depth in question is more than 100 kPa, the CRR value is modified by using the following equation: In this equation, the CRRj is corrected cyclic resistance ratio. Furthermore, the Kσ parameter is a coefficient based on the effective vertical stress is calculated by the following [26]: Where Kσ is the overburden correction factor, σ΄V is the effective vertical stress and f is an exponent that is a function of site conditions including relative density, stress history, aging and over consolidation ratio. For the relative densities between 40% and 60%, f= 0.7-0.8 and for the relative densities between 60% and 80%, f= 0.6-0.7 (Figure 7). Safety factor (Fs) against liquefaction in soil layers is calculated using the following equation: Liquefaction occurs when the amount is Fs ≤ 1; when it is Fs >1 there is no probability of the occurrence of liquefaction.

Liquefaction potential index (LPI)
The researchers presented several methods for the assessment of the rate of liquefaction and the level of occurrence. One of the common methods is proposed by Iwasaki et al. [27], [28] presented in the following equation: Where, Z is the depth of midpoint in question layer. The Liquefaction intensity is stated between zeros and 100. The liquefaction risk can be obtained using Table 3 based on the liquefaction potential index (LPI) value.  Table 3. The limitations of the LPI and severity categories (Table 3) were discussed in detail by Sonmez [29] . To overcome these limitations, Sonmez modified F(Z) term appearing the equation of LPI by considering the threshold value of 1.2 between the non-liquefiable and marginally liquefied categories as follow:

Evaluation of settlements in soil layers due to liquefaction
In this study, calculation of the settlement value (volumetric strain) in soil layer after liquefaction have been performed in two sections. The first part is evaluation of the soil layers above the groundwater level and the second part is assessment of the soil layers under the water table. In first part, The Tokimatsu and Seed [15] method have been used for determining the volumetric strain of soil layers above the ground water level in the boreholes (54 boreholes along Tabriz Metro Line 2). The process is described as follows: 2. Assessment of effective shear strain due to earthquake in soil layer with using following relationship that is proposed by Tokimatsu and Seed [4]: Where, rd is the stress reduction factor, amax the peak ground acceleration, σv represents the total stress at the depth in question, and Gmax is the maximum shear modulus calculated by below equation that is proposed Tokimatsu and Seed based on kN/m 2 : = 4400 × (( 1 ) 60 ) 0.33 × ( ′ ) 0.5 ) 3. After the calculation of the volumetric strain (εc) by using the Figure 8 diagrams in each soil layer of boreholes, settlement value in each layer is determined using the following equation: Where, h is the thickness of layer in question. Finally, for each borehole log, the total settlement of soil layers above the groundwater level is accumulated in meter.
Also, the Tokimatsu and Seed [4] method was used in order to determine soil layers settlement at the below of the groundwater level in 54 boreholes of the study area. The procedure is described as follows: 1. The cyclic stress ratio (CSR) due to earthquake in soil layers is estimated with using equation No. 2, 2. Then with using Equation No.7, clean sand equivalent of number standard penetration resistance test ((N1)60CS) for soil layers below the water table is calculated,

3.
The volumetric strain value (εv) is assessed by determining both the CSR and (N1) 60CS for each soil layers with using the diagrams in Figure 9, 4. Finally, after calculating the volumetric strain (εv) in each soil layers, settlement values is determined in meters with using equation No.17 and then total amount of settlement have been calculated.
In final, after summation the amount of settlements in soil layers in the above and below the groundwater level in each boreholes, the total value of settlement calculated.

Results
The results of this study can be explained in below: 1. In 54 of borehole along the Tabriz Metro Line 2, generally the type of the soil layers in 170 samples was sandy, in 213 cases was silty and in 22 cases was gravelly. Moreover, the variation of the number of the standard penetration resistance test results are between 5 and 85. The value of safety factor versus liquefaction in soil layers below ground water level is shown in Figure 10. Accordingly, it can be explained that approximately in 30% to 40% of the data safety factor less than one. This conditions indicant that liquefaction hazard in study area is low to moderate. 2. Effects of fines content and relative density on volumetric strain in soil layers in Mw=7.5 can be illustrated in Figure 11. Accordingly, it is seen with increasing relative density and fines content in soil layers volumetric strain decrease. This result display with reducing void ratio in soil particles and growth fines content settlement hazard due to liquefaction go down. Figure 11: Variations of volumetric strain versus relative density in soil layers (Mw=7.5).

3.
Variations of volumetric strain versus effective shear strain (γeff) due to earthquake in soil layers in Mw=7.5 is observed in Figure 12. As seen on diagrams generally with growing effective shear strain value of volumetric strain trend to rising. But, similarity as mentioned above with increasing fines content volumetric strain declined. 4. The effect of the effective shear strain (γeff) on volumetric strain (εc) in soil layers above ground water level in two section of relative density variations are illustrated in Figures 13 a and b. As can be seen on diagrams, volumetric strain between 40% to 70% relative densities is more than when relative density grow up to 70%. Also, effects of effective shear strain due to earthquake on volumetric strain in very dense soils is less than moderate dense soils. Although, with increasing fines content in dry soil layers volumetric strain approximately climbed up. Figure 13: Variations of volumetric strain versus effective shear strain in soil layers above ground water level based relative density a: 40%≤Dr≤70%, b: Dr>70%.

5.
Comparison between settlement values of soil layers above and below ground water level can be seen in Figure 14. As shown in diagrams settlement in saturated soil layers due to liquefaction and earthquake is more than dry soils. 6. Rate of liquefaction potential index (LPI) along Tabriz Metro Line 2 is observed in Figure 15 and Table 5. Accordingly Iwasaki et al. method, almost 78% of boreholes are included in the range of low to moderate hazard of liquefaction. Also, 22% of boreholes are located in the category of high risk of liquefaction.  Figure 15: Values of LPI in boreholes in study area.

7.
The total settlement values in soil layers of boreholes in study area is illustrated in Figure 16a. Also, variations of LPI is proposed in Figure 16b. With comparison between each other can be seen a suitable correlation. As shown in Figures with increasing in LPI similarity total settlement growth up and vice versa too. Therefore, a range of settlement variations in soil layers in accordance with LPI for study area have been proposed in Table 6. Accordingly, maximum settlement can be happened in along Tabriz Metro Line 2 is more than 9 cm.

Conclusion and discussion
As mentioned before parts, main goal of this study is evaluation of settlement in soil layers due to liquefaction along Tabriz Metro Line 2. Accordingly, first the liquefaction potential of the layers of soil along the Tabriz Metro Line 2 assessed based on the standard penetration resistance test (SPT) results with using Idriss and Boulanger [4], [5] method. Second LPI determined by Iwasaki et al. [24], [25] procedure and p settlement (volumetric strain) of soil layers due to liquefaction using the Tokimatsu and Seed [15] method. Generally, the results of study showed that: 1. With considering ground water level and type of soils, liquefaction potential in west part of Tabriz (Bazaar to Tabriz Train station) and Metro line 2 is more than east part, 2. Also, there is a good agreement between LPI and total settlement in soil layers due to liquefaction can be seen in west part of Tabriz Metro line 2 the most settlement and volumetric strain values will occur (up to 50 cm), 3. Furthermore, different parameters such as the relative density of the soil, the fines content of soils, the type of fines in soil (clay or silt), the maximum shear strain, and the structure of the grain can affect the value of settlement in soil layers, 4. Therefore, with observing the results suggested that by conducting exact studies through other field tests and analysing them using software more accurate results can be achieved for this field and some methodology proposed for retrofitting critical regions.