FLUID BEHAVIOUR ANALYSIS ON LIQUEFACTION USING KORINOFACTION DEVICE
Ari Sentani, S.T.,M.Sc.1 , Dr.Ir.H.Soedarsono,M.Si. 2 and Eka Koestiana Soeridjal3
1 Civil Engineering Program, Engineering Faculty, Sultan Agung Islamic University
2 Civil Engineering Program, Engineering Faculty, Sultan Agung Islamic University
3 Civil Engineering Program, Engineering Faculty, Sultan Agung Islamic University
Abstract. Earthquakes can be followed by liquefaction, which is a response of saturated soil when it is subjected to shock or stress that cause loss of soil strength or bearing capacity as an impact of the increasing of soil pore water and the loss of the soil stress’s effectiveness. This research using Korinofaction that work to cause cyclic loads or vibrations that come from DC servo motor with an adjustable speed and force. The earthquake’s strength is measured by the number of rpm measured on the digital tachometer. Korinofaction is equipped with plumbing system to observe fluid behaviour during liquefaction. The results of research showed that silty sand and silt was liquefied in VIII Modified Mercalli Intensity earthquake and cause the occurrence of water flow on the surfacedue to increase soil pore stress. The flow rate that triggers liquefaction in the silty sand is 6,769 x 10-5
⁄ , and siltl is 5,0 x 10-5 ⁄ . The water flow that flows in the silty sand had permeability of 4,76 x 10-4 ⁄ while on the silt is 6,09 x 10-4 ⁄ . After liquefaction, gradient hydraulic of silty sand is 4,76 mm and silt is 6,09 mm. Based on this research liquefaction caused mobilized debris flow and muddy debris flow.
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Keywords : earthquake, liquefaction, soil, water, flow, gradient hydraulic, permeability, debris
1. Introduction
Indonesia is at the meeting of three plates of the earth crust, which is Pacific Plates, Eurasia Plates and India Australia Plates. Indonesia also is at the meeting of two main earthquakes lines which is Mediterranean earthquake line and Circum Pacific earthquake line, so that Indonesia has the potential for an earthquake. Such as the earthquake in Palu, Sigi, Parigi Moutong and Donggala, Center Sulawesi Province. This earthquake has a magnitude 7,4 SR/Mw in depth 10 Km earthquake’s center in Koro fault path. This earthquake followed by tsunami in Teluk Palu. This disaster is caused by a sediment avalanche from seabed with depth 200 – 300 m. Sediment from rivers which empties into Teluk Palu have not strongly consolidated, so it collapsed and landslide during the earthquake and triggered tsunami. Apart from earthquake and tsunami, it also occurred liquefaction in Petobo, Palu.
Sand in residential areas turned into mud and lost its bearing capacity. According to Arthur Casagrande (1976), liquefaction is the response of loose, saturated sand when subjected to strains or shocs that results in substantial loss of strength and in extreme cases leads to flow slides. Liquefaction causes soil structure’s damage. The soil layer becomes slurry and almost have no bearing capacity caused by the loss of effective soil stress. Liquefaction also causes land subsidence, soil cracks, discharge of fine sand slurry to sthe soil surface, the loss of soil friction against the pile until the building is overthrown (Prawirodikromo, 2012). Liquefaction is closely related to water due to the amount and movements of water due to vibrations that cause liquefaction. This research will analyze
the fluid behaviour on liquefaction using Korinofaction device. Besides that, this research also analyze the impact of liquefaction on soil structure and water flow.
2. Literature Review 2.1. Liquefaction
Liquefaction as one of the earthquake’s impact does not occur in all types of soil. Generally, liquefaction occurs in loose to moderate consistency of saturated granular soil with drainage properties in the soil. Soil deposit that have the potential for liquefaction when given cyclic loads include sand and silt. Because it only occurs on saturated soils, liquefaction generally occurs near rivers, bays or other water bodies (Kramer, 1996).
Changes in soil conditions of water saturated sand become liquid due to increased pore water pressure to a point equal to the total stress due to cyclic loads causing the effective soil stress to decrease to zero. Thus, liquefaction is the occurrence of a soil losing strength and stiffness in a short time. When an earthquake occurs, the shear forces cause the sand to react, so that the pore water pressure increases. Due to the cyclic vibrations that occur for a short period of time, the soil loses its strength or stiffness and cannot support the structure above it to remain stable (Jefferies and Been, 2006).
Increased pore water pressure causes water flow rises to surface in the form of mud or sand jets.
In this situation, effective soil stress is zero and soil’s particle apart as if floating in water. Structure above liquefied sand deposit when earthquake occurs will sink and buried channels will calcify on the surface (Seed, 1970).
Soil response is showed by deviation of speed, acceleration and stress that arises on the surface or in the ground caused by earthquake energy waves. This response is so complex, because in addition to the incoming energy wave in three dimensions, sedimentary soils where the energy waves pass may have a non – linier response, inhomogeneous soils and influential groundwater.
To simplify the problem, an assumption is taken that the direction of wave propagation is considered to be purely vertical with the motion of soil particles in the horizontal direction.
Earthquake waves are not necessarily pure vertical but have a certain angle to the vertical line. The wave motion in the vertical direction has a shear effect on the soil element. The soil elements will alternately change shape and cause shear stress and shear strain
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ss. To stimulate the shear sress and shear strain, a simple cyclic shear test can be used. The ratio between shear stress and shear strain is known as soil shear modulus (G).The main distortion component is the relative displacement/change of place between the grains of soil/sand which is expressed in the unit of soil shear strain, (Vucetic, 1992). The amount of shear strain is considered to have the most influence on changes in the grain structure of the soil, including the breakdown of particle bonds between the grains, the occurrence of slips between two or more particles that are in contact with each other and a further consequence, namely the possibility of developing soil volume elements and varying pore water pressures.
The boundary shear strain is the limit of shear strain when the volume of the soil changes in volume. If the soil or sand shear strain is greater than the boundary shear strain, the soil or sand changes in volume. If a unit volume or sample of water saturated soil is given a cyclic load with a greater shear strain than the boundary shear strain, the volume of sand will expand. The expansion of the san sample volume is due to the increase in pore water pressure as a result of laboratory tests by Peacock and Seed (1981) presented by Prakash (1981). The dynamic loading is cyclic dynamic loading that is close to harmonic load. The loading on the specimen is expressed in terms of the stress deviation.
Based on the research, immediately after the cyclic loading was carried out, the pore water stress increased almost linearly.
The effective grain stress σe decreases until the minimum limit is even zero, so that the shear strain is so large and the sand grains are no longer touching. As a results, the sandy soil loses its bearing capacity or the sand is liquefied. The pore water tension increases because there is no drainage
(undrained) during cyclic loading. The combination of the deviation stress, the pore number e, the number of loading cycles, the confining stress, the loading frequency and the relative density of Dr, the soil will affect the density of water saturated loose sand so that it experiences liquefaction.
There is a certain threshold at which liquefaction will not occur. Liquefaction does not occurs when :
a. Earthquake magnitude <5 on the Richter scale (M<5).
b. Earthquake intensity below VI (IMM<VI).
c. It is a deep earthquake (depth of focus>70 km)
Wag and Law (1994) said that based on the results of field observations of more than 100 years and more than 100 liquefaction events, it shows that liquefaction does not occur if the epicentre distance is more than :
(1)
Liquefaction occurs when the groundwater is below 3,0 meters deep. Liquefaction will not occur if the groundwater depth is more than 5,0 meters (Wang and Law, 1994).
Citeria for liquefied soil according to Yound and Gilstrap (1999) and Day (2002); Perlea et al, 1999 in Prakash and Puri 2003 :
a. Soil diameter D50 between 0,02 – 1,0 mm.
b. Content of fines, grain D < 0,005 mm less or equal to 20%.
c. The coefficient of uniformity D60/D10 < 10.
d. Relative density of Dr <75%.
e. IP plasticity index <13%.
Figure 2.3. Liquefaction Criteria
(Source :
[20] )3. Research Metodology
4. Results
4.1. The Basis of Liquefaction
This study uses a Korinofaction tool with instruments in the form of water, soil type and cyclic vibrations/loads due to the earthquake. The purpose of this study is to analyse the process of liquefaction based on the Korinofaction tool, analyze fluid behaviour during liquefaction, analyze the time it takes for liquefaction to occur during an earthquake, analyze the amount of water that causes liquefaction and analize the liquefied area and also analyze the type of soil that has the potential for liquefaction.
The soil used in this research is silt and silty sand. The silt and silty sand is used based on previous research conducted in the Bantul Regency area, Yogyakarta by Titi et al, in the journal Wahana Fisika, 2 (1), 2017.8-27 using the SPT (Standard Penetration Test) method and Ungkap M.
Lumbanbatu and Suyatman Hidayat, who conducted an initial evaluation of the liquefaction vulnerability of Kendal area and its surroundings, which was published in the Indonesian Geology Journal, 2 (3), September 2007. From the results of this study, it was found that the two areas (Kali Opak and Kendal coast area) have the potential for liquefaction.
Liquefaction occurs due to an increase in pore water tensions/cyclic loads caused by an earthquake which then causes a loss of soil stress which changes the behaviour of the soil to become liquid. Based on the book Seismology Engineering & Seismic Engineering by Widodo Prawirodikromo, the conditions for liquefaction are as follow:
a. Earthquake magnitude>5 SR b. Earthquake intensity>VI (MMI>VI)
c. Includes shallow earthquakes (depth of focus <70 km) d. Initial Relative Density
e. Type of soil
Table 4.1. Scale Comparison
Modified Mercalli (MMI) Ground Acceleration*) %g (+) Ground Speed*) cm/dt(+)
I <0,17 <0,10
II 0,17 – 1,4 0,1 – 1,1
III 0,17 – 1,4 0,1 – 1,1
IV 1,4 – 3,9 1,2 – 3,4
V 3,9 – 9,2 3,4 – 8,1
VI 9,2 – 18 8,1 – 16
VII 18 – 34 16 – 31
VIII 34 – 65 31 – 60
IX 65 – 124 60 – 116
X >124 >116
*) based on Wald et al. (1999)
(Source : [20] Prawirodikromo W., 2012, Seismiligi Teknik & Rekayasa Kegempaan,)
In this sudy, to determine the earthquake speed using a speed measuring device in the form of a digital tachometer, which is how it works using a sensor that is directed at the rotation of the driving pulley on the servo, then from the digital tachometer, the rotational speed of the pulley is obtained in units of rpm. The rpm unit is converted into cm/s using the physics formula. The formula used to convert this speed is as follows :
Speed =
(2)
With :
n = number of rpm r = radius (m)
The resulting unit in m/s is then converted into units of cm/s.
4.2. Korinofaction Tool Work System
The working system of the Korinofaction tool is to create a vibration/cyclic load resembling an earthquake. The cyclic load comes from the DC servo motor which is at the bottom of the Korinofation tool. There is a water installation that acts as groundwater in the experiments carried out.
The water installation consists of a bucket (water basin), a water pump and a plumbing system located at the bottom of the tool. The bucket functions as a water reservoir which will be channelled by a water pump through the plumbing system to the glass tub where the liquefaction experiment is located.
In the plumbing system, there is a pressure gauge that is used to measure the level of water pressure flowing into the liquefaction experiment area.
In previous research, Korinofaction was used to created cyclic loads/vibrations as a source of earthquakes which caused the pore water’s increase in the liquefaction experiment area. When the pore water tension increases, the effective soil stress will decrease to its lowest point, resulting I liquefaction. The previous Korinofaction work system with the current one is still the same, only a few modifications were made to improve the tool.
In this study, the modifications made to improve the tool in order to get better research results include changing the dimensions of the tool. The new Korinofaction has scalatic dimensions that are larger than the previous dimensions. The DC servo motor replaces the single phase dynamo engine as a turbine driving machine. This aims to facilitate testing using various variations in the speed and strength of cyclic loads that cause earthquakes, because variations in speed and strength of cyclic loads on a single-phase dynamo are limited and not constant (rpm becomes difficult to measure). Another disadvantage of a single phase dynamo motor is that the engine temperature will increase quickly when the trial time is too long. By using this DC servo motor, rpm is calculated by digital calculation.
We can adjust the percentage of power and speed of the DC servo motor as needed. The current Korinofaction tool uses 3 pulleys from the previous 4 pulleys to streamline tool performance.
The size of the glass tub as a liquefaction analysis medium was calculated scalatically and was bigger than the glass tub for the previous analysis media. The area that is being scaled is the area in Petobo, Palu, Central Sulawesi which is experiencing liquefaction. Previously, Korinofaction used a water barrel that was at the top of the tool and its position was higher than the glass tube as a plumbing system. In the modified tool, the bucket as a place for water is at the bottom of the tool and its position
is lower than the bottom of the glass tub. In addition, in this new Korinofaction, water enters the glass bath, unlike the previous Korinofaction where water enters through the side of the glass bath. To analyze the pressure level of the water entering the glass bath, it can be observed the pressure gauge that has been added to Korinofaction. The length of the iron plate which is likened to the earthquake’s trajectory is changed based on the calculating of torque. This torque calculation functions to calculate rpm manually without a digital achometer. The position of the iron plate which was originally on the side of the glass tub was moved to the center of the glass tub so that it was closer to real earthquake simulation.
4.3. Results and Discussion
The results of the research that has been carried out, there was liquefaction in the two soil samples used. The occurrence of liquefaction is caused by several factors, such as cyclic loads/vibrations and soil saturated water. Cyclic loads cause soil saturated water to increase and seep to surface. Increased soil saturated water causes the pore water tension to increase. As a result, the effective stress decreases with increasing pore water tension. The effective stress of the soil, which decreases over time, will reach its lowest point and make the soil change like slurry (dissolve in water). This soil behaviour causes the above structures to lose their strength and balance, so they are swallowed up by the liquefied soil.
The liquefaction event that occurred in the study had several aspects that could be analyze based on the data obtained. Based on the results of the experiment, several things can be studied, including the process of liquefaction, fluid behaviour during the liquefaction and area affected by liquefaction.
4.3.1 The Process of Liquefaction in 2 Types of Soil Samples
Liquefaction occurred in the two types of soil used for the experiment with several different results. However, it has the same behaviour when experiencing liquefaction. When the experiment was carried out by applying a cyclic load/vibration to the analysis medium, the silt and silty sand were saturated with water. Then the pore water content increases at the same time as the cyclic/vibratory loading takes place. This results in a reduced effective soil stress. When the effective stress of the soil reaches its lowest point with increasing pore water, the soil turns into a slurry. This kind of soil texture causes the structure above to lose its balance and strength, so that it collapses, breaks down and shifts from its original position.
4.3.2 Flow of Water in a Liquefaction
The water flow that occurs in a liquefaction can be observed when conducting research. Based on the research results, it can be calculated that the flow rate that causes liquefaction with the data obtained, which is the water volume and time. The water flow pressure can also be analysed from a pressure gauge. The results of the observations on the experiments carried out, the water flow that occurs during the liquefaction is a type of viscous flow, which is a type of water flow that causes shear stress between moving liquids due to cyclic loads at different speeds. In addition, the flow which was originally laminar flow, when given a cyclic load/vibration, changes to turbulent flow. Turbulent flow is a flow that has irregular movements, intersecting trajectories and large flow velocities.
Flow velocity can be calculated by the formula of discharge and cross sectional area. For the size of the trial model, the speed obtained from the two trials is a fairly large speed. This velocity causes the flow motion to always be irregular in turbulent flow, so that it is included in the unsteady flow and non-uniform flow categories.
Although the water flow on the ground due to liquefaction is a turbulent flow, initially the water flow is laminar flow, based on the Reynolds Number value which is below 2300. Meanwhile, based on the calculation of the Froude Number, this flow is a subcritical flow, with the Froude Number value at below 1. And according to the flow regime, with the Froude Number and Reynolds Number values, the water flow that occurs is subcritical laminar. As a result of a cyclic load and an open flow, the flow turns turbulent.
The flow of water in the soil and porous sand that occurs during the liquefaction event has a quantifiable energy. However, because this water flow continues to move, it loses energy based on the
elevation of the ground surface. This head loss is called a hydraulic gradient. Water originating from the plumbing system then enters the analysis medium has a change in the velocity value. The velocity of the water flowing in a porous medium can be calculated using Darcy’s Law and paying attention to the permeability coefficient.
In addition to the flow types above, liquefaction also causes debris flow. Debris flow is a flow that has a high speed and carries objects in its path. There are several types of debris flow that are known, but in this liquefaction, the debris flow that occurs is a type of mobilized debris flow.
Mobilized debris flow is a debris flow that is formed due to changes in pore water pressure and forces that experience dispersion in certain layers. Meanwhile, based on the material carried in the experiment, the debris flow that occurs is a muddy debris flow, because the material that forms it or the material carried consists of mostly small (fine) particles.
Tabel 4.2. Water Flow Data
Soil Types Q v (Darcy)
Silty Sand 0,00006769 ⁄ 4,76 mm 0,003 4,67 ⁄
Silt 0,00005 ⁄ 6,09 mm 0,002 6,09 ⁄
Tabel 4.3. The Value of Reynolds and Froude Number Soil Types
Silty Sand 2169 0,054
Silt 1601 0,04
4.3.3 Earthquake Strength
From the research results, it was noted that the rpm value has a small difference in the two samples used. In silty sand, 45 rpm is recorded and on silt 43 is recorded. The earthquake strength that causes the liquefaction can be calculated based on the rpm value. The calculation results show that the same earthquake strength occurred in the two samples, which was VIII MMI. At the stat of the VIII MMI magnitude earthquake, the behaviour of silt and silty sand still tends to remain. The two soils began to show changes in behaviour when water began to enter the test medium and the soil pore water increased. Then the effective soil stress decreases until the soil melts as water reaches the surface. This is called liquefaction.
4.3.4 Liquefied Area
Based on the results of their results, silty sand is a medium that is prone to liquefaction. This is due to the characteristics of sand, which is a non-cohesion medium, which is soil that has no or very little attachment between grains and contains almost no clay. This is also applies and proven in this study.
Table 4.4.Liquefied Area
Soil Types
Area (cm2) % Area
Silty Sand 7073,31 88,42
Silt 6806,27 85,08
4.3.5 Depth of the Liquefied Area
When the soil begins to lose its effective stress due to increased soil pore water, the building or structure above it loses balance and damaged. Some of them even sank into the ground that had a pulp like texture. Soil that the texture have ben changes will decreases automatically. This land subsidence was recorded differently at several points. Based on data that recorded after conducting the experiment, the silty sand decreased with a higher value than silt, which is 2,1 cm and silt decreased by 1,42 cm. this is directly proportional to the small value of sand cohesion (non-existent)
Table 4.5. Depth of the Liquified Area
Soil Types Settlement (cm) Depth (cm)
Silty Sand 2,1 7,9
Silt 1,42 8,58
5. Conclusion
From the results of data processing, an analysis of the liquefaction events that had been modeled was carried out. The results of the analysis showed that at the beginning of the VIII MMI magnitude earthquake, the soil behaviour of the silty sand and silt tended to remain unchanged or permanent. The soil begins to change its behavior when the pore water begins to increase and the effective stress of the soil begins to decrease. It showed when water reaches the surface and the soil loses its effective stress to its lowest point, the soil turns like slurry. This is called liquefaction.
The water flow in liquefaction is a turbulent viscous flow, unsteady flow and non-uniform flow.
The discharge value in silty sand is greater than silt. That is 6,769 x 10-5 ⁄ . Whereas on silt, the discharge value is 5 x 10-5 ⁄.. The Reynolds Number value for pipe flow when the experiment was carried out on both soil types was 2169 for silty sand and 1601 for silt. This shows that the flow that occurs in the pipe when the experiment is carried out is the same as the laminar flow. The flow that occurs in the pipe during the experiment using silt and silty sand is a subcritical flow, based on the calculation of the Froude Number, which is a Froude Number below 1, 0,054 for silty sand and 0,04 for silt.
According to the flow regime, with the Froude Number and Reynolds Number obtained, the water flow in the pipe in two experiments carried out was subcritical laminar, with the Froude Number less than 1 and the Reynolds Number value less than 2300.The characteristic of silty sand, which has almost no cohesion value, causes water to seep slower into the silty sand. It is proven that the sand permeability value is 4,67 ⁄ , slower than the permeability value on silt, which is 6,09
⁄ .The hydraulic gradient value in silty sand is greater than the hydraulic gradient in silt. The hydraulic gradient values for silty sand are 3 mm and on silt are 2 mm.
The water flow in the silty sand experiences a greater loss of energy during liquefaction, which is 0,003. Meanwhile, water flow in silt loses energy by 0,002. This is due to the characteristics of the sand which has a low cohesion value. The flow of water during liquefaction forms a debris flow. The type of debris flow that is formed is based on the way it is formed and the material carried by the flow is the mobilized debris flow and the nuddy debris flow.
The liquefaction experienced by the two types of soil samples resulted in damaged to the analysis medium. The area affected is different for each soil. Silty sand has an area 7073,31 cm2 of the liquefied area, or 88,42% of the total area of the analysis media. Meanwhile, silt has a liquefied area of 6806,27 cm2 or 85,08% of total area of analysis media. In addition to the area of the liquefaction area, the depth of the liquefied area is also different. In the silty sand, there was a decrease of 2,1 cm and in the silt, it was 1,42 cm. the difference in the extent and depth of this liquefaction is influenced by the value of soil cohesion. The smaller the value of soil cohesion, the greater the impact of liquefaction that occur.
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