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Literate: Jurnal Ilmiah Indonesia p–ISSN: 2541-0849
e-ISSN: 2684-883X
Vol. 6, No. 8, Agustus 2021
ANALYSIS OF SUBSOIL LIQUEFACTION POTENTIAL IN THE REGION OF MATARAM CITY IN INDONESIA
Intan Puspitaningrum
Warsaw University of Life Science, Poland
Email: intanpuspita2105@gmail.com
Abstract
One of the reasons of subsoil liquefaction
are cyclical loads induced from earthquake. It generally take in the subsoil
when there is a loose saturated granular soil. Loose, sand and silty sand have
the highest probability of liquefaction. Most places prone to this event are
the subsoil that is close to water source, namely river or bay area. Mataram
city is located on the west coast of Lombok Island. It acts as the capital and
economic powerhouse of the region. The 2018 M7.0 earthquake showed how
devastating the earthquake effect on people’s livelihood. Understanding the
potential of subsoil liquefaction to happen is crucial to help government and
people on the potentially affected area to adjust the proper mitigation
actions. Upon analysis of soil data taken from 9 SPT sites and 22 CPT sites, it
is concluded that the subsoil of Mataram city is prone to exposed with
liquefaction with the most severe area is the west coast of the city and the
least probable is the eastern part. Maximum settlement is forecasted to be
0.458 m taken from CPT-21 site.
Keywords: liquefaction potential, Cyclic Stress Ratio
CSR, Cyclic Resistance Ratio CRR, displacement, Mataram
Introduction
The 2018 earthquake that happen on Sunday 29^{th}
of July with magnitude of M6.4 in Lombok and Bali, or to be precise on 47 km
away from Lombok’s capital of Mataram with epicenter on 24 km deep left a
devastating effects. With reportedly a considerable amount of death toll and
people suffering from injury, it is one of the strongest that ever occurred in
the area. Thousands of people were left with no place to return, while
infrastructure are being not functional and further threats from landslide
increasing the possibility of worsening situation (Putra, Kiyono, Ono, & Parajuli, 2012).
The earthquake occurred in several series, namely
foreshock (July 29^{th}) with M6.4, main shock (August 5^{th})
with M7.0, and aftershock (August 9^{th}) M6.2. Lombok is surrounded by
several active earthquake sources, Back Arc Thrust Zone in the north,
megathrust in the south, and faults on both west and east sides (Lonteng, Balamba, Monintja, &
Sarajar, 2013).
After the M7.0 mainshock, several phenomenons happen
in numbers of locations scattered along the west coast to north coast of
Lombok. Landslide was the major phenomonon occurred along the west coast
resulting in cut of transportation and access from the main port in west coast
to the most affected area in the north. Land subsidiary and uplift were also
spotted in several areas. Land subsidance was identified mostly occurred along
the west coast with signs of small tsunami. Vertical uplift mainly happen on
the northern part of the island, close to the epicenter with recorded number of
0.44 m higher prior to the earthquake (Saut, 2015).
Signs of liquefaction is observed on smaller scale. It
happen as a result of a strong earthquake’s vibration in an area with mostly
containing aluvial sedimentation combined with fine particles of soil,
saturated, and typically shallow groundwater depth (Lonteng et al., 2013).
Looking back at the 2018 earthquake, Mataram as the
capital of Lombok acts as economic powerhouse and the most populous city in the
island shall be suffering from the effects of the upcoming earthquake in the
future. Therefore understanding the potential possibility of the upcoming aftermath
of earthquake in the form of liquefaction is one among many things we can do to
mitigate the outcome.
Methods
The aim of this research is to analyze the
liquefaction potential of the subsoil of Mataram city in Lombok using the soil
data taken using SPT and CPT in several locations. Soil data then being
analyzed to calculate the cyclic resistance ratio (CRR) and stress ratio (CSR)
in order to obtain the factor of safety (FS). For the further analysis we only
use FS value from CPT result to obtain vertical (S) and lateral displacement (LD). CPT has become very popular for
site characterization because of its greater repeatability and the continuous
nature of its profile as compared with other field test (Zhang, Robertson, & Brachman, 2002). The liquefaction degree was
assessed by using the Liquefaction Potential Index (I_{L}). Lateral
displacements (LD) were also evaluated based on the known ground slope (S) and
lateral displacement index (LDI).
(Warman & Jumas, 2013) did research on three locations to
identified the factor of safety over the potential of liquefaction in Padang
city, Sumatra on 2009 after the M7.6 tectonic earthquake. Soil investigation is
done using CPT referring to ASTM D 3441-86 Standard. The result then analyzed
using equation to calculate the Cyclic Stress Ratio (CSR) and Cyclic Resistance
Ratio (CRR). Result shows area with relatively safe from liquefaction potential
is having cone resistance (q_{c}) > 100 kg/cm^{2 }(10 MPa),
and dominated by soil types of sandy silt and silty sand.
(Obermeier, 1996) also describe the term “Liquefaction
potential” relates to the likelihood of liquefaction occurring during a
specific earthquake at a particular strength of shaking. Even a saturated, very
loose sand has no liquefaction potential if the severity of shaking is low enough.
Calculation or an estimation in determining the potential of a soil to
experience liquefaction requires two variables: (1) the seismic demand on a
soil layer, or CSR, and (2) the capacity of the soil to resist liquefaction,
expressed in Cyclic Resistance Ratio (CRR). Composed the following formula for
calculation of Cyclic Stress Ratio (CSR):
Where
a_{max} = peak horizontal acceleration at the
ground surface generated by the earthquake
g = acceleration of gravity
σ_{v0} = total stress
σ’_{v0} = total effective stress
r_{d} = stress reduction coefficient.
Accounts for the flexibility of the soil profile. (Blake,
1996) provides with approximation formula
to determine the r_{d} value derived from the mean curve formula by and
further developed by:
Where z is
the depth beneath ground surface in meter.
As for the CRR value, several field test that are
common to be used have gained evaluation of liquefied resistance, including the
Standard Penetration Test (SPT), the Cone Penetration Test (CPT), shear-wave velocity
measurement (Vs), and the Becker penetration test (BPT). SPT and CPT are
generally preferred due to the more extensive database and experience (Tijow, Sompie, & Ticoh, 2018).
Criteria for evaluation of liquefaction resistance
based on the SPT is largely constitute of CSR versus (N_{1})_{60} plot as shown in Figure 1. (N_{1})_{60 }is the SPT
blow count normalized to an overburden pressure of approximately 100 kPa (1
ton/sq ft) and a hammer energy ratio or hammer efficiency of 60%. Curves were
made to accommodate granular soils with the fines contents of 5% or less, 15%,
and 35% as shown. The CRR curve for fines contents <5% is the basic
penetration criterion for the simplified procedure and is referred as “SPT
clean-sand base curve”.
Figure 1
SPT
Clean-Sand Base Curve for M7.5 earthquake with data from liquefaction history
(Source: Journal of geotechnical and geoenvironmental
engineering, 2001)
Further developed an approximated formula for
clean-sand base curve plotted in Figure 1 by the following equation:
The above equation valid for (N_{1})_{60 }< 30. For (N_{1})_{60 } ≥ 30, clean granular soils are too dense
to liquefy and are classed as non-liquefiable. (Youd & Idriss, 2001) on the Summary Report from the 1996
NCEER and 1998 NCEER workshop recommend the following formula as correction for
the influence of fines content (FC) on CRR:
Where α and β is coefficient obtained from
the following relationship:
Other correction due to the additional factors involve
to fines content and grain characteristics influence SPT result, as shown in
Table 1. Equation below constitutes the corrections:
Where
N_{m} = measured standard penetration
resistance
C_{N} = factor to normalize Nm to a common
reference effective overburden stress
C_{E} = correction for hammer energy ratio
(ER)
C_{B} = correction factor for borehole
diameter
C_{R} = correction factor for rod length
C_{S} = correction for samplers with or
without liners
Table 1
Corrections
to SPT
Factor |
Equipment
variable |
Term |
Correction |
Overburden
pressure Overburden
pressure Energy
ratio Energy
ratio Energy
ratio Borehole
diameter Borehole
diameter Borehole
diameter Rod
length Rod
length Rod
length Rod
length Rod
length Sampling
method Sampling
method |
- - Donut
hammer Safety
hammer Automatic-trip
Donut-type hammer 65 – 115
mm 150 mm 200 mm <3 m 3 – 4 m 4 – 6 m 6 – 10 m 10 – 30 m Standard
sampler Sampler
without liners |
C_{N} C_{N} C_{E} C_{E} C_{E} _{ } C_{B} C_{B} C_{B} C_{R} C_{R} C_{R} C_{R} C_{R} C_{S} C_{S} |
C_{N}
≤ 1.7 0.5 – 1.0 0.7 – 1.2 0.8 – 1.3 1.0 1.05 1.15 0.75 0.8 0.85 0.95 1.0 1.0 1.1 – 1.3 |
As for liquefaction analysis using CPT data, A primary
advantage of using CPT is that a nearly continuous profile of penetration
resistance is developed for stratigraphic interpretation. The result is
generally more consistent compared to that of SPT. The stratigraphic capability
of CPT makes it particularly good for assessing liquefaction-resistance
profile. Figure 2 given by (Robertson, 2016) for direct determination of CRR for
clean sands (FC ≤ 5%) from CPT data is valid for M7.5 earthquakes only.
It shows calculated cyclic resistance ratio plotted as a function of
dimensionless, corrected, and normalized CPT resistance q_{c1N} from
sites where surface effects of liquefaction were or were not observed following
past earthquakes (Sasmi et al., 2020).
Figure 2
Recommended cyclic resistance ratio
(CRR) for clean sand under level ground conditions based on CPT
The clean-sand curve at Figure 2 may be approached by
the following equation (Robertson, 2016).
If
If
Where the (q_{c1N})_{cs} is the
clean-sand cone penetration resistance normalized to approximately 100 kPa (1
atm).
The CPT procedure requires normalization of tip
resistance. This corrections lead to normalized, dimensionless cone penetration
resistance q_{c1N}.
Where
Where
C_{Q} =
normalizing factor for cone penetration resistance
P_{a} =
1 atm of pressure in the same units used for σ’_{v0}
n =
exponent that varies with soil type
q_{c} =
field cone penetration resistance measured at the tip
(Robertson, 2016) give Figure 3 for estimation of soil
type. The boundaries between soil types 2 – 7 can be approximated by concentric
circles and can be used to account for effects of soil characteristics on q_{c1N
}and CRR. The radius of circles, is referred as soil behavior type index I_{c},
is calculated by the following formula:
Where
and
Figure 3
CPT-Based
Soil Behavior-Type Chart
The soil behavior chart in Figure 3 was developed
using an exponent n of 1.0, which is appropriate value for clayey soil types. However,
for clean sand, an exponent between 0.5 is more appropriate, and a value
between 0.5 and 1.0 would be appropriate for silts and sandy silts (Jefferies & Been, 2019). Differentiation is performed by
assuming an exponent n of 1.0 (characterized as clay) and calculating the
dimensionless CPT tip resistance Q from the following equation:
If the I_{c} calculated with an exponent of
1.0 is >2.6, the soil is classified as clayey and is considered too
clay-rich to liquefy, and the analysis is complete. If the calculated I_{c}
is <2.6, the soil is most likely granular in nature, thus C_{Q} and
Q should be recalculated using an exponent n of 0.5. I_{c} then shall
be recalculated. If the recalculated Ic is <2.6, the soil is classified as
non-plastic and granular. However if the I_{c} is >2.6, the soil is
likely to be very silty and possibly plastic. In this case, q_{c1N}
should be recalculated using an intermediate exponent n of 0.7 (Aryastana, Ardantha, Eka Nugraha, &
Windy Candrayana, 2017).
In order to normalized penetration resistance (q_{c1N})
for silty sands is corrected to an equivalent clean sand value (qc1N) cs by (Castelli & Lentini, 2010) the following equation:
Where K_{c}, the correction factor for grain
characteristics, is defined by the following formula:
for
for
Since the clean-sand base or CRR of SPT and CPT on the
above section of this chapter is only apply to magnitude 7.5 earthquakes. To adjust
the clean-sand curves to magnitude smaller or larger than 7.5, (Green et al., 2017) introduced correction factors coined
‘magnitude scaling factors (MSF)’. Therefore, the equation on finding the
safety factor FS of the potential of liquefaction to be happened is written as
follows:
Where
CSR =
calculated cyclic stress ratio generated by the earthquake shaking
CRR_{7.5 }= cyclic resistance ratio for magnitude
7.5 earthquakes
Several scaling factors are proposed by researches as
provided in Table 2. For engineering practice purpose, it is recommended for
magnitude <7.5 the lower bound for the recommended range is the new MSF
proposed by Idriss in column 3 of Table 2.
Table 2
MSF Value
Defined By Various Investigators
Magnitude (1) |
Seed
and Idriss (2) |
Idriss (3) |
Andrus
and Stokoe (4) |
5.5 6.0 6.5 7.0 7.5 8.0 8.5 |
1.43 1.32 1.19 1.08 1.00 0.94 0.89 |
2.20 1.76 1.44 1.19 1.00 0.84 0.72 |
2.8 2.1 1.6 1.25 1.00 0.8 0.65 |
To estimate the severity of liquefaction degree at
given site, proposed the liquefaction potential index (I_{L}) as
follows:
IL =
For sites with level ground, far from any free face,
it is reasonable to assume that little or no lateral displacement occur after
earthquake, such that the volumetric strain will be equal or close to the
vertical strain. If the vertical strain in each soil layer is integrated with
depth using this equation, the result should be an appropriate index of potential
liquefaction-induced ground settlement at the CPT location due to the design
earthquake.
S =
Where
FL = 1 – FS for FS ≤ 1.0 and F=0 for FS > 1.0
W (Z) = 10 – 0.5Z (Z in meters and the depth
20 m is decided considering where
the liquefaction happen during the
past earthquake phenomena)
The following simplified procedure
for assessing soil liquefaction based om the IL can act as the preliminary
guideline.
I_{L} = 0 Liquefaction risk is
very low
0 < I_{L} ≤ 5 Liquefaction
risk is low
5 < I_{L} ≤ 5 Liquefaction
risk is high
15 < I_{L} Liquefaction risk is very high
Where S is the calculated liquefaction-induced ground
settlement; ɛ_{vi }is the postliquefaction volumetric strain for
the soil sublayer i ; Δzi is the
thickness of the sublayer i; and j is the number of
soil sublayers.
Figure 4
Relationship
Between Postliquefaction Volumetric Strain and Equivalent Clean Sand Normalized
CPT Tip Resistance For Different Factors Of Safety (FS)
Generally, liquefaction-induced ground failure include
flow slides, lateral spreads, ground settlements, ground oscillation, and sand
boils. Lateral spreads are the pervasive types of liquefaction-induced ground
failures for gentle slopes or for nearly level ground with free face
Lateral displacement index (LDI) is defines as follows:
Where
Z_{max} =
maximum depth below all the potential liquefiable layers with a calculated SF
< 2.0
γ_{max} = maximum cyclic shear strains
Where γ_{max} be
approach by the following mathematical expressions:
if
if
if
if
if
if
if
if
if
if
if
if
if
Approach for Lateral Displacement (LD) is recommended
for use based on the research mostly in Japan and America with its earthquake
properties and ground condition, moment magnitude between 6.4 and 9.2, peak
surface acceleration between 0.19 g and 0.6 g, and free face height less than
18 m (Juang, Ching, Wang, Khoshnevisan, &
Ku, 2013). Lateral Displacement can be
estimated with equation bellow:
LD = (S +
0.2) LDI
Where
LD =Lateral
Displacement
S =Knowing
ground slope
Figure 4 shows several locations in Mataram city in
where soil data is being taken by using SPT or CPT. Most locations are being
tested by either SPT or CPT and some are taken by both SPT and CPT. The
earthquake profile is based on the 2018 M7.0.
Figure 5
Locations of
Data SPT and CPT Taken
Results and Discussion
In this chapter of journal, results of factor of
safety acquired from the calculations of CPT and CPT data in accordance to the
depth of observation are depicted in Figure 6 to Figure 7.
Figure 6
SF on
Location 04 and 19 (Downtown and West Coast)
Figure 7
SF on
Location 14 and 15 (North Area and Eastern Area)
Table 3
Recapitulation
of Settlement and Lateral Displacement
Location |
Coordinates |
Depth ( z ) max |
Σ I_{L} |
Σ S |
Max LD |
(m) |
(m) |
(m) |
|||
CPT - 01 |
-8.5777629 , 116.086348 |
16.8 |
35.62 |
0.35 |
0.015 |
CPT - 02 |
-8.5777486 , 116.0867196 |
22 |
31.70 |
0.40 |
0.010 |
CPT - 03 |
-8.5777486 , 116.0867196 |
22 |
31.40 |
0.40 |
0.010 |
CPT - 04 |
-8.5790728 , 116.0886944 |
22 |
32.30 |
0.10 |
0.010 |
CPT - 05 |
-8.6200626 , 116.0822933 |
8 |
13.58 |
0.09 |
0.010 |
CPT - 06 |
-8.6181029 , 116.1648509 |
9 |
16.81 |
0.10 |
0.0064 |
CPT - 07 |
-8.6176298 , 116.1649291 |
8.4 |
9.21 |
0.06 |
0.0064 |
CPT - 08 |
-8.6176298 , 116.1649291 |
6.6 |
13.40 |
0.09 |
0.0065 |
CPT - 09 |
-8.6055768 , 116.0904813 |
10.8 |
36.73 |
0.21 |
0.010 |
CPT - 10 |
-8.5733601 , 116.1022052 |
6 |
7.19 |
0.05 |
0.0064 |
CPT - 11 |
-8.5945499 , 116.102548 |
17.6 |
36.85 |
0.37 |
0.010 |
CPT - 12 |
-8.5971415 , 116.1601164 |
5.6 |
15.90 |
0.12 |
0.010 |
CPT - 13 |
-8.6194192 , 116.0975514 |
11.6 |
31.95 |
0.23 |
0.010 |
CPT - 14 |
-8.5664244 , 116.1131799 |
9 |
29.36 |
0.19 |
0.010 |
CPT - 15 |
-8.5927082 , 116.1559701 |
3.8 |
6.42 |
0.06 |
0.0064 |
CPT - 16 |
-8.5844697 , 116.1286235 |
12.4 |
22.0 |
0.21 |
0.010 |
CPT - 17 |
-8.5641529 , 116.0981165 |
14.4 |
37.12 |
0.28 |
0.010 |
CPT - 18 |
-8.5955247 , 116.1126641 |
8.6 |
13.22 |
0.10 |
0.0064 |
CPT - 19 |
-8.6003905 , 116.0836751 |
7.4 |
19.69 |
0.12 |
0.0064 |
CPT - 20 |
-8.5842564 , 116.1072639 |
12 |
16.51 |
0.14 |
0.010 |
CPT - 21 |
-8.5705941 , 116.728371 |
2 |
38.80 |
0.46 |
0.010 |
CPT - 22 |
-8.588308 , 116.1453149 |
4.8 |
6.29 |
0.05 |
0.010 |
Conclusion
Generally speaking, Mataram city is prone to
liquefaction with most of the liquefaction potential may happen from at 2
meters below the surface. Figure 5 shows that downtown area is heavily prone to
liquefaction starting from 2 meter below until more than 20 meter. While the
west coast is also have a high potential of liquefaction according to Figure 5
and having the biggest settlement potential (0.46 m) according to data obtained
on CPT-21. This may due to the its location close to the epicenter of the past
earthquake and the location of most of rivers downstream, the place where most
of the soil being soft. It shows a significant potential up to 20 m below. The
same pattern is also observed on the northern part of the city, where the FS
< 1 is commonly observed from 2 m up until 13 m. Nevertheless, it shows a
smaller magnitude compared to that of the western coast.
Moreover, the eastern part shows the least potential
of liquefaction as shown in Figure 6 and the least settlement potential on only
0.05 m obtained at CPT-22 site. This can happen due to the higher altitude and
the presence of strong soil in the surrounding area. Lastly, the southern part
of the city indicates a relatively medium potential of liquefaction, although
it is worth noticing the effect of liquefaction might increase due to its close
location to the west sites. Finally, looking at the calculation results, it is
can be concluded that Mataram city has a high potential of liquefaction and it
is recommended to take a further actions.
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