10th Asian Regional Conference of IAEG (2015)

Liquefaction Characteristic Based on Ground Response Linier Equivalent Analysis andCyclic Stress Concept on Young Merapi Volcanic Deposit in Bantul Regency, Yogyakarta,
Indonesia

TAUFIQ Wira Buana and Muhammad WAFID Agung
Geological Agency of Indonesia
E-mail:wafid.mhd@gmail.com
Abstract.
Bantul regency is one of earthquake prone area due to subduction activities ofIndo-Australia and Eurasia crust on the south and Opak fault on the east. The morphologyof this area is low land between hills on the west and the east which called as Bantul Graben.
Bantul Graben consists of Young Merapi Volcanic Deposit. Devastated area and buildings
as severe earthquake evidence on May 27th, 2006 was caused by some factors i.e.:
amplification and liquefaction on Young Merapi Volcanic Deposit. From the confirmed
deterministic seismic hazard analysis study which indicated that the most responsible
source of earthquake at the time of hazard event is an Opak fault with strike-slip type which
gives the highest value of peak ground acceleration on bedrock compared with other sourcei.e.: subduction type Liquefaction phenomena based on cyclic stress analysis by using theresult of ground response linier equivalent analysis was investigated and the result showsthat safety factor and liquefied lithology are verified. The amplification average of Young
Merapi Volcanic Deposit is two times and amplification values tend comparable withsediment thickness. The amplification value of each lithology will decrease with increasing
density or consistency. The amplification could trigger the liquefaction phenomena only on
sandy and silty deposits. On the other hand, high density or consistency (N1(60) > 20) andclayey deposits tend not liquefied. Sand deposits will liquefy if the level of density fromloose to medium (N1(60) between 5-20) with cyclic stress ratio (CSR) between 0.18 to 0.53.
Whereas silt deposits will liquefy if its consistency level from very soft to stiff (N1(60)between 3-15) with cyclic stress ratio (CSR) between 0.16 to 0.69.
Keywords: cyclic stress concept, amplification, N-SPT, liquefaction
1.

Introduction

Devastating earthquake on 27 May 2006 in
Yogyakarta has severely damaged the ground surface.
BAPPENAS data (2006) attributed the loss due toinfra structure destruction causes 4,121 people diedand 79,889 buildings were severely damaged.
Furthermore, the relatively shallow of the earthquake
source, the condition of buildings that do not meet therequirements for earthquake resistant buildings, and a
relatively dense population is the reason why so many
casualties in that event (BAPPENAS, 2006).
Earthquake at that time also cause liquefaction insome places on young Merapi volcano deposit(Figure 1).
The intent of this study was to determine thecharacteristics of liquefaction in two types of young
Merapi Volcano deposit.

Figure 1. Liquefaction evidence due to Yogyakarta
Earthquake1

10th Asian Regional Conference of IAEG (2015)

2.

Gravelly sand: grey, fine to coarse sand and consist ofgravel, sub rounded to rounded, various density frommedium to very dense.
Silt deposits are limited and founded as a thinlayer within sand deposits which could be found near
Opak river on the east and near Bedog river on thewest. Silt deposits could be divided into two groups
of material based on grain distributions, i.e; silt andsandy silt.
Silt: pale brown, low plasticity, soft to stiff.
Sandy silt: pale brown color, low plasticity to nonplastic, soft to stiff, consist of little very fine to finesand.
Clay are deposited near surface with browncolor (sandy clay). Clay deposits with blueish blackcolor are found at the depth of more than 15 m below
the surface with limited locations. Clay deposits
could be devided into two groups, i.e: clay and sandyclay.
Clay: blueish black, medium to high plasticity, stiff tohard.
Sandy clay: brown color, medium plasticity, soft tostiff, consist of very fine to fine sand.
During field survey, liquefaction phenomena
could be identified in several locations (red crosssymbols in Figure 2).

Geology and liquefaction at Bantul Regency

Research area is low land which set between
two mountains on west-east, coastal plain on thesouth, and Merapi Volcano on the north direction(approximately 60-80 km). Major fault system has
two orientations on southwest-northeast that divide
between South Mountain-Bantul Graben andsoutheast-northwest
that
separate
Progo
Mountain-Batul Graben (Rahardjo et.al, 1995).
Sudarsono and Sugiyanto (2007) stated that
Young Merapi Volcanic Deposit (Qmi) has highpotential to liquefy. Qualitative description from
borehole log shows that Quaternary deposits untill 20
meter depth consist of unconsolidated material(uncementated), i.e.: sand, silt, and clay deposits.
Sand deposits have wide spread horizontally andvertically compared with silt and clay deposits.
Based on grains size distributions sand deposit
can be classified into three groups of material, i.e:sand, silty sand, and gravelly sand with physicalproperties as follows.
Sand: grey color, fine to coarse sand, less gravel, subrounded to rounded, various density from veryloose to very dense.
Silty sand: brownish grey, fine to coarse sand, consistof silt and less gravel, sub rounded to rounded,various density from very loose to very dense.

AA’A

A’

100 m0m-100 m?

OPAK FAULT

Figure 2. Regional geological map (Rahardjo et al., 1995) and identified liquefaction evidence on May 27th,
2006210th Asian Regional Conference of IAEG (2015)

Idriss, 2001).
According to NCEER (1997), factor of safety(FS) against liquefaction is needed for correction
factors called “magnitude scaling factors (MSF)” toadjust the simplified base curve to magnitudes
smaller or larger than 7.5 with recommended formula
as follows;

2.1 Ground Response Linier Equivalent Analysis
Ideally a complete ground response analysis
needs the rupture mechanism at the source of anearthquake for modelling, the propagation of waves
from the earth source to the top of bedrock beneath aspecific site and then determine how the surface
deposit motion is influenced by soils between it andthe bedrock (Kramer, 1996)

MSF =Based on deterministic sesmic hazard
analysis from Buana & Sadisun (2013a),FS =Yogyakarta earthquake that controlled by fault typesource (Opak Fault) give the largest peak groundacceleration on bed rock (0.34 g) than the subduction
type (0.15 g). Amplification due to Opak Fault shows
value between 1.3 to 2.92 times or average value
around two times (Buana & Sadisun , 2013b).

σ′= 0.653.

σ. ′ . r ....................1σ4.

Discussion

Conclusions
1.

2.3 Standard Penetration Test
Standard Penetration Test is one method tocalculate a cyclic retention ratio. As suggested by
NCEER (National Centre for Earthquake Engineering
Research, 1997), Cyclic Retention Ratio (CRR) value
could be calculated by Blake’s formula as follows;.

MSF.......................................... (4)Liquefaction analysis shows that sand and silt
layers are potential to liquefy. Behavior and respondof each deposit may influenced by density andamplification. When the calculate amplification(Buana & Sadisun, 2013b) to be correlated withdensity of sand or consistency of silt deposit shows
the positive correlation. Amplification factor tend todeclined with increased of density or consistency(Table 1 and Table 2). Even though, no significant
value difference between sand and silt deposits.
Analysis result shows that liquefaction on sandand silt deposits are suitable with the threshold
reccommended by NCEER (1997). Furthermore, sandand silt deposits with high N1(60) value need more
acceleration to liquefy (Figures 3 and 4).

in which: amax = maximum acceleration at the groundsurface; h = total overburden pressure on sand layer
under consideration; ’0 = initial effective
overburden pressure on sand layer; and rd = stress
reduction factor. In this case, amax value on analysis
refer to Ground Response Linier Equivalent Analysis
value from Buana & Sadisun (2013b).

=.

................................................ .(3)where MSF is magnitude scaling factor and M isearthquake magnitude ; CRR7.5 is the cyclic resistanceratio determined for 7.5 earthquake magnitude(obtained from equation 2) ; CSR is cyclic stress ratio
developed in the field due to earthquake shaking(calculated by equation 1) ; FS is factor of safety
against liquefaction for the specific earthquake
magnitude.

2.2 Cyclic Stress
Cyclic stress concept in this paper is an
improvement concept from the Seed and Idriss’s
concept (1971) i.e.: Cyclic Stress Ratio (CSR) thatdefine on Seed et.al. (1983). The cyclic stress ratio isthe ratio of the average cyclic shear stress hdeveloped on horizontal surfaces of the sand as aresult of the cyclic or earthquake loading to the initial
vertical effective stress ’0 acting on the sand layer
before the cyclic stresses were applied. The cyclic
stress ratio developed in the field due to earthquake
shaking can readily be computed from an equation by
Seed et.al. (1983):
CSR =.
.

2.
3.

...........................(2)4.

where CRR7.5 is the cyclic resistance ratio for 7.5
earthquakes magnitude; x = (N1)60; a = 0.048; b =-0.1248; c = -0.004721; d = 0.009578; e = 0.0006136;
f = -0.0003285; g = -1.673E-05; and h =3.714E-06.
(N1)60 is the standard penetration test blow countnormalized to an overburden pressure of
approximately 100 KPa (1 ton/sq ft) and a hammer
energy ratio or hammer efficeincy of 60% (Youd and3Amplification from Opak fault triggeredliquefaction phenomena on sand and silt
deposits of Young Merapi Volcano.
Both deposits have shown potential toliquefy, even though the silt deposit shows
little more retention compared with sand.
Sand deposit will liquefy when N1(60) valueless than 20 (very loose to medium dense)with CSR less than 0.53.
Silt deposits will liquefy when N1(60) valueless than 15 (very soft to stiff) with CSR
triger less than 0.69.

10th Asian Regional Conference of IAEG (2015)

Table 1. Amplification Factor and N-SPT Correlation for sand deposits
N VALUEDENSITYAMPLIFICATION FACTOR AVERAGE<4Very loose
2.22< 10Very loose-loose

1.77

10-30

Medium dense

1.330-50
> 50

Dense
Dense-Very dense

1.25
1.14

Table 2. Amplification Factor and N-SPT Correlation for silt deposits
N VALUECONSISTENCYAMPLIFICATION FACTOR AVERAGE<4Very soft-soft
2.51
5-8
9-15

FirmStiff

2.46
1.67

16-30

Very Stiff

1.41> 30

Hard

1.21

Figure 3. Correlation between CSR and N1(60) on sand (A) and silt (B) deposits

Figure 4. N1(60) value and safety factor against liquefaction correlation4

10th Asian Regional Conference of IAEG (2015)edited by Youd, T.L. and Idriss I.M. (1997),
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Geologi, Bandung.
Seed, H.B. dan Idriss I.M. (1971): Simplified
Procedure for Evaluaiting Soil Liquefaction
Potential, Journal of the Soil Mechanics andFoundation Design, ASCE, Vol. 107, 1249-1274.
Seed, H.B., Idriss I.M., dan Arango I., (1983):
Evaluation of Liquefaction Potential Using Field
Performance Data, Journal of GeotechnicalEngineering, Vol.109 No.3, pp458-482.
Sudarsono, U. dan Sugiyanto (2007): Zonasi
Likuifaksi Daerah Bantul, Daerah Istimewa
Yogyakarta,
Proceedings
AnnualJoint
Convention Bali 2007 The 32nd HAGI, The 36thIAGI, and The 29th IATMI Annual Conferenceand Exhibition, Bali.
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resistance of soils: summary report from the 1996
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Journal of Geotechnical and Geoenvironmental
Engineering, 127:4, 297-313.

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