STUDY ON SEISMIC PERFORMANCE AND
STRUCTURAL INSPECTION OF RETAINING WALL
Kosh Nath Adhikari*
MEE06001

Supervisor: Dr. Masahito TAMURA**ABSTRACT
Seismic performance of concrete block retaining wall with concrete backing was evaluated by the shaking
table test and compared for acceleration and displacement at various nodes with the theoretical
calculation by finite element method. Two dimensional elasto-plastic dynamic analyses were performedby using recorded Kobe NS component acceleration as input motion for both cases. Structural inspection
helps to alleviate secondary disaster after an earthquake. A regular inspection of retaining walls in thevicinity of inhabitants is necessary not to allow the situation of retaining walls to worsen by the naturalagencies. Proposed manual for inspection will be helpful for the inspection and rating the level of risk
which will inspire residents for regular upkeep of retaining walls. Finally, a newly developed nondestructive Surface Wave Method (SWM) for the evaluation of stiffness of retaining wall is introduced.
Keywords: Seismic performance, elasto-plastic, structural inspection, surface wave method.

INTRODUCTION
Nepal is located at the boundary of Indian and Tibetan tectonic plates which are colliding with each other.
Because of its location, Nepal is prone to large magnitude earthquakes and earthquakes in the past haveclaimed thousands of lives and properties. Retaining wall is an indispensable structure whether it is for theconstruction of building and other physical infrastructures or for the slope stabilization measures in Nepal
owing to its fragile and difficult nature of topography. A retaining wall is a structure whose primary
purpose is to provide lateral support for soils at slopes steeper than their angle of repose.
Shaking table tests for both concrete block retaining walls without concrete backing and with concrete
backing were observed during the study and the obtained results were compared with Finite Element
Analysis for the case of retaining wall with concrete backing. FEM is the only reliable method to observe
behaviors of such structures and it has importance for our country where there is no lab for conducting theshaking table test. Generating awareness about the surrounding environmental condition of retaining wall
plays a vital role among the parties concerned for the regular inspection of retaining walls not to allow thesituation to worsen. A inspection guideline is utmost important and the practice has been started in thispaper with the help of already prepared manuals of Japan and field visit right after occurrence ofearthquakes at Noto and Niigata in 2007.
Survey data analyzing part is an important work which I have performed here about structural inspectionof retaining walls for Ueno and Akabane areas of Japan. This survey was conducted by Building ResearchInstitute (BRI) with cooperation of housing companies and universities of Japan. Newly developed nondestructive test for structural evaluation of retaining wall by Surface Wave Method (SWM) was studied as
an introductory part to continue it further in my future study.
* Engineer, Dept. of Urban Development and Building construction, Nepal, ** Chief Research Engineer,International Institute for Seismology and Earthquake Engineering

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SHAKING TABLE TEST AND FINITE ELEMENT METHOD
Shaking table testPreview
Concrete block retaining wall without concrete backing and with concrete backing were two types ofspecimens for one dimensional full scale shaking test. The inclinations of walls were nearly 780 toresemble the field practices in Japan though the prevailing code has restricted the construction of former
type retaining wall. Overall dimensions of specimen are illustrated in figure 1.

CONCRETE BACKING

SAND

STEEL
TANK

0.3

CONCRETE
BLOCK
0.4X0.25X0.35

SECTIONAL ELEVATION OF BLOCK RETAINING WALL
WITH CONCRETE BACKING

Figure 1 Cross section of specimen(All the dimensions are in meter)

Photo 1 specimen after
application of 818 gal

Photo 2 Specimen after
application of 1000gal

The design compressive strength of concrete block was 18N/mm2. Edosaki sand having physicalproperties as wet density of 16.3KN/m3, dry density 14.29 KN/m3 and the natural water content 14.1%
was used as backfill material.

Test procedure
Shaking was performed by applying Kobe earthquake NS component recorded by the Japan Metrological
Agency (JMA). Input level of acceleration was 100gal, 200gal, 400gal and 818 gal. After the applicationof acceleration of 400 gal, some displacement on the top of the soil and the retaining wall was noticed.
After the application of 818 gal acceleration, settlement in soil and gap between the wall and soil werewidened though the specimen still survived as shown in photo 1.
Surface Wave Velocity method for the evaluation of stiffness of the retaining wall and impulse hammer
test to evaluate the response of the retaining wall was performed as non destructive tests before
commencement of shaking table test. A Swedish Weight Sounding test was performed for the backfill
material to know the soil stratification, N values and unconfined compressive strength of soil. The sameprocedures were followed for both specimens. Wall without concrete backing completely collapsed after
the application of 1000gal acceleration but wall with concrete backing was tilted away from the backfill asshown in photo 2.
Finite Element MethodPreview
Elasto-plastic two dimensional dynamic analyses were performed for the concrete block retaining wallwith concrete backing by the use of Amiko software.

2Procedure
SWS N-value No-2(1)(2)
fitted curve
N-value0 1 2 3 4 5 60
Depth in m

0.5
1
1.5
2
2.5
3

N=3.1√Depth

There were two sets of test data from Swedish Weight Sounding test which
are plotted as scattered plot. Finally a best fit curve as shown in figure 2. Nvalue is assumed to be constant for the depth of each 0.5m. Young’s
Modulus of Elasticity, Eo,in MN/m2 of soil is determined by the equation(1) for each 0.5 meter depth. Cohesion for soil and concrete were assumed as
1Kpa and 1.0E+20 Kpa for blocks and concrete respectively.
Internal frictional angle, Ø, of soil was assumed as 35° for the calculation.
Dilatancy angle, ψ was calculated by the equation 2 which was taken fromthe Introduction to soil strength and ground failure published by Japan
Geotechnical Society (JGS) in 1995, unit weight of soil and concrete block, γ,
is assumed as 16 KN/m³ and 20 KN/m3. Poisson’s ratio n, was 0.35 for soiland for concrete 0.15.

Figure 2 Best fit SWS profile

E o = 1.4 N

Eq.(1)(Relation is taken from Recommendation for the design of Building Foundations in 2001 published byAIJ.)ψ = φ − 30 0Eq.(2)Assigned different material properties are illustrated in figure 3. Legends 1 and 4 to 8 denote soil 2
represents the concrete block wall 3 represents the filter medium gravel and 9 indicate concrete backing.
No 315No 432

No 493Figure 3 material properties

Figure 4 Mesh and node numbers

Comparison of test and calculated results
Comparison of Results at Node 493
1500

5001000

Accleration in gal

Acceleration in gal

Comparison of Acc. Results at Node 432
10000-500 4142434

Test Result
Calculated-1000-1500-2000

500
Test Result0-500 4142434-1000-1500

Time in sec

Time in sec

Figure 5 Comparison of acceleration with the input acceleration of 818 gal

3Test ResultComparison of Horizontal Displacement at Node493

50

Displacement in mm

Displacement in mm

Comparison of Horizontal Displacement at Node43204142434

Test Result-50

Calculated Result-100-150

10-14142434

Test Result
Calculated Result-2-3-4Time in sec

Time in secs

Figure 6 Comparison of displacements with the input acceleration of 400 gal

800
600
4002000-200 4-400-600-800

Comparison of horizontal accleration at Node 493
600
Accleration in gal

Accleration in gal

Comparison of horizontal accleration at Node 315

Test Result
142434

Calculated

400200
Test Result0-200 4142434

Calculated-400-600

Time in sec

Time in sec

Figure 7 Comparison of accelerations with the input acceleration of 400 gal
Comparison of horizontal displacement at Node493

4Displacement in mmDisplacement in mm

Comparison of horizontal displacement at Node432

20-2 4142434

Test Result
Calculated-4-6-81.5
1
Test Result

0.5

Calculated

04142434-0.5

Time in sec

Time in sec

Figure 8 Comparison of displacements with the input acceleration of 400 gal
The wave forms of both the figures 5 and 7 are consistent for acceleration but the displacement in figure 6were not consistent. FEA was again run with changing angle of internal friction from 350 to 300 by using
the same 818 gal acceleration, noticeable difference was not obtained. Finally, Acceleration of 400 gal
was used for analysis and the obtained displacements at various points were nearly consistent as shown infigure 8. It is found that when the attached surfaces within the body of retaining wall are separated then it
does not show displacements in big quake.

STRUCTURAL INSPECTION
Structural inspection helps to alleviate or avoid secondary disasters right after an earthquake. A standard
inspection and evaluation procedure is necessary to conduct a survey on the damaged area of earth
retaining structures to determine the degree of hazards. The life of structure decreases with increase intime. Environmental factors such as rain, sun and vegetation will deteriorate the structures and cracks willappear. Building Research Institute had conducted survey of Akabane and Ueno area in Tokyo
metropolitan area with the cooperation of housing companies and Universities of Japan. Nearly 230
retaining walls were surveyed and analysis was performed and the findings about weep holes, drainage
condition and all types of cracks based on Yokohama City’s criteria were presented.

4Checking algorithm for proposed manual of Nepal
Checking for the surrounding environmental conditions and othera) Weep hole b) Exuded water c) Drainage facility
A (Reference points of retaining wall) = (The largest one is used out of scores in a) to c)

2. Checking based on the retaining wall for any:a) Cracks b) Horizontal displacement c) Differential settlementd) Clearance at the external cornere) Bulgef) Inclination/ Breakage
B (Changed points of retaining wall) = (the largest one is used out of scores in 1) to 6)

3. Finally, evaluating the retaining wall comprehensively
According to the obtained total score mentioned {(reference points of retaining wall A +(Changed points of retaining wall B)}, the safety of the retaining wall is ranked into any ofthree levels:
I Almost safe, score<=5 II Relatively unsafe, Score >5 to<=9 III High risk, Score >9Namely, the retaining wall of house lot with higher total score is at higher risk

FIELD INVESTIGATION OF RETAINING WALL DAMAGED BY EARTHQUAKES
Noto and Niigata
A magnitude of 6.9 earthquakes hit the Hokurika region of Japan near thePeninsula by 2007, Noto Hanto Earthquake on 25 March, 2007. One person
was killed in Wajima city and 170 people were injured. Toge area has a steep
slope more than 200 and retaining walls are found to be constructed to support
the approach roads embankment and houses. Failures of retaining walls arefound due to increased earth pressure during earthquake as shown in photo 3
Photo 3 Failure of wall
A couple of powerful earthquakes of 6+ in Shindo scale jolted in northwestern coast at Niigata prefecture
about 14 hours interval, death toll of 11 and injured more than 1890 in Japan by Niigata Chuetsu-Oki
Earthquake 2007 on 16 July, 2007.

Photo 4 Overturned wall

Elderly people who were living alone in the old wooden houses were badly
suffered in the earthquake. The soil of the affected area consists of fine sand.
The outward thrust from the back fill soil was found to be the main cause of
failure of retaining walls with slip of the soil being low shearing resistance.
Most of the walls which were erected vertically without provision of front
slope were severely affected. So, failure of retaining structures was the slip ofthe earth surface followed by the inadequate capability of retaining walls tosustain the earth pressure during earthquake as shown in photo 4.

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CONCLUSIONS
Seismic performances of retaining walls prepared by dry concrete blocks without concrete backing andwith backing were viewed for shaking table tests by applying recorded Kobe NS component.
1) The wave form of acceleration obtained from the both shaking table test and finite element method is
consistent at different measured nodes.
2) The displacements obtained at different locations in two cases are different for 818 gal input level ofacceleration and found nearly same for 400 gal acceleration.
Structural inspection helps to alleviate loss of lives and properties caused by secondary disaster after anearthquake and this manual will be a means to judge the level of risk.
3) To generate and disseminate awareness for earthquake disaster mitigation about the existing retaining
wall to residents to encourage for regular environmental inspection.
Field investigation right after earthquakes in 2007, Noto Hanto Earthquake and 2007, Niigata ChuetsuOki Earthquake has found similar types of impacts on retaining walls and the major causes of failure werefailure of substructures, front slope more than 600, insufficient thickness of wall, out-of-plane and in-plane
failures and slip of ground surfaces.

ACKNOWLEDGEMENT
I would like to express my thanks to Associate Professor Dr Akihiko WAKAI of Gunma University, Dr.
Tsutomu HIRADE, Senior Research Engineer of Building Research Institute, Japan, Dr. Akio ABE ofTokyo Soil Research Company Limited for their guidance.

REFERENCES
AIJ, (2001) Recommendation for Design of Building Foundation, 2nd edition Tokyo: Architectural
Institute of Japan.
Bajra M. Das., (2007) Principal of Foundation Engineering, 6th edition A division of Thomson CanadaLimited.
Coduto D.P., (2001) Foundation Design/ Principal and Practices, 2nd edition Prentice-Hall,Inc.,New Jersey.
JGS, (1995) Introduction to Soil Strength and Ground Failure: Japan Geotechnical Society.
Joseph E. Bowels, (1996) Foundation Analysis and Design, 5th edition McGraw-Hill Companies,Inc.
MLIT, Retaining wall-Quick Inspection Guide, Japan. Evaluation of degree of hazard for the residentialsite by the council on network of evaluation of degree of hazard for the residential site.
National Building Code of Nepal (2004).

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