Volume 6, Issue 2

June 2024

Special Contribution

Residual bearing capacity of scoured shallow foundations andtheir reinforcement effect by press-in sheet piles
Hidetoshi Nishioka
1. Introduction

Professor, Department of Civil and Environmental Engineering
Chuo University

There have been increasing cases of damage to bridges thatcross rivers as flood disasters have become more severe due toclimate change in recent years. Many of the bridges built inJapan prior to World War II are still in use today, but many ofthese bridges are supported by shallow-spread foundations orwood piles, making them particularly susceptible to scouring.
Large displacement occurs in scoured foundations, making thebridge impassable even if the bridge does not collapse. The
following two contradictory points need to be consideredwhen managing the restoration of bridges, which areimportant infrastructure, for a country or local community tosteadily rebuild after a disaster. The first is for paths to be madetemporarily accessible as soon as possible after a disaster, evenin cases of limited performance to some extent. The second isto ensure the structure to be strengthened so that the damage
does not happen again.
This article introduces examples where this form ofmanagement was successful in railway bridges in Japan [1],[2].
Fig. 1 shows the situation during flooding, where a shallow
foundation was scoured, resulting in a residual settlement ofapproximately 300 mm and a residual inclination of
approximately 50/1000. Emergency restoration wasimplemented at an early stage (approximately one month afterthe disaster) after confirming with loading tests up to 90% of
the maximum train load (Fig. 2) that the structure had theminimum bearing capacity to withstand slow train operation.
In this case, in exchange of forgoing foundation reinforcement
work, train speeds were restricted, and an electronicmonitoring system was also used, thereby ensuring safety.
Afterwards, steel pipe piles were pressed in around thescoured foundation in order to reinforce and integrate thestructure (Fig. 3). These types of reinforcement work were
carefully conducted during periods of low rainfall (autumn andwinter in Japan) while trains were running. The reinforcement
work was completed, and the train speed restrictions andelectronic monitoring systems were lifted approximately sixmonths after the disaster.
Generalizing this form of good practice experience requires
clarifying the mechanism by which the residual bearing
capacity of shallow foundations is expressed even after being
damaged by scouring and establishing quantitative design
methods for the reinforcement effect by press-in sheet piles. In
this article, we introduce model experiments using an
aluminum-rods model ground that we are conducting for these5

Fig. 1. View during the flood [1]Fig.2. View of Static loading test by water tank [2]

Fig.3. General drawing of the restoration withPress-in Steel pipe piles [1]Volume 6, Issue 2

June 2024

purposes.

2. Overview of experimental technology using aluminum-rods model ground
The aluminum-rods model ground is a mixture of aluminum rods (specific gravity 2.7) of different diameters of several
millimeters and stacked together with the rods aligned in the same direction as shown in Fig. 4, where a two-dimensional
granular material is simulated. The nonlinearity of the stress-strain relationship and the dilatancy properties are similar
to those of a medium dense sand. Another major advantage is that the information on displacement and porosity changes
in the model ground can be easily obtained from the end face of the aluminum rod through image analysis. Because of
these features, aluminum-rods model ground has sometimes been used as one of the basic techniques for conducting
model experiments targeting two-dimensional boundary value problems in the field of geotechnical engineering sincethe 1960s[3],[4].
Enlarged view
In this experiment, we used a mixture of three types ofaluminum rods with diameters of 1.5 mm, 2.0 mm, and 3.0mm in a weight ratio of 1:1:1. A comparison of its grain size
distribution with that of Toyoura sand, which is widelyused in model experiments in Japan shows that theuniformity coefficient Uc is similar, with a mean diameter
D50 of around 2 mm, which is about 10 times that of
Toyoura sand. Furthermore, the unit weight of thealuminum rods model ground with this grain size
distribution is still within a relatively narrow range ofaround 21–22 kN/m3 even after changing the compaction
method and degree. Additionally, the grain size effectshould be noted in conditions where clear shear bands
tend to occur in the ground, such as in bearing capacity
View of Aluminum Rods
tests. This grain size effect is thought to occur because theFig. 4. Overview of aluminum rods model groundshear band thickness is proportional to the grain diameter.
For example, Toyosawa et al. reported from bearing
capacity tests in centrifuge model tests using dense river sand that the bearing capacity may increase due to the grain
size effect in cases where the breadth of footing B is less than 50 times the D50 value[5]. Using this as a reference, thenwhen conducting bearing capacity tests with a shallow foundation in this aluminum-rods model ground with these grain
size distributions, then the breadth of footing B should be at least 100 mm (approximately 50 times the D50 value).

The internal friction angle of the aluminum rods model ground is often easily estimated by an angle of repose test inwhich an aluminum rod is placed in a sample box and tilted, and the angle of repose obtained as the tilt angle at the timeof collapse is around 29–33°. Additionally, back calculation using the Rankine earth pressure equation from the passive
coefficient of earth pressure and active coefficient of earth pressure obtained from a retaining wall earth pressure
experiment conducted separately yielded an internal friction angle of 28–31°, which is about the same as the angle ofrepose[6]. Furthermore, back-calculation of the bearing capacity factor Nγ, and Nq from bearing capacity tests on the same
aluminum rods model ground also resulted in an internal friction angle in the range of 29-30 degrees.[7]Additionally, in model experiments using aluminum rods model ground, the ground materials themselves considerably
differ, making it difficult to determine a scaling factor of various physical quantities based on a strict similarity rule.
Therefore, the similarity rule in aluminum-rods model ground generally uses the idea that a dimensionless quantity (e.g.,bearing capacity factor Nγ, Nc, Nq, coefficient of earth pressure K) for the physical phenomenon to be reproduced is madeequal between the prototype and the model as needed.

3. Bearing capacity tests of scoured shallow foundation
We conducted vertical loading tests under conditions that artificially simulated the condition in which the ground at thebottom of the foundation was washed away by scouring in order to determine the relationship between scour depth andresidual bearing capacity exerted after a disaster [8],[9].
Fig. 5. shows a reproduction of scouring. The condition in which a hole is created by scouring on the upstream side of a6

Volume 6, Issue 2

June 2024

bridge pier is reproduced by removing an aluminum rod, andthe scour depth d is used as the main parameter. After thescouring was simulated, vertical load was applied by using ajack, and the relationship between the vertical load andsettlement was measured. Fig. 6 shows the results.
In Fig. 6, the settlement was set to zero before simulating
scouring (extracting aluminum rod). Additionally, the dead load(65 N) was approximately 1/3 of the ultimate bearing capacity
under conditions where no scouring occurs. The displacement
when this dead load was applied represents the residual
displacement after a disaster. The results showed that theresidual displacement after a disaster increased with increasing
scour depth d.

Fig. 5. Modeling scour by removing aluminum rods

Meanwhile, as the loading continued, the load tended to increase from the vicinity of the residual displacement, showingthat the maximum load (i.e., residual bearing capacity after disaster) increased to about the same level as in the casewith no scouring. In other words, even if scouring caused damage that induced residual displacement, the expected
residual bearing capacity itself after the disaster may not decrease by much. This is consistent with the actual cases of
emergency restoration shown in Figs. 1–3, where the residual bearing capacity needed for train operation could be
expected without reinforcing the foundation.
300

After disaster250

Before disaster

Load (N)

200
150
100

Dead load (65 N)

50002468Residual settlement(d=32mm)
d=0mm

d=13mm101214161820

10% of the foundation width (10 mm)

Settlement (mm)d=19mmd=26mm22d=32mm

Fig. 6. Load-settlement curves after scour
4. Bearing capacity test of shallow foundation reinforced with press-in sheet piles
The restoration of foundations damaged by scouring (or preventative reinforcement prior to the disaster) requires workin rivers, so construction methods that do not use cement or concrete (or use only a small amount) are advantageous forwater quality conservation, and the use of press-in construction methods is expected. Sheet pile foundations thatcombine sheet piles and footings have previously been developed[10],[11],[12], and they are also used to reinforce river
bridge piers[1]. In general, sheet piles and footings should be rigidly connected and integrated in order to obtain highreinforcement effect with less material. However, in the case of reinforcement for scoured foundations in the river,construction efficiency and economy can be improved by eliminating the construction period, cost, and space requiredfor integrating the existing footing with the additional sheet pile. Therefore, an experiment was conducted to confirm thereinforcement effect when integration with the existing footing was omitted (however, the heads of the added sheet piles
were connected)[13].
Fig. 7 shows the experimental setup. A sheet pile model (aluminum plate) was installed around the footing with a breadthof footing B = 100 mm, and a vertical load was applied. The sheet piles and footing were not connected, but the sheet
piles on both sides were connected at the head. The main experimental parameter was the penetration length L of thesheet pile model.
7Volume 6, Issue 2

June 2024

Fig. 7. Experimental setup for confirming reinforcement effect by sheet piles

Fig. 8 shows the load-displacement relationship fromunreinforced cases (Case L0) to cases reinforced withsheet piles with penetration length L = 20–140 mm(0.2–1.4 times the breadth of footing B) (Case L20–CaseL140). It can be confirmed in the figure that the bearing
capacity increases due to penetration.
Fig. 9 shows the relationship between penetration
length L and ultimate bearing capacity (maximum
vertical load up to 10 mm settlement). The results
confirmed that the reinforcement effect is notproportional to penetration length L but rather changes
in a step-wise manner with 0.8 times the breadth offooting B (L = 80 mm) as the boundary.
Fig. 8. Load-settlement curves (effect of penetration length L)

5. Conclusion
The press-in construction method is very much suitedfor post-disaster restoration and preliminary
reinforcement work for river bridge pier foundations,where there is increasing risk of scouring damage, butthere is still insufficient accuracy of evaluating themechanical performance before and afterreinforcement. In this article, we introduced
experimental examples of residual bearing capacity
immediately after a disaster and the effects ofadvance reinforcement. Evaluations are currently
limited to those that are independent from eachother, but in the future, we plan to conduct combinedevaluations as well as consider seismic performanceafter reinforcement. We hope to continue these
examinations and establish a logical evaluation anddesign method to contribute toward the realization ofa safe and more secure society.

Fig. 9. Relationship between L and ultimate bearing capacity8Volume 6, Issue 2

June 2024

Acknowledgements
The author expresses sincere gratitude to the following graduate students of Chuo University who performed the model
tests shown in this article: Ms. Yuna Sasaki, Ms. Moeka Hirano, and Mr. Ryuto Shikakura. And some part of this work was
supported by the JSPS KAKENHI Grant-in-aid for Scientific Research (c) (Grant Number JP20K04687).

References[1] Kuroki, Y., Nishioka, H., Kumabe, K. and Koda, M. (2018): Reinforcement of Damaged Bridge Pier by Scouring Using
Steel Pipe Piles, Proc. of the First Int. Conf. on Press-in Engineering 2018, Kochi, pp.549-554.
https://www.press-in.org/ja/publication/freefile/86/180[2] Nishioka, H., Shinoda, M., Sumi, Y. and Yamate, H. (2013): The Vertical Loading Test and Train Running Test to theBridge Pier of Spread Foundation which was Damaged by Scouring Settlement, 48th Japan National Conference on
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[8] Sasaki, Y., and Nishioka, H. (2022): Vertical Loading Tests on Local Scoured Spread Foundation on Aluminum Rods
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[10] Nishioka, H. (2017) Case-history Development of Sheet Pile Foundation, IPA Newsletter, Vol.2, Issue 4, pp.13-16.
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Int. J. of Physical Modelling in Geotechnics 10: 25-34, 2010. https://doi.org/10.1680/ijpmg.2010.10.2.25[13] Shikakura, R., Nishioka, H., Yamakuri, Y., Arai, Y. and Hirano, M. (2024): Experimental study on the influence ofpenetration length and eccentricity on the bearing capacity of sheet pile foundations, Proc. of the 3rd Int. Conf. on
Press-in Engineering, Singapore. (in press)9