Risk Assessment and Mitigating Measures
Regarding Pile Installation at EBS Biohub Jetty
Ir. drs. R.E.P. DE NIJS a , ir. A.C.A. NAP b , W. KORTE c and W.J. NEDERLOF c a
Witteveen+Bos, Deventer, The Netherlands
b
Port of Rotterdam, Rotterdam, The Netherlands
c
GEKA, Dordrecht, The Netherlands
Abstract. During the design phase of the reconstruction of the existing mooring facility “EBS Biohub” at the port of Rotterdam, the Netherlands, a high risk profile was recognized in the proximity of four new dolphins next to the existing jetty. The risk originated from the greater installation depth of the dolphins related to the limited foundation depth of the existing foundations and from the structural analysis which indicated that the superstructure could only withstand limited settlement differences. Risk management aspects were incorporated in the contract by defining an observational approach involving monitoring of the structure and the subsoil. The contractor Geka offered a pile test at dolphins to be installed at a safe distance from the jetty. A number of test piles were installed at varying distances from the dolphins in order to detect settlements. The induced settlements were compared with a contractually defined criterion. Based on the test, two installation methods were in place. Two dolphins were installed under a slope in order to create a minimum of 5 m distance between the installation trajectory and adjacent piles. Two dolphins at shorter distance were placed while two adjacent piles per dolphin were cut from the jetty platform with an alternative support by a reaction frame with active hydraulic compensation at service load. All criterion on bearing capacity, vibration levels, settlements, deformations and cone resistance were met. No damage on nor deformation of the existing structure was encountered resulting in a success in terms of budget, planning and risk management.
Keywords. risk management, settlement, capacity, contract, observational
1. Project Introduction
The project comprised the reconstruction and extension of the existing mooring facility “EBS Biohub” at the port of Rotterdam, the Netherlands, see Figure 1.
Figure 1. Initial northern aerial view jetty EBS “Bio Hub”.
The jetty had an initial length of 230 m and a width of 16 m. Part of the reconstruction involved the installation of seven mooring dolphins of which four dolphins were foreseen at close proximity of the existing jetty, see Figure 2.
Figure 2. Existing jetty and new dolphins M1 to M4 The existing jetty structure consisted of two longitudinal loader beams with a crane rail on top, supported at 4.5 m intervals by a joke of two hollow pre stressed piles per loader beam. At each joke a pre stressed cross beam was mounted,
© 2015 The authors and IOS Press.
This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License.
44 in total, with a length of 11 m. According to the as built information of the jetty, the piles were founded at 3 m depth in the first sand layer, see Figure 3. The foundation level was estimated at NAP -23 m, 6 m below dredging depth. The new dolphins had a tip level of NAP -35 m.
Figure 3. loader beam, cross beam and pile foundation
2. Risk Analysis
A high risk profile was recognized in the proximity of four new dolphins next to the existing jetty. The geotechnical risk originated from the installation depth of the dolphins related to the limited foundation depth of the existing pile foundations and low density of the sand layer at pile tip level, see Figure 4. Further analysis was based on a potential volume loss in the trajectory from pile tip level (NAP -23 m) to dolphin installation depth (NAP -35 m), marked in Figure 4. Based on previous projects involving (sheet)pile driving in granulair soils (de Nijs 2003, de Nijs 2015) an estimated lower boundary on compaction was used in a settlement analysis. The lower boundary was chosen in order to define a minimum risk level in the analysis of the
structural risk assessment. The following paragraphs will explain the analyses.
Figure 4. electrical Cone Penetration Test 15 2.1. Effect of New Pile Driving
During pile installation, either by use of a hammer or a vibratory hammer, volume loss due to compaction caused by the driving energy should be anticipated in loosely packed granular soils. A lower bound compaction of 1% over 0.75 meter from the dolphin had been estimated. This volume loss was applied over the installation depth below pile tip level. Since the dolphins were to be installed within 2 m from the piles, a lower boundary settlement of 20 mm at pile tip level was to be considered in the structural analysis, see Figure 5.
Figure 5. Predicted volume loss, lower bound -30 -25 -20 -15 -10 -5 0 0 5 10 15 s e ttle m e n t (m m )
distance between dolphin and pile (m)
Prediction installation effect NAP -23 m in trajecory NAP -23 m to -35 m
1% volume loss Pile tip
Sand layer wedge
2.2. Impact on Superstructure
The main elements form the superstructure, the longitudinal loader beams, had a rigid behavior. The connection with the cross beams and piles acted as hinges. The limited thickness, 8 cm, of the in situ casted deck implied it couldn’t redistribute alternative loading caused by deformations. At pile settlement, the 4 to 1 slope of the piles would cause a movement in horizontal direction or would require a reaction force, see the red marked pile in Figure 6.
The horizontal displacement would initiate a tension load in the cross beam, since the unaffected surrounding piles outside the active wedge would not respond to the deformations. The connections with the loader beams could not withstand this additional load. As a result the deck would be damaged and the crane rail distance would exceed the horizontal tolerance. From the structural analysis in FEM it was found that the jetty could only withstand limited settlement differences between piles to a maximum of 10mm.
Figure 6. Impression on structural deformation
3. Contractual Approach
3.1. Contractual Approach on Risks
The analysis indicated a high risk on damage to the jetty as a result of pile settlements induced by dolphin installation by means of hammering. From a clients point of view The Port of Rotterdam perceived an uncertain outcome if only the requirement of a maximum settlement of 10 mm would be contracted. The specification itself could not guarantee that the requirement would be met by the contractor. Exceeding the criterion was a most unfavorable outcome for the client as well as the contractor, introducing new uncertainties on repair and future usability of the jetty.
The alternative however, a reliable and cost effective design or procedure on pile installation to mitigate this risk, proved to be very difficult to design and define in a contract. Especially the practical aspects of pile installation or structural mitigation were difficult to estimate or to define.
In response to this uncertainty the client adopted the strategy to invite contractors to offer a risk management procedure. In the selection procedure the contractor would benefit from the offered quality regarding settlement control and preservation of structural integrity. The offered risk management was therefore incorporated in the bid and ultimately in the contract between client and contractor.
In order to allow the contractor to optimize and demonstrate its risk management procedure, the observational method was in place. Since three dolphins could be installed prior to the four critical ones, a learning curve on site was possible. The contractor was also allowed to reduce pile driving energy by means of flushing the dolphin internally up to 2 m above tip level.
3.2. Preservation of Structural Integrity
The contractor GEKA offered a plan which consisted of a monitored trial run on two dolphins, M6 and M7, to be installed at a safe distance from the jetty and its pile foundation. The dolphin positions would be surrounded by nine test piles in comparable size and depth to the piles of the existing jetty. The test piles would be installed at varying distances from the Sand layer
new dolphins. The settlements of the piles induced by the installation of the dolphins would be recorded. Vertical load was applied at piles P3 to P5. The site overview is presented in Figure 7.
Figure 7. Overview test piles at dolphins M6 and M7 Based on this test the recorded settlement would be compared with the contractually pre defined maximum settlement criterion of 10 mm. In case the criterion would be exceeded, two alternative methods would be in place for the remaining four critical dolphins to be installed.
The first alternative was to position the dolphins under a slope 7 to 1 in order to optimize the distance of the installation trajectory to adjacent piles, see Figure 8. The second option, see Figure 9, was to cut the piles loose from the jetty platform and use a hydraulic compensation frame in order to pressurize them.
Figure 8. Dolphin install4ed with slope 7 to 1.
4. The Works
4.1. Test piles
First a series of nine test piles were driven in the vicinity of the installation positions of dolphins M6 and M7. The impact on the subsoil after installation of the dolphins was recorded by leveling the surrounding test piles, see Figure 10.
Figure 10. Dolphin installation with adjacent test piles The results were collected and interpreted, see Figure 11. The results proved that the lower bound prediction of 1% volume loss was exceeded significantly on all test piles, loaded as well as unloaded. The results indicated a volume loss in the range of 2 to 3%, regardless of the installation equipment of the dolphins.
Figure 11. Settlements test piles after dolphin installation
This implied that the encounterd settlement was most certainly too big in order to allow regular dolphin installation at the jetty. Based on the results at dolphins M6 and M7 the contractor concluded that both alternative dolphin installation methods were in place.
4.2. Installation of Dolphins at Slope 7 to 1
The dolphins M3 and M4 were installed by a diesel hammer type Delmag D100 under a optimized slope of 7 to 1, see Figure 12. At this slope, the distance at the foundation depth between existing piles would meet the minimum distance of 5 m. At a distance of 5 m or more between the existing and new piles , the slope solution was sufficient. The hammer operated at 75% of its capacity, since the application of full energy seemed to increase settlements at M6 en M7. Another important aspect was the reduction of soil resistance in the dolphin by means of flushing the piles internally up to 2 m above the tip. This remedy was in place in case the blow count exceeded 65 blows / 25 cm. At dolphin M3 the blowcount reduced from 70 to 21 blows / 25 cm after flushing. Non continuous hammering did not cause additional problems.
Figure 12. Dolphin installation with slope 7 to 1.
-100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 0 2 4 6 8 10 12 14 s e ttle m e n t (m m )
distance test pile - dolphin (m)
Settlement testpiles
lower bound prediction -1% P1 -23 unloaded P2 -23 unloaded P3 -23 pile load P4 -23 pile load P5 -23 pile load P6 -25.8 unloaded P7 -25 unloaded P8 -23 unloaded vibratory P9 -23 unloaded vibratory postdiction -3% and -2%
4.3. Active Support During Hammering Dolphins
The dolphins M1 and M2 were installed in the same manner and with the same equipment as M3 and M4, although vertical. At these positions the jetty was also equipped with a rigid connection between the two loader beams, see Figure 13. The connection was also supported by piles, hindering the 7 to 1 slope solution.
Figure 13. Dolphin positions M1 and M2, vertically installed
Figure 14. Frame for hydraulic compensation
5. Resume
In the reconstruction of the existing mooring facility “EBS Biohub” at the port of Rotterdam, the Netherlands, a high risk profile was recognized in the proximity of four new dolphins next to the existing jetty. The contractually requested risk management allowed the contractor sufficient freedom to implement specific knowledge on pile installation and settlement control. During the works, the importance was confirmed at the installation test of three dolphins at safe distance from the jetty. The mitigating measures, slope installation and hydraulic compensation, proved to meet the requirements on settlement control and structural integrity. All criterion on bearing capacity, vibration levels, settlements, deformations and cone resistance were met.
References
Nijs, R.E.P. de (2003). Installation of sheetpiles in granulair soil, Geotechniek, october 2003, 46-54. (in Dutch) Nijs, R.E.P de, Kaalberg, F.J, Osselaer, G, Couck, J. van
Royen, K. (2015). Full scale field test (sheet)pile drivability in Antwerp (Belgium), Proceedings of XVI ECSMGE 2015, Edinburgh, 13-17 September 2015. Per dolphin the adjacent piles with a slope
towards the dolphin were cut and hydraulically
compensated, see the marked piles in Figure 13.
Before cutting the piles, a reaction frame was
installed, see Figure 14. The piles were loaded up
to service load. During installation of the dolphins the system was kept at its original load by hydraulics. During driving of the dolphins approximately 40 mm settlement occurred.
