During recent construction of an overpass bridge on soft ground, piled ground improvements were used to reduce the settlement of the bridge abutments and approach embankments. The piles were installed prior to construction of the approach embankments, under which significant consolidation settlement would occur due to placement of the embankment and surcharge fill. During construction, the horizontal movements of the pile groups in response to lateral loading from soft soil consolidation were significantly higher than predicted and beyond acceptable limits.
This paper presents the observed pattern of lateral ground movement across the pile group due to soft soil consolidation, supported by instrumentation records gathered during construction. The deformation mechanism, and suitability of various calculation methods for predicting the lateral movement of pile groups due to soft soil consolidation will be discussed. The remedial design process will be presented, with the range of remedial options considered before the ultimate decision was made to overload the pile groups, then deconstruct and re-pile the abutments.
As part of a recent transportation project, a new overpass bridge was required. The site ground conditions comprised approximately 6.5m of soft compressible soil overlying loose to dense silty sand and sand. The groundwater table varied seasonally, from approximately 1.5m below ground level to near the ground surface.
The chosen design comprised a bridge superstructure of precast pre-tensioned Super-T beams on shallow bankseat pad foundations. The bankseat pad foundations were in turn supported by mechanically stabilised earth (MSE) abutments with a raft of six rows ground improvement timber piles and a load transfer platform supporting the overlying MSE abutment and bridge. The ground improvement piles were driven approximately 3m into the sand to found on a dense layer.
The structure was well instrumented for construction, with instrumentation comprising three inclinometers, survey prisms on the facing of the MSE wall, and settlement plates beneath the approach embankments.
The structural arrangement, monitoring instruments and ground model are shown in section in Figure 1 below.
Figure 1 : Ground model and structural arrangement
The intended construction sequence was as follows:
- Ground improvement piles are installed beneath the retaining structure and bridge abutments at 1.4m c/c spacings, together with wick drains beneath the approach embankment at 2m c/c spacings.
- A load transfer platform is constructed over the pile group.
- The approach embankment and retaining structure are built at the same rate, to reach final embankment height.
- Surcharge is placed over the wick drained area and a surcharge hold period completed.
- Surcharge is removed, the bridge bankseat is cast and precast beams placed prior to final paving and finishing works.
A key feature of the site was the soft compressible soils beneath the approach embankment, where up to 1.6m of consolidation settlement would eventually occur in the upper 6.5m of soft soil. Due to the construction sequence, the bridge abutment piles were installed prior to the embankment construction and the soil was not pre-loaded prior to pile installation, so consolidation settlement occurred during construction of the retaining wall and approach embankment. Both abutments behaved in a similar manner, so results of one abutment only are presented.
LATREAL DEFORMATION MONITORING
As construction progressed, it became evident that the lateral deformations of the retaining wall and piles would exceed acceptable limits. Lateral deformations began to quickly accumulate once the approach embankment height exceeded approximately 3m, progressing at a ratio of approximately 20% of the vertical settlement of the approach soils. Construction was paused at a height of 4.0m while a solution was found to limit, or accept, the lateral deformations of the system. Plots of lateral deformations and vertical approach settlement with time are shown in Figure 2 below.
Figure : Observed settlement and lateral displacement during early construction
The lateral deformation resulted from bending of the ground improvement piles due to horizontal load from the consolidating soils. The pattern of deformation varied significantly across the pile group, as shown by the modelled deformation pattern in Figure 3, which is supported by the inclinometer records. The maximum lateral ground movements occurred behind the pile group at approximately a third the depth of the soft soil layer with the back row of piles being put into double-bending. The front row of piles were deflected in a typical cantilevered shape with the base of the piles fixed in the sandy soils at depth, and the load applied to the pile head causing pile movements.
Figure : Modelled pattern of lateral deformation through the pile group
Once construction had been paused, options were considered for remediating the bridge structure to achieve a compliant and resilient design. Through investigating remedial options, key features were observed regarding the lateral deformations that guided the design solution:
- The lateral movement of the structure occurred in conjunction with approach embankment settlement. Stopping settlement of the approach embankment would also stop the lateral movement.
- The ratio of lateral movement to vertical settlement was only slightly affected by the drainage state and excess pore water pressure in the approach embankment. During undrained initial loading, when excess pore water pressures were high, the ratio of lateral movement to vertical settlement peaked at approximately 30%. During ongoing settlement after dissipation of the excess pressure, this ratio dropped to approximately 15% to 20%. Drainage measures or changes to construction rates were therefore likely to have only a limited impact on the lateral movements.
- The lateral movement was not easily limited through physical restraint. Structural solutions were investigated for restraining the abutment retaining wall, but even large diameter contiguous bored pile walls were found to be unlikely to stop the lateral movement. To limit remaining movement to less than approximately 30mm required extremely rigid solutions such as contiguous CFA pile cell structures or similar extensive cement stabilisation.
Following investigation into available designs, the potential design solutions were narrowed down to the following three options:
- Option 1) Construct the approach embankment from Polystyrene Geofoam, stopping settlement and lateral deformation by removing the additional weight of the approach embankment.
- Option 2) Install rigid ground improvement through the approach embankment from the partial construction height to prevent further settlement.
- Option 3) Drive out the consolidation settlement through surcharging to above the design height, then excavating the abutment retaining wall, installing replacement ground improvement piles and re-building the abutment.
Following assessment and pricing of the remedial options it was decided to pursue Option 3. By driving out the existing settlement and then simply reconstructing the ground improvement piles and MSE wall this option was assessed as having the greatest cost-certainty, with a highly resilient end design that was not sensitive to the details of the numerical modelling of the abutment. The key challenge and risk with this design option was the temporary stability of the abutment during construction. The construction staging for Option 3 is shown in Figure 4 below:
Figure ) Construction staging for re-piling abutment
To meet programme constraints, the embankment and surcharge was required to be constructed from 4m to over 11m at a rate of almost 1m per week. This construction rate was expected to generate excess pore water pressures exceeding 100kPa in the consolidating soils, cause lateral deformations in the order of 0.3 to 0.5m and significantly exceed the structural capacity of the existing ground improvement piles which were to be replaced following consolidation settlement.
Additional high-strength geotextiles were specified and installed to improve the stability of the abutment during overloading, allowing the rapid construction rate without resulting in slope failure.
OBSERVATIONS DURING SURCHARGING
During the rapid filling of the bridge abutment prior to re-piling, large settlements and lateral deformations occurred, with the pattern of deformation continuing to develop as outlined in Section 2 above. Lateral movements averaged slightly over 30% of the vertical approach settlement during filling, as pore water pressure built to approximately 100kPa (10m excess pressure).
Figure : Pore pressure, horizontal and vertical movement during surcharging
Based on the deflections indicated by the inclinometer, the deflected shape of the back row of piles continued to develop following the pattern shown in Figure 3, exceeded the section capacity in two locations near the centre of the soft soil and at the top of the underlying sand layer. Significant settlement of the back row of piles occurred during construction, with the pile heads approximately 200-300mm lower than installed height when they were excavated during re-piling operations. Despite being predicted to have exceeded their section capacity, the piles were not observed to buckle or deform in an uncontrolled manner. Based on analysis of the soil lateral capacity, it is considered that the soft soil had enough strength to restrain the piles from buckling despite their significantly deflected shape.
Following surcharging of the approach embankment, the excavation, re-piling and reconstruction of the abutment retaining wall occurred without significant further movements. Soil rebound and reconsolidation resulted in approximately 40mm horizontal movement of the retaining wall during reconstruction, which was predicted and well within acceptable limits.
SUITABILITY OF DESIGN METHODS
The design relied heavily on finite element modelling through Plaxis and RS2, which was able to closely match the observed behaviour and deformations of the system. In the authors view, there was no force-based technique that adequately captured the complex deformation pattern of the pile group or enabled meaningful predictions of the total deformation and pile actions. An elastic plastic finite element model was useful for showing the system behaviour, although to produce a closely matched back-analysis, and to predict the time dependent construction behaviour, a soft soil creep model was used for the detailed design.
The primary factor influencing the horizontal deformations of the system was the compressibility and total settlement of the approach embankment. Despite the large number of piles included in the original construction, the piles provided relatively little restraint to lateral movement of the abutment, and force-based approaches were not useful for making design predictions. The piles deflected as individual bending elements within the moving soft soils, resulting in the piles cantilevering from the sand layer at 6m depth below ground. With the large cantilever length, the force of each pile stabilising the slope was only in the order of 15kN once large displacements had already occurred.
Due to the limited pile restraint provided by the group, methods designed for assessing the bending of individual bridge piles under lateral deformations could have been used as a preliminary screening tool to evaluate the feasibility of installing a similar settlement reducing pile group system. It may have been appropriate to use the following design methodology to check the feasibility of the system:
- FHWA driven pile manual Section 7.3.8 (FHWA 1996b). Evaluate the potential for soft soil squeezing by confirming that embankment loading is greater than three times the undrained shear strength of soft soils. The soft soil lateral movement is then estimated as 25% of the approach settlement to evaluate the deflected pile shape and resulting pile actions.
- For a more specific discussion of the pattern and magnitude of lateral movements relative to the approach embankment settlement, the reader could refer to Stewart et al 1994. For detailed design purposes, or for assessing the feasibility of stronger, stiffer piles that may restrain movement due to consolidation, it would be advised that the slope deformations and pile actions be calculated by careful finite element modelling of the abutment system.
A case study is presented of lateral movement of a group of settlement reducing piles due to soft soil consolidation and associated ground squeezing. A prudent design was developed and implemented to remediate the bridge abutment, by driving the soft soil consolidation out and then re-piling the structure. This approach created a resilient end design that was not sensitive to the soil properties or modelling procedures, due to the minimal movements during reconstruction of the abutment retaining wall.
In future as a preliminary analysis to confirm the feasibility of installing a similar system without pre-loading, analysis techniques for individual piles could have been used based on estimating the movement as a proportion of the vertical approach embankment settlement. For detailed design of a similar system, careful finite element modelling would be advised, particularly if the designer expects the piles to be strong enough to restrain the abutment laterally.
P. J. Hannigan, 2016. Geotechnical Engineering Circular No. 12 – Volume I Design and Construction of Driven Pile Foundations, FHWA-NHI-16-009
Stewart, D. P., Jewell, R. J. & Randolph, M. F. (1994). Design of piled bridge abutments on soft clay for loading from lateral soil movements, Geotechnique 44, No. 2, 277-296