Table of Contents
To develop a reasonable design strategy, designers examined the frequency, the likelihood of wind speeds, different earthquake intensities, and the depth of river bed scour, and established the probabilities of these environmental events occurring at the bridge site.
This article describes how designers combined these various events of varying probabilities of occurrence to determine a sensible combination of the various environmental events to use as the load cases in the design of the bridge foundations. The article then details the structural as well as geotechnical aspects of the piled foundation design.
SITE CONDITIONS FOR FOUNDATION OF PADMA BRIDGE
Geology and ground conditions
The bridge’s alignment is in Bangladesh, between Mawa and Janjiar along the Padma River. In order to present the ground conditions, designers created geological sections across the river (Figure 2) using the information from the projects Main Bridge Stage 1 – Ground Investigation (GI) Works and Padma Bridge Approach Road Geotechnical Investigation Works. Then the completed Stage-2 ground investigation work including sophisticated in-situ tests down to 150 meters below the river bed level (De Silva, Wightman & Kamruzzman, 2010).
When the stage 2 GI information became available, the designers further examined the pile design and improved it. Moreover, Sand dominated the soil in the main bridge area, with a thin layer of clay or silt covering it. As shown in the section (Figure 2), the ground condition consists primarily of Unit 2 (less than 20% fine materials) with a thin layer of Unit 1a (fine content greater than or equal to 50%).
Geological unit descriptions are provided in Tables 1 and 2.
Between Unit 1a and Unit 2, on the Char area near the middle of the Padma River, there was a thin layer of Unit 1b (soil with 20% to 50% fine materials). But it was absent on either bank or even deep beneath the river.
Designers found the Unit 3 (soil with less than 20% fine materials) stratum from typically 70m PWD to 90m PWD, and it was about 20m thick. SPT-N values typically rise with depth as a result of increased compaction.
Although sub-unit stratigraphy varies with depth, the south bank typically exhibits lower SPT-N values than the north bank at the same depth. This might be because Unit 1a is situated relatively deeper on this bank. Moreover, there are more fine materials visible on the Mawa side.
The section below shows that the River’s north bank (Mawa) contains superficial strata Unit 1a that can be up to 15 meters thick.
| Geological Unit | Criteria * | Criteria * | Typical Descriptions of the Materials Recovered from the Boreholes* |
|---|---|---|---|
Unit 1a | 50%≥finer than 0.06mm | Fine soil dominated by clay and silt. Commonly silty CLAY, clayey SILT, and slightly sandy SILT, with trace of Mica. Various colors including brownish Grey, dark grey, grey | Soil with fine content (Clay & Silt) larger than or equal to 50% |
| Unit 1b | 20%≥finer than 0.06mm ≥50% | Soil with 20% to 50% fine materials (Clay & Silt) | Generally coarse soil dominated by fine sand with much silt. Commonly very silty SAND. With a trace of Mica. Mainly grey color. |
| Unit 2 | finer than 0.06mm≤50% Coarser than 2mm≤10% | fine materials (Clay & Silt) and more than 10% of materials are coarser than Coarse Sand | Coarse soil dominated by fine and medium sand, with much coarse sand and gravel. Commonly slightly silty SAND and gravelly SAND. Poorly graded with race of mica. Mainly grey but randomly brownish-grey. |
Unit 3 | finer than 0.06mm≤50% Coarser than 2mm≥10% Soil with less than 20% | Soil with less than 20% fine materials (Clay & Silt) and less than 10% of materials are coarser than Coarse Sand | Coarse soil dominated by fine and medium sand, with much coarse sand and gravel. Commonly slightly silty SAND and gravelly SAND. Poorly graded with race of mica. Mainly grey but randomly brownish-grey. |
| Geological Sub-unit | Typical SPT N | Classification of soil |
|---|---|---|
| a & b | 0 < N ≤10 | Very loose to loose |
| c | 10 < N≤17 | Medium dense |
| d | 17 < N ≤ 32 | Dense |
| e | 32 < N ≤ 50 | Very dense |
| f | N > 50 | Very dense |
Site seismicity for designing the foundation of Padma Bridge
Based on research by the Bangladesh University of Engineering and Technology (BUET), designers used the design PGA values at an elevation of -120 m PWD (in “bedrock”) in the Padma Bridge design which was the maximum PGA values using the Abrahamson & Silva attenuation relationship. Furthermore, Table 3 displayed these suggested values.
| Return Period (Years) | Recommended Horizontal PGA in cm/s2 (Abraham and Silva 2008) at -120m PWD | Recommended Horizontal PGA in terms of “ g “ at -120m PWD | Recommended Horizontal PGA in terms of “ g “at riverbed/ground level |
|---|---|---|---|
| 2 | 4 | 0.004 | 0.008 |
| 10 | 11 | 0.011 | 0.022 |
| 50 | 32 | 0.032 | 0.064 |
| 100 | 51 | 0.051 | 0.102 |
| 200 | 80 | 0.080 | 0.160 |
| 475 | 141 | 0.141 | 0.282 |
| 1000 | 230 | 0.230 | 0.460 |
River scour situation for designing the foundation of Padma Bridge
Due to the importance of scour in the design of the foundations, sub-consultant Northwest Hydraulics Consultants conducted extensive research. They determined general scour level close to the river bank for a return period is -47m PWD close to the banks. So, they anticipated a local scour depth of an additional 15 meters at the piers for the steel raking piles or in addition to the general scour, at least 21 meters where they installed vertical concrete bored piles. Besides, Table 4 specifies the scour levels that they did take into account in the pile analysis for the various pile types.
| Pile Type | Design Period | Scour Level at Pier Location (Close to the Bank) | Scour Level at Pier Location (Mid River) |
|---|---|---|---|
| Raking Steel | 1 in 100 yrs | -62m PWD | -62m PWD |
| Tubular Pile | 1 in 500 Yrs | -70m PWD | -70m PWD |
| Vertical | 1 in 199 Yrs | -68m PWD | -68m PWD |
| Concrete Bored Pile | 1 in 500 Yrs | -75m PWD | -75m PWD |
Designers considered the soil material above the scour level as specified not to contribute to the positive skin friction of the pile capacity. They also took into account an extreme scour level, the check flood scours, at -70m PWD close to the bank and at – 55m PWD at mid-river in the pile design.
OPTIONS FOR PILE FOUNDATION OF PADMA BRIDGE
Pile arrangements for designing the foundation of Padma Bridge
For the main bridge piled foundation, there were totally two types of options.
-steel raking piles
-vertical concrete bored piles.
They analyzed both of them properly. The analysis of the total no of piles was as follows:
– 6 Steel Raking Bored Piles (for the seismic isolated bridge deck) – Figure 3.
– 8 Steel Raking Bored Piles (for the bridge deck without seismic isolation) – Figure 4.
– 12 Vertical Concrete Bored Piles (without seismic isolation) – Figure 5.
Geotechnical pile design for the foundation of Padma Bridge
The designers imposed the forces on a model of the foundations using the soil structure interaction program PIGLET, to determine the overall group behavior of the piles, the settlement of the pile cap, the individual pile settlements, lateral deflections, and the bending moments. They carried out the geotechnical design of the piles and determined the pile length and the required geotechnical capacity of the piles by the axial forces on the piles determined from PIGLET.
| Details | Design 1 (6 Piles with seismic Isolation) | Design 2 (12 Piles without seismic Isolation) | Design 3 (bored piles without seismic Isolation) |
|---|---|---|---|
| Type of Piles | Steel Raking | Steel Raking | Vertical Concrete |
| No. of Piles | 6 | 8 | 12 |
| Diameter (m) | 3 | 3 | 3 |
| Scour Level (100 year) | -62m PWD | -62m PWD | -68m PWD |
| Pile Founding Level (mPWD) | -114mPWD | -96mPWD | -121mPWD |
| Critical Load (MN) | 81.2 | 64.7 | 67.4 |
| Shaft Resistance (MN) | 58.2 | 38.8 | 40.6 |
| End Bearing (MN) | 83.7 | 83.7 | 84.8 |
| Geotechnical Capacity (MN) | 83.7 | 68.3 | 67.8 |
| Ratio Geotechnical Capacity/Pile Design Load | 1.03 | 1.05 | 1.01 |
Structural design of piles for foundation of Padma Bridge
For the structural design of the foundations, designers carried out a 3-dimensional non-linear time history dynamic analysis to ascertain the effect of seismic action on the structure. When the subtended angle for the main bridge’s plan alignment was less than 18 degrees (the radius is 3000 meters, and one Main Bridge module is 900 meters), they modeled the structure as a straight line.
When they calculated the foundation and/or column connection forces based on the plastic hinging of the columns, they ignored the combined load cases when calculating the resulting force effects. So, using an appropriate range of soil parameters that reflect the actual site conditions, the seismic analysis took soil structure interaction into account.
The vertical component of seismic action does not need to be taken into account when the bending moments in the piers due to permanent loads are small because there is no active fault within 5 km of the construction site (BS EN 1998-2, 4.1.7).
Furthermore, they used the bending moment and curvature relationship to describe the nonlinear behavior of the plastic hinge in the nonlinear dynamic analysis. So, based on anticipated material properties, each column had a minimum lateral flexural capacity to withstand a lateral force of 0.1 Pdl, where Pdl is the tributary dead load applied at the center of gravity of the superstructure. Finally, they took P-effects into account for deformations with a large amplitude. In accordance with AASHTO LRFD Clauses 5.10.11.4 and 5.10.12, plastic hinges have been described.
Because of its height (120 m) and the substantial mass of the superstructure pile cap and pile, this main bridge behaves in a complicated way. In addition, the designers carried out the design using a modified Penzien model and a three-dimensional non-linear time history dynamic analysis (see Figure 6).
The structure and the free field soil are the two components that make up this model.
Lattice spring links simulate the interactions between the structure and the free field. In relation to the free field component, program SHAKE had already performed a free field analysis to ascertain the equivalent shear modulus and effective damping ratio between each layer of soil. Subsequently, they carried out a 3-dimensional dynamic analysis using the equivalent shear modulus and effective damping as input data.
The depth of the springs, masses, and dashpots along the piles thus served as a representation of the variations in the river bed with scour. In order to simulate earthquake events for the dynamic analysis load cases, the design of strong ground motions had then been imposed on the model at the predetermined depth (see Figure 8 for the seismic ground motions).
The piles’ structural design had since made use of the design forces and stresses derived from the model (see Figure 9 for bending moment diagrams). Moreover, each case represents a unique scour combination for a specific bridge module. As you can see, it is impossible to reduce the structural thickness over depth for piers with deep scour because significant bending moments extend far down the pile.
Effect of deck isolation on the Foundations of Padma bridge
The initial seismic design strategy was to dissipate seismic energy through plastic hinging in the columns. To achieve an efficient seismic design, all columns were engaged evenly and simultaneously by Shock Transmission Units (STUs) in the longitudinal direction and by concrete shear blocks in the transverse direction. In addition, all plastic hinge regions in the columns were to be properly detailed to ensure ductile behavior over many cycles of earthquake excitations. The foundations were to remain elastic and be designed for the plastic moments and shears developed in the columns above. Adopting this system requires a piled foundation of 8 steel raking piles. Each pile is 3.0 m in diameter and filled with sand for stability. The pile cap is an octagon shape with an overall dimension of 20 m x 20m x 7 m thick.
An alternative pile arrangement was investigated using 3m diameter concrete bored piles.
Rather than using the rake of the piles to resist the lateral loads from seismic events and ship collision, lateral loads would be taken in bending by the piles. In order for the piles to have sufficient capacity to withstand the large lateral loads the piles would need to be heavily reinforced. With reference to Figure 8, it can be seen that the large bending moments extend a long way down the piles, to -70m PWD.
In order to resist this moment, the temporary casing used to construct the piles would have to become permanent down to -70mPWD and act compositely with the reinforced concrete infill. There would need to be at least fifteen such piles and the piles would need to be founded at -120m PWD. The final problem with this system of foundations was the large displacements that would be experienced by the deck from seismic effects. The displacements were determined to be in excess of the criteria set for the railway. Consequently, the use of concrete bored piles has not been pursued in the design.
A further alternative investigated has been the adoption of a seismic isolation system for the bridge.
Isolation bearings have been used worldwide to mitigate seismic response by isolating structures from seismic input. Isolation bearings can accommodate thermal movements with minimum resistance but will engage under seismic excitations. In this strategy, all primary structural members will remain elastic without any damage (or plastic hinging). So, the action of friction pendulum bearings has been studied. In order to decrease the input of earthquake forces, friction pendulum bearings extend the isolated structure’s natural period by resembling the characteristics of a pendulum. The damping effect due to the sliding mechanism also helps mitigate earthquake response.
The amount of lateral loads and shaking movements transmitted to the structure is significantly reduced because earthquake-induced displacements mostly affect the bearings. It has been found that the loads transmitted from the deck to the substructure are significantly reduced if a seismic isolation system is adopted for the bridge. If the seismic isolation system is employed the number of 3m diameter steel tubular piles can be reduced from eight, for each pier, to six.
This reduction in the number of piles more than offsets the cost of the isolation system. It should also be noted that plastic hinges should not form in the substructure in a seismic event, consequently reducing the potential for major maintenance in the future. Seismic isolation does not benefit the bored pile solution, as the piles are too flexible, leading to a long period and failure to activate the isolation system during a seismic event. When considering all the above factors it has been decided that the seismic isolation system is adopted, with six 3m diameter steel tubular piles per pier.
DRIVABILITY OF STEEL TUBULAR PILES FOR THE FOUNDATION OF PADMA BRIDGE
As the axial forces generated on the raking piles will also be large, when combined with the vertical loads, the piles need to be embedded deep in order to be able to carry these axial loads with the required factors of safety. The lengths of the piles that were expected to penetrate into the soil were of the order of 115m, depending on the riverbed level at the time of driving. Since these were raking piles, they could only be installed by driving. Therefore, drivability analyses had been undertaken in order to check if it was feasible to drive these piles to the required depths at the proposed rake angle in the soil strata encountered at the site. The drivability assessment was carried out using the software program GRLWEAP.
The drivability analysis involved first of all conducting a static soil analysis based on the underlying soil conditions representing the Soil Resistance at Time of Driving (SRD). When the proposed pile was raked at 1 in 6, the soil stresses along the pile shaft at any increment depth in the pile were considered purely due to pile shaft resistance under fully open-ended pile base conditions (cleaning and removing of soil from within the annuls of the pile) with no contribution arising from friction from the intrados of the steel tubular pile and from end bearing.
This could e achieved by constantly clearing and removing the soil from within the pile annulus.
The dynamic analysis had been conducted by using the commercially available computer program called “GRLWEAP” to simulate the drivability of the pile. In order to simulate the pile driving conditions, the following parameters were required and entered into the program:
1. Static soil analysis based on API code
2. SPT N value based on β-method
3. Driving system characteristic
4. Gain/Loss factor of pile resistance as a result of soil disturbance during driving the pile The predetermined resistance distribution for any particular depth was calculated based on unit shaft resistance and end bearing (SRD) with respect to depth input into the program.
A wave equation analysis was then performed with the gain/loss factor during pile driving into the ground was assumed to be 0.8. Since the shaft resistance and toe resistance would be reduced when the soil was being disturbed during driving. Four cases of drivability assessment were performed each considering a different pile hammer driving system to install the 3m OD steel tubular pipe pile.
The pile driving systems were selected based primarily on considering the factor of pile driving efficiency, stroke, and energy output of the hammer system. It was found that there are hammers available for driving the piles to the required depth. The most suitable hammer has a weight of 1130kN, a stroke of 2m, and a driving energy of 2260kJ. With such a hammer the maximum driving stress on the pile is 196MPa, well within the allowable stress of 300MPa. The drivability had subsequently been confirmed with the relevant hammer manufacturer.
CONCLUSIONS
It’s already proved that the foundations for the Padma Multipurpose Bridge are one of the most difficult design elements due to the challenging ground conditions and harsh environmental conditions. Piles must be designed for large lateral loads with the piles unsupported for a length of 65 meters due to deep scour and earthquake loading. AECOM used modern geotechnical and structural technologies to create appropriate design solutions, giving the bridge a stable foundation system that should require little future maintenance.
REFERENCES
-Sham, S.H.R. & Tapley, M.J. 2010. The design of Padma Multipurpose Bridge – challenges and solutions in design of the river spans. Proc. IABSE-JSCE Conference, Dhaka, 10-12 August 2010
-Sham, S.H.R. , G.X. Yu & S. De Silva 2010. Foundation design methodology for the Padma Main Bridge-IABSE-JSCE Joint Conference on Advances in Bridge Engineering-II, August 8-10, 2010, Dhaka, Bangladesh.
-De Silva, S., Wightman, N.R. & Kamruzzaman, Md. 2010. Geotechnical ground investigation for Padma Main Bridge. Proc. IABSE – JSCE Conference, Dhaka, 10-12 August 2010
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