How can scenario development help to deal with uncertainties in shoring design using flexible walls?
Ehsan Moradabadi
CEng MIEI | PhD in Civil Engineering (UCD) | Registered Geostructural Engineering Consultant | Technical Design (Geotechnical and Structural) | Feasibility Studies | Team Building | Project Management | Training
In the realm of scientific engineering, flexible walls are typically characterized as sheet piling or an amalgamation of sheet piling and soldiers (i.e., derived single piles), with or without the inclusion of bracing elements (e.g., tie-backs, anchors, chains, props, or rakers).
When juxtaposed with other alternatives, the advantages of sheet piling, such as recyclability, reusability, lightweight nature, ease of long-term installation, and length adjustability with minimal maintenance both above and underwater, render them one of the most favored types of temporary and permanent retaining structures among contractors.
From an environmental perspective, the primary source of emissions from flexible walls is ‘steel materials’, accounting for 96% of emissions, equivalent to approximately 2 tons of CO2 per ton of steel (this can be reduced to one-third when the steel is recycled). The use of conventional driving machinery can also result in noisy and disruptive installation processes. These drawbacks place them among the most costly and least environmentally friendly methods for stabilizing deep excavations. However, when employed for temporary purposes and reused multiple times, or for permanent structures with limited accessibility for maintenance, their long-term benefits may outweigh their drawbacks, particularly in cases where the use of steel is optimized.
Similar to other geostructures, the successful implementation of flexible walls depends on a thorough understanding of the long-term behaviors of the other main players, namely soil and water, prior to installation. Material optimization is not feasible without a comprehensive soil investigation and a study of the long-term effects of climate changes on the complex structure composed of the wall and the ground.
By adapting the process of designing flexible walls to the observational method and the iTwin framework, the design process proposed in this edition of GDPs addresses the question of how sustainability assessments and scenario developments in the design stages can reduce the cost of installing sheet piles and increase their acceptability level by reducing their carbon footprint, in addition to reducing uncertainties in the entire process.
A short summary of the method
In the context of scientific engineering, a flexible wall represents a shoring technique that incorporates sheet piling (as depicted in Figure 2) or a combination of sheet piling and soldier piles (as illustrated in Figure 3) for the purpose of stabilizing deep excavations. When soldier piles are utilized, they are driven vertically at regular intervals, accompanied by horizontal planks, referred to as lagging. The soldier piles are predominantly steel H-piles, but can also encompass precast concrete, micropiles, and conventional pipe sections.
The design procedure
In alignment with the previous edition, the design procedure for flexible walls is examined here, commencing with a desk study. In comparison to prior editions, to achieve enhanced coherence between the desk study and the preliminary and detailed design phases, the sustainability assessment is scrutinized in greater detail. Given that these structures can be employed in both temporary and permanent conditions, the structural categorization has been taken into account throughout the entire process.
Desk study
Assuming the documents and information received from the owners, permanent designers, or design coordinators are sufficient for a comprehensive desk study and subsequent analysis, the suitability of the method for the project may need to be demonstrated from a sustainability perspective (i.e., constructibility, economy, and HSE), akin to other geostructures. A desk study for flexible walls typically encompasses:
- Review of earthwork design in addition to landscape/architectural planning drawings
- Investigation for buried services and existing structures
- Study of previous geotechnical investigations and experiences from the area (i.e., flexible walls may not be suitable for very loose ground, as the toe of the wall should be able to fix in stiff ground to provide adequate passive resistance. Sheeting is particularly suitable in coarse materials with maximum sizes roughly less than 6 inches, or stratified subsoil)
- Evaluation of the suitability of machinery and sheet-pile driving for the ground (i.e., drivability of the ground, e.g., driving sheet-pile may not be possible when N-SPT of the soil is roughly greater than 50, or the ground strata is rocky or extensively contains pebbles and boulders)
- Noise and vibration assessment
Sustainability is assessed through:
- Service life expectation
- Durability and maintenance
- Material use and carbon footprint
- Fuel saving
- Reuse and Recycling
For assessing the sustainability of the excavation method and comparison with other alternatives, it is suggested that the sustainability team employs one of the following carbon calculators, available as reporting standards, software packages, or spreadsheets:
- EFFC DFI Carbon Calculator (supported by EFFC and DFI)
- GHG Protocol Corporate standard and GHG Protocol Product Life Cycle Accounting and Reporting Standard (supported by European Commission)
- PAS 2050 (supported by UK DEFRA, DECC, BIS)
- ADEME Bilan Carbone and BP-X-30-323 (supported by French ADEME and AFNOR)
- ISO 14064 and ISO 14067
- ENCORD
- Product environmental footprint (supported by DG Environment and JRC IES)
The criteria for the entire process of desk study and design should be based on approved standards and guidelines such as EC 00, 01, 02, 03 and 07, BS EN 12063, BS 8002, BS 8081, CIRIA RP 1122, CIRIA C760, FHWA-NHI-16-009 and 010, FHWA GEC 009, TII 1600, EM 1110 and AASHTO LRFD 8, among others.
The desk study may indicate that the flexible wall is not an effective method for the project. Consequently, the designer(s) may suggest other available alternatives.
It is recommended that any geotechnical site investigation be planned only after a comprehensive desk study to optimize the costs that may be dictated to the project from site investigation.
Categorising the structure
In the context of scientific engineering, the life expectancy of flexible walls can categorize the structure as either temporary (i.e., when the life expectancy is less than a year or the structure serves as a falsework supporting a permanent structure) or permanent, when the reference period exceeds a year. The design process depicted in Figure 1 applies the recommendations of the Eurocode for categorizing permanent flexible walls. For temporary applications, the recommendation of BS 5975 is adapted (similar categorization may be adapted in accordance with other relevant codes and standards). When the reference period exceeds 50 years, the design may necessitate additional considerations and expert judgment beyond the Eurocode.
The project’s importance level may dictate varying levels of supervision and inspection. When the structure is temporary, the design check should be conducted in accordance with the categories summarized in Table 1. BS 5975 does not recommend reliability classes and inspection levels for temporary structures. It is suggested that the corresponding reliability classes and inspection levels of permanent structures be applied to temporary flexible walls as well (an arrangement for inspection between the project supervisor for the design stage and the construction stage may be necessary. A typical arrangement was previously suggested in GDPs at https://lnkd.in/eackD8Nt). Tables 2, 3, 4, and 5 summarize the definition of consequence classes, corresponding reliability classes (and their corresponding modification factors for actions), and supervision and inspection levels, respectively, for permanent structures in accordance with the EC.
Predesign process
In alignment with the discourse on other geostructures in preceding editions, the designer may necessitate a pre-design process prior to executing a geotechnical design. This process is intended to:
Geotechnical design (ULS)
Following the pre-design process, a geotechnical design would necessitate execution in eight steps:
1- Assessment of the initial ground condition, which includes:
Note: If the flexible walls were employed for near-shore or off-shore applications, such as cofferdams, sea walls, or quay walls, while the design process remains similar, additional considerations may be necessary that have not been explored in this technical note.
2- Generating construction scenarios that may include following:
3- Defining problem geometry and the preliminary sheet-pile configuration with/without support by soldiers, tie-back or anchors that may consist of:
4- Generating load combinations and scenarios, consisting of:
5- Calculating active and passive pressure to the walls and evaluating the overall and rotational stability of the wall
The primary output of a geotechnical analysis for flexible walls involves the computation of active and passive pressures exerted on the walls, followed by an examination of the overall stability and rotational stability of the wall. Figure 5 presents an exemplar of a simplified method for calculating active and passive pressures on the wall, while Figures 6 and 7 depict the failure modes that typically require verification by the designer(s).
As the flexible walls are usually designed as retaining structure, vertical failure of the flexible walls (Figure 8) is not always a concern. Exceptions to this occur in rare circumstances where the wall is designed to resist axial loads, such as when sheet piles are directly used as abutments for securing Bailey bridges, or when the self-weight of the sheet piles is of a magnitude that could induce vertical failure in the ground.
6-Seepage analysis
If seepage is a significant factor in the project, additional analysis may be required to assess the impact of seepage on the project. The designer may propose suitable measures such as the application of a sealing layer or a concrete blanket to prevent seepage heave and to dry out the excavation area. The temporary and permanent effects of any measure should be taken into account in the geotechnical analysis. In instances where H-piles are used as soldiers and lagging walls in excavation below the groundwater level, a proactive measure may be necessary to control seepage. Figures 9 and 10 depict the piping failure modes and various scenarios for hydrostatic pressure, respectively, that necessitate further consideration by a geotechnical designer.
7-Calculating factor of safety (FS) or utilisation factor (UF) for assessing resistance against active pressure and seepage for each scenario
The designer has the option to employ either a traditional method or a phi/ci reduction method for calculating either the Factor of Safety (FS, also known as FoS or SF: the ratio of the ultimate strength of a member or piece of material to the actual working stress or the maximum permissible stress when in use; or the ratio of passive actions to active actions), or the utilisation Factor (UF, also known as the ratio of the actual to maximum allowable performance). In the traditional method, the engineer calculates the ratio of passive actions to the active actions on the structure, while in the c-phi reduction method, the soil strength is gradually reduced and when failure occurs, the corresponding strength reduction factor can be considered as a factor of safety in soil strength. While the traditional method can be simplified and executed manually, for c-phi reduction, computer utilisation is necessary, especially for complex geometries. Notably, FS (a safe ratio is more than 1) or UF (a safe ratio less than 1) derived at this stage are based on the satisfaction of the internal stability of the flexible walls. Acceptable FS or UF at this stage do not definitively indicate that the flexible wall is structurally safe, and further structural analyses will be necessary, which are summarised in subsequent sections.
It is worth mentioning that, when a deterministic analysis is under consideration, there are two major approaches among engineers for considering uncertainties in active and passive actions. Some traditionally assume nominal values for material strengths and loadings, and subsequently calculate a factor of safety based on un-factored values. The acceptable factor of safety in this approach, also known as Allowable Stress Design (ASD), should be a large value, such as 2 or 3. On the other hand, some scale up the active pressure by multiplying internal and external loads by specific factors and scale down the passive pressures or resistance of the structure by applying partial factors to materials and soil strengths or resistance of the structural elements. The acceptable factor of safety in this approach, also known as Load and Resistance Factor Design (LRFD) method, is greater than but close to 1. (See more in Appendix A of Eurocode 7 or AASHTO LRFD-8).
In addition to load and resistance factors, the engineer may consider an additional reduction factor for material strength or resistance of the structure to cover uncertainties in modelling of a phenomena/mechanism in Geotechnics, also known as modelling factors (e.g., see Irish national annex to Eurocode 7).
Furthermore, reusing sheet piles (e.g., for temporary applications) may cause the designer to assume an additional partial factor for theoretically reducing the strength of the steel material affected by either deformation or corrosion, according to engineering judgment, economic and environmental considerations, or approved laboratory tests.
8- Calculating bending, axial and shear forces diagrams on the structural elements for each scenario
Assuming the general factor of safety for the flexible wall was deemed satisfactory in the previous step, additional analyses are required. These analyses aim to calculate the bending, axial, and shear stress/forces in the structural elements of the wall across all considered scenarios. This ensures a comprehensive understanding of the wall’s performance under various conditions, contributing to a more robust and reliable design.
Structural verification of sheet-piles/soldiers
Upon ensuring the general stability of the flexible wall, it is incumbent upon the designer(s) to demonstrate that the structural performance of the wall is also acceptable. Figure 11 provides illustrative examples of potential failures in a retaining wall, underscoring the importance of thorough structural analysis in the design process. This step is crucial in mitigating the risk of such failures and ensuring the safety and effectiveness of the flexible wall.
The geostructural verification of flexible walls may include verifying of:
Structural/Geostructural verification of?tiebacks/anchors/chain/props (struts)
If any axial element was used in the arrangement of flexible walls as tie-backs, chain, props or rakers, the engineer may need to verify below:
Deformation control (SLS)
The deformation of the wall should generally be evaluated against predefined performance criteria (contractual ratios) for the projects. Various finite element or finite difference models may be required to assess the performance of the wall, including lateral and vertical displacements. In certain scenarios, such as bridge abutments, the designer may need to evaluate the lateral squeeze of the wall to ensure it falls within the acceptable range of performance criteria. The analysis necessitates the inclusion of adjacent buildings and amenities. To determine the design’s suitability for the project, particularly in urban areas, the designer may need to conduct a more comprehensive assessment of the long-term effects of the method on adjacent buildings, amenities, roads, or railways. More information about the assessment procedure can be found in a previous article published in a GDPs newsletter. at https://lnkd.in/eNC7ijNU)?
In the event that the Serviceability Limit State (SLS) performance, as determined from the deformation analysis, is deemed unsatisfactory, it may be necessary to modify the structural elements and repeat the design process. This iterative process should continue until the performance level of the structure is numerically satisfied, thereby ensuring the structure's safety and functionality.
Seismic design (ULS)
In regions susceptible to earthquakes, the implementation of a seismic design becomes a necessity. This involves:
- Determining seismic design parameters, which include Peak Ground Acceleration (PGA), Site class, return period, average shear wave velocity or an equivalent parameter, and other modification factors
- Adjusting the seismic design coefficients to account for the specific seismic conditions of the site
- Reconstructing the models and scenarios for further geotechnical and stability analysis to ensure the design can withstand the anticipated seismic forces
The aforementioned steps may lead to updates in the geotechnical models used for structural and geotechnical verification. This is to ensure that the ultimate limit state of the structure is at an acceptable level for resisting the probable seismic loads that the structure may encounter during its lifetime.
Groundwater and drainage design
The designer is required to consider appropriate drainage features to manage both surface and subsurface water during and post-construction, provided the wall was not designed as a cofferdam. The design for groundwater and drainage for flexible walls may encompass:
Hydrological considerations may necessitate the designer(s) to either revise the geotechnical models or generate new ones for executing stability analysis and structural verification. This is to address different scenarios, such as rapid drawdown analysis, ensuring the design is robust and capable of handling various hydrological conditions.
Frost protection
Under specific conditions, a desk study and geotechnical site investigation may reveal that the soil is susceptible to frost. In such instances, the designer may propose one of the following measures to safeguard the wall against potential frost post-construction:
These measures ensure the structural integrity of the wall even under frost conditions, thereby enhancing its durability and performance.
Evaluating the constructibility of the design
The design output necessitates an examination for constructibility prior to the preparation of the final drafting. This involves a thorough review of the design and the incorporation of additional safety features, which are crucial for the successful implementation of the project. These measures ensure that the design is not only theoretically sound but also practically feasible, thereby enhancing the overall success of the project. Further details on these considerations and safety features would be project-specific and determined by the designer based on the unique requirements and constraints of the project. The designer may consider below safety features for ensuring about the constructibility of the design:
Load Testing and monitoring action plan
Prior to finalising the design, it is imperative to validate the adequacy of the design during the construction phase and to provide pertinent information for the project’s mitigation program. To this end, the designer(s) must devise a load testing and monitoring action plan, which includes:
Taking into account the concept of the Internet of Things, within a general iTwin framework, (see previous article published in GDPs for more information at https://lnkd.in/eFb8d8NJ) the instruments proposed for live monitoring of the structure, in conjunction with new survey technology (particularly LiDAR drone surveying), can generate a real-time database. By synchronising these data with a digital replica of the flexible wall (e.g., FEM and FD models), it is possible to predict the system’s behaviour and formulate a robust action plan for the mitigation program and post-validation of the design. In the event of unforeseen occurrences, or the presence of any defect in either the design or construction, an action plan based on the iTwin framework can signal warning signs and prevent catastrophic collapse.
The aforementioned knowledge has been successfully applied in the design, peer review, or advisory checks of various projects, some of which are summarised below:
In conclusion, while the flexible wall is a favored option among contractors, it is one of the least environmentally friendly methods. The design process outlined in this edition has been crafted to incorporate sustainability assessment and scenario development at the design stages. This approach is based on the concept of the iTwin framework and the observational method, addressing concerns related to the depletion of material resources.
See more information on Geostructural Design Processes and previous newsletters at ??https://www.dhirubhai.net/newsletters/6985944134142283776/