OTC 23635

Identifying Some Knowledge Gaps in Marine Foundation Practice - A Design and Construction Perspective Gerry Houlahan, Moffatt & Nichol; Paul Doherty, University College Dublin; Robert F. Stevens, Fugro-McClelland Marine Geosciences, Inc.

Copyright 2012, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2012. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract The Marine Foundations Committee of the Deep Foundations Institute in the United States (DFI) has been actively advancing the state of the art in marine applications of deep foundation design and construction for the past 8 years. The scope of these efforts has ranged from driven pile foundations similar to those used in offshore applications to those used only in traditional onshore and near shore marine structures. There is now a significant overlap between these two distinct reference sectors of technology. This paper presents the opinions of the committee in identifying the major recent advancements, specifically in the sector more similar to offshore driven pile technology. Furthermore, remaining areas of uncertainty are identified and possible solutions are suggested in order to fill these knowledge gaps. Applications of driven piles as diverse as offshore oil & gas platforms, large bridges, ports and harbours, and the offshore wind industry are often seen to share similar design and construction issues. However, in many aspects these sectors suffer from unique and specific design and construction challenges that are not addressed individually. This paper explores the need to investigate some of these recent issues that have arisen in practice. The authors discuss challenges such as (i) the conservatism of traditional pile design methods, (ii) the difficulties in sizing hammers for large diameter (>4 m) monopiles and predicting pile driveability, (iii) the difficulty in predicting set-up effects and the long term capacity of offshore platforms, (iv) the impact of cyclic loading on foundation resistance, and considerations for short grouted connections on high diameter/thickness ratio piles. Considering the array of issues facing the industry, this paper also identifies possible areas of future research that could lead to improved industry practice and better efficiency.!!!Introduction Designing marine and offshore structures is a complex process with many interfaces between different engineering processes from structural design issues to fabrication and installation details. However, one of the greatest sources of uncertainty can be attributed to the geotechnical design of the supporting foundations. Determining an appropriate soil reaction (both ultimate capacity and stiffness) is complicated by the fact that soil is a naturally occurring, non-linear, anisotropic, non-homogenous material, which is both non-linear and viscous when loaded. As a result, offshore foundations are often designed using a series of simplifying assumptions, which are based on past experience and empirical observations. The accuracy of these simplifying empirical approaches varies in different foundation designs and can impact the constructability as well as the underlying factors of safety and the accuracy of predicted stiffness and dynamic responses of the supported structures. It is clear that there is significant room for improvements in marine foundation design to ensure that there is a consistent reference for factors of safety and reliability across every offshore or marine structure, while adopting efficient engineering designs.!!!!!!There are many aspects to consider in this wide field of deep marine foundations, such as foundation type selection, underwater ground improvement using piles or other methods, pile-to-pile soil consolidation effects, interaction between axial and lateral pile loadings, the need for testing of pile bearing capacity and what should be

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the preferred site soil investigation techniques. The scope of this paper, however, presents a focus on driven pile foundations and the adequacy of present practice. Several technical challenges (TC) have been highlighted that reflect current and future issues facing the industry, where research is needed to improve the current foundation design and construction procedures. These technical challenges are listed below. !TC 1: Foundation Codes for Offshore Axial Pile Design The method of securing offshore structures to the seabed has been a crucial consideration for the success of offshore oil/gas developments over the past sixty years. Piled foundations often represent the most suitable method of transferring large structural loads to competent bearing strata. The most common foundations employed by the offshore petroleum industry are large diameter open ended steel piles driven to a required penetration. For example, Chow (1997) reported that over 90% of oil and gas platforms in the North Sea are founded on open ended piles. More recently Overy (2007) described the installation of nine platforms installed by Shell UK in the North Sea which all employed steel piles ranging from 600mm to over 2m in diameter. The rapidly developing wind energy sector has also recently led to further expansion in the offshore driven pile industry. In shallow waters, the most common foundation solution for wind turbines are single piles with diameters up to 6m, commonly referred to as monopiles. In deeper waters, tripod or jacket structures transfer the loads to small groups of 3 or 4 pile elements. !!!!The range of geometries and capacities of these piles have increased dramatically in recent years to meet the demands of highly loaded offshore developments and have been facilitated in this regard by a parallel expansion in the technology available, including more powerful and efficient piling hammers. However, research into the controlling mechanisms governing pile-soil behavior has lagged well behind the technology advancements and as a result offshore pile design is still heavily reliant on empirical design methods which have not been developed for the specific loading conditions or geometries encountered offshore (Jardine and Chow, 2007). The poor reliability of traditional empirical design methods has been demonstrated through database studies, such as that conducted by Briaud and Tucker (1988), with measured and predicted pile capacities differing by up to 60%. It is worth noting that reliability studies of this type only consider the uncertainty in the design method for the range of piles and test methods within the database, which are typically small diameter closed ended piles failed under statically maintained load conditions. However, the realistic offshore situation is considerably different. As pile design for axial loads evolves past traditional working stress design and moves toward more rational reliability based design, quantifying the inherent uncertainties and biases in the existing design method will become paramount for efficient bearing capacity design. On this basis additional and more complex factors need to be considered such as the impact of end condition, cyclic loading, rate effects and time-capacity relationships. !!!!The most commonly adopted pile design guidelines used for offshore foundations in cohesive soils are those specified by the American Petroleum Institute (API), which were formally introduced into practice in 1969. These guidelines suggest a design approach based on the traditional “alpha” method, which relates the unit skin friction directly to the undrained shear strength for piles in clay. The caveats of this method are well documented by a number of researchers, not least of which is the difficulty in obtaining a representative undrained shear strength value, as demonstrated by Lehane (1992). However due to the relative ease of application and the long term track record in industry this method has remained the most popular design method in practice. Similarly, an empirical approach relating the unit shaft friction to the vertical effective stress has remained the API industry standard design approach for piles in sand and similar caveats apply, with the design issues for piles in sand described in detail by Gavin et al. 2011. ! Recognizing the limitations of such an empirical approach sparked a concentrated research effort to establish the underlying mechanisms controlling driven pile behavior. This pioneering research was conducted at the Norwegian Geotechnical Institute (NGI), the Imperial College London (IC), MIT and Oxford University, University of Western Australia (UWA), amongst many other institutions. Parallel investigations conducted by these research bodies all employed highly instrumented small scale closed ended model piles to measure the stresses acting on the pile shaft throughout installation, consolidation and loading in a range of clay sites. The measured radial stresses confirmed that the pile resistance was directly related to the effective stress acting on the pile shaft at failure and thus spurred the transition from total stress to effective stress design. One such design method to emerge from this period of research was the Imperial College (IC-05) design approach. The reliability of IC-05 was tested against a database of load tests by Jardine et al (2005) and found to be significantly more reliable than the then current API-93 method, with a mean calculated to measured value (Qc/Qm) and coefficient of variation (COV) of 1.01 and 0.2 respectively, in comparison to a mean Qc/Qm of 0.99 and COV of 0.33 for the API approach. This suggests a significant improvement for predicting the capacity of closed ended piles installed in clay subject to static loading, due to the underlying rational effective stress framework.

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!!!!However, recent amendments to the API guidelines were published in 2007 and included an updated commentary on axial pile capacity in clay, which specifically addresses the limitations of such database studies. The concerns detailed in this commentary include:

(i) The difficulty in selecting soil parameter design values, (ii) Potential measurement error in the measured capacity, (iii) The load tests considered in the database are typically smaller geometries than those used offshore

and are primarily closed ended, (iv) Realistic offshore loading conditions (rate and cyclic influences) are not well represented by static

maintained load tests in the database, leading to potential biases when applying database studies to actual offshore design.

Additional factors contributing to the uncertainty in calculating offshore capacities were described in API (2007), which included length effects and capacity-time relationships.!!!!!!Recent projects such as the reconditioning of the San Francisco- Oakland Bay Bridge (Mohan et al 2002, Mohan et al 2004) have used full scale measurements to demonstrate the conservative nature of the existing API codes. The east span of the bridge was replaced due to seismic concerns, with the new bridge founded on high capacity driven steel piles, which were designed for tension loads of 90MN and compression loads of 140MN. Dynamic Load tests complete with signal matching, conducted on these 1.8-2.5m diameter piles driven up to 100m into the underlying clay formations measured the unit skin friction, which was found to be 40% higher than predicted by conventional API design. This suggests considerable scope for more efficient design of large diameter piles in similar ground conditions. Going forward, a rational effective stress based design is required that better captures the additional factors relevant to offshore design situations. By conducting instrumented tests that more closely represent the offshore pile situations, the impact of end condition, length effects, time-capacity dependency, cyclic loading and rate effects can be quantified. The effective stress framework already established for closed ended model piles subjected to static loading can then be extended to the design of offshore piles with improved reliability and efficiency. ! For piles in sand, there have recently been significant strides in developing CPT-based effective radial stress solutions for predicting pile capacities and these are now included in the commentary of the main API text, where any of five methods (ICP, UWA, NGI, Fugro and API) can be adopted as alternatives in predicting the design capacity of offshore piles. However, despite a similar underlying framework these methods can give drastically different design pile penetrations, with no guidance given as to when one method is more applicable than another. Underlying biases in these methods are not addressed and inherent flaws in each method are not discussed, leaving the designer with no guidance as to when one method should be used rather than another. Comparison of the pile penetrations required to resist a 10 MN design load (assuming a 2.5m diameter pile installed in dense sand) is shown in Figure 1 below, where pile lengths range from 4m to 22m. As a geotechnical designer, the API code allows a considerable range in pile lengths – but which one is correct? Clearly, there is considerable scope for improvements in these prediction methods and urgent research is needed to develop an accurate pile design code. TC2: Efficient Foundation Options for the Offshore Wind Sector A variety of sub-structure options is currently available to support offshore wind turbine generators (WTG), with many more emerging concepts undergoing rapid development. Existing concepts include gravity bases, monopiles, jackets/tripods and more recently, floating turbines tethered to the seabed with anchor lines, which are illustrated schematically in Figure 2. At shallow water sites with competent seabed conditions, gravity bases have proved successful provided they can generate enough bearing resistance. These concrete foundations resist the applied wind and wave loads through the dead weight of the concrete base and the bearing resistance of the foundation soils. A competent bearing stratum at seabed level is therefore a necessity to facilitate a gravity base. Monopiles, which consist of a single large diameter steel tube driven into the seabed to a specified penetration, have proven to be an efficient and competitive solution in water depths up to 35m and have formed 75% of existing turbine foundations worldwide. The Danish design code (DNV 2007) states that monopiles are suitable to !

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!Figure 1: Pile lengths predicted according to the API design code

water depths up to approximately 25 m, however recent wind farm construction has gone beyond this limit, extending the range in excess of 35m (partly due to the availability of high capacity modern hammers). These piles resist the lateral wind and wave loadings (and resulting moments) through cantilever action, which generates horizontal earth pressures in the direction opposing the applied loads. It is significant that the lateral and rotational foundation stiffness, which controls the dynamic response of the turbine-tower-foundation system, is the driving criterion that limits the economic use of cantilevers with increasing depth, rather than capacity. !!!!From 35m to 60m water depths, jacket structures have been used to support the wind turbine super structure. The jacket consists of a steel lattice frame founded on piles under the legs of the structure. The lateral applied loads are transferred through the jacket structure into the foundation piles, which resist the loads using push-pull action through the axial skin friction developed along the pile shaft. In a similar manner, tripod sub-structures rely on the axial pull out resistance of three supporting pile elements. A recent pilot project off the Norway coast has demonstrated the technical possibility of using deep water floating turbines. However, the commercial viability of floating designs still warrants significant investigation.

!Figure 2: Foundation dependence on water depth (after Doherty et al, 2010)

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!!!In recent years, there has been a significant push to reduce the cost of offshore wind energy and make it competitive with alternative fuel sources such as oil and gas. In order to meet this objective, it is recognised that the capital expenditure of the infrastructure needs to be reduced. The foundations can cost up to 40-45% of the total development cost of a wind farm and are therefore one of the primary areas requiring improved efficiency. The Carbon Trust in the UK has identified foundation substructures as a key barrier to reducing the cost of

offshore wind energy and one which requires urgent research. As a result, a number of emerging concepts are being trialled at wind farms across Europe as demonstrator projects that could dictate the future of offshore wind substructures. These emerging concepts include single large diameter suction caissons (see Figure 3 below), tripod suction systems, braced monopile jackets and integral concrete bases with the turbine preinstalled. The key design issues for these emerging concepts include; accurately predicting the system failure mechanisms, determining the modal frequencies of the structure and avoiding the resonance frequencies of the supporting turbines, predicting the response of the foundation to long term cyclic loads, ensuring the foundation is robust enough to withstand the installation process and critically reducing the installation cost by removing the dependence on heavy lift jack-up vessels. It is crucial for the success of the wind industry that these concepts receive adequate testing at the demonstration stage to ensure a robust design framework that will allow the novel concepts to become serious contenders for large scale wind farm production. !!!

Figure 3: Suction Bucket Concept TC3: Pile Driveability There are a series of ongoing concerns regarding driving piles offshore including structural damage from overstressing the pile material, premature refusal, hammer availability and environmental concerns from piling noise. !!!!In Europe, many countries are considering banning pile driving completely due to the negative impact of the piling noise on marine mammals, while other countries are introducing strict noise limitations that are difficult to achieve with conventional piling technology. Currently, piling operations in the North sea include environmental mammal surveys that ensure specific marine life are not in the vicinity of the pile driving operations before the hammer impacts can commence. Soft-starts are also common-place whereby the hammer is run at a low initial energy to provide marine mammals with an opportunity to swim away from the site before being subjected to the full shock of the maximum pile driving noise. There are several studies underway to assess the benefits of providing noise reduction screens or bubble curtains that can reduce the decibels transferred into the water. Noise transfers from the pile-hammer assembly into the surrounding air, radially through the structure and also through the underlying soil into the water. A recent report by Stokes et al (2010) examined a number of mitigation measures to reduce the pile driving noise generated during monopile installation and determined that the radial structure borne sound transmission dominated the noise level in the untreated pile scenario. Using theoretical energy dissipation models, the impact of bubble curtains, coffer dams and damping layers were investigated. These methods were determined to reduce the noise level by approximately 10dB, however the magnitude of the reduction was somewhat limited by the seismic transmission of noise through the ground. This noise transmission route contributed less to the untreated piling case but became the dominant means of propagation when the noise reduction measures were in place. There is varied evidence on the success of these reduction methods. For example, pile driving noise for the Benicia Bridge project (Rosta 2003) in Northern California caused serious delays to the project because of rare fish species that were living in the area. A bubble curtain was adopted on this project to dampen the sound transmission, with a measure 26dB reduction in noise, which allowed the project to be completed. In sharp contrast, recent wind farm projects in the German North Sea have not been able to progress with construction because, despite adopting similar noise mitigation measures, they have been unable to meet strict environmental noise limits for the project due to the propagation of noise through the underlying soils. To date there is no universally accepted approach for reducing piling noise.!!!!!!Another concern is the availability of offshore hammers with sufficient capacities to install the number of offshore piles that are likely to be driven to support offshore wind farms over the next ten years. The current state of the art hydraulic hammers can accommodate a 5.2-m-diameter pile, with plans to fabricate a 6-m-diameter pile anvil-connector sleeve. These hammers have rated energies of 1900 to 2000 kJ. Some specific highlights from a number of offshore wind farms constructed to date include:

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!• The Lynn and Inner Dowsing Wind Farms off the Lincolnshire coast of England used 4.74-m-diameter

monopiles driven to a maximum depth of 26 m in hard chalk with an MHU 1900S hammer in a maximum water depth of 13 m.

• The Princess Amalia Wind Farm off the Dutch coast used 4-m-diameter monopiles driven to a maximum

depth of 32 m with an MHU 1900S hammer in a maximum water depth of 24 m.

• The Burbo Bank Wind Farm in Liverpool Bay used 4.7-m-diameter monopiles driven to a maximum depth of 24 m with an MHU 800S hammer in a maximum water depth of 8 m.

• The North Hoyle Wind Farm off the coast of North Wales used 4-m-diameter monopiles driven in dense

sand, sandstone and mudstone with an MHU 600B hammer in a maximum water depth of 15 m.

• The Arklow Banks Wind Farm used 5.1-m-diameter monopiles driven with an IHC hammer.

• The Greater Gabbard Wind Farm used 5.1-m-diameter monopiles driven with an IHC hammer. !!!!The scale of typical monopiles is illustrated in Figure 4 below. Determining an appropriate hammer for each site is a complex process that requires the geotechnical engineer to determine the likely blow-count and driving stress for a given hammer-soil-pile configuration. If the predicted response is within acceptable limits then the hammer is appropriate for the task. Part of this process involves predicting the static resistance to driving (SRD), which is calculated using empirical design approaches such as Toolan and Fox (1977) and Stevens et al (1982). These design methods are subject to similar sources of uncertainty as highlighted in TC1 for axial pile design and as a result are likely to give a broad range of results. !!!

! !Figure 4: Offshore monopiles !!!!!

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!!!There are currently several methods available to assess the driveability of offshore piles. These include methods proposed by Toolan and Fox (1977), Stevens et al. (1982) and API (2007) along with more recent CPT based soil resistance models such as UWA05 and IC05. These approaches adopt different assumptions and can predict considerably different driving requirements at specific sites. Specific limitations of the previous methods include the inconsistent treatment of friction fatigue, the limited scope of the formation database and the poorly considered pile end-condition effects. These problems are exacerbated when working in atypical sites, such as the Clair oil field, which exhibited very high soil strengths (Aldridge et al, 2010), or when using extreme geometries such as the large diameter piles commonly adopted by the wind sector. This problem is illustrated in Figure 5 below, which plots the predicted blow counts required to install a 6m-diameter monopile to a depth of 35m into both dense and very dense sand (Further details are provided in Doherty et al, 2011). Two separate models were used to predict the soil resistance to driving, with the API and IC05 predictions shown in Figures 5a and 5b, respectively. The API model indicates that the driving resistance will increase consistently with depth but the pile can be installed to the final depth in both the dense and very dense sand, without any problems as the blow counts remain well below 80 blows/250mm. By sharp contrast, the IC05 approach suggests that this pile can be installed to 35m depth in dense sand but will reach early refusal at approximately 10m depth in very dense sand. This is a significant problem for monopile designers and a huge risk for wind farm developers because both install-ability and metal fatigue are key design considerations affected by resistance to driving. These design problems often manifest in the field as premature refusal due to pile damage. There needs to be significantly more certainty as to whether large diameter monopiles can reach the target penetration using existing technology or whether alternative foundation concepts need to be considered that require less extreme hammer energies. Research is needed to develop a more scientifically accurate driveability model that removes the uncertainties introduced by existing approaches. !

! !Figure 5: The blow counts required to install a 6m-diameter monopile into dense and very dense sand, using (a) The API model and (b) The IC05 model. !TC4: Pile Lateral Loading The industry standard design approach for predicting the response of piles to lateral loads is described in the API (2007) guidelines, which adopt non-linear soil springs to represent the soil reaction to an applied load. This process is illustrated schematically in Figure 6. For piles in sand, the shape of the springs is hyperbolic, with the ultimate resistance (pc) predicted based on a theoretical wedge failure at shallow depths and flow failure in deeper strata. pc is subsequently adjusted to match the ultimate resistance measured in experimental results (pu). The experiments conducted were lateral pile load tests conducted on instrumented 610mm diameter flexible piles installed in medium-dense sand at Mustang Island in the 1960s (Reese et al 1974). The theoretical resistance was multiplied by the empirical factor A, which was dependent on normalised depth below ground surface. This A factor shown in Figure 7 is explicitly developed for a single set of tests on piles now considered small in diameter and this is now being extrapolated to monopiles with diameter an order of magnitude larger. This was recognised as a concern in the 1980s and the API sponsored a database study, reported by Murchinson and O’Neill (1984), which described the limitations of the existing p-y approach. A clear conclusion of this work was that existing pile load test databases were poorly populated and more large scale load test data was needed to validate the existing design procedures. This is clearly illustrated in Figure 8, which plots out the validation database compiled by Murchinson and O’Neill (1984) and shows the relatively small number of piles available

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within the database and the small diameters of those piles. Similar issues are observed when adopting the API approach for cohesive soils. !!!!The relatively small diameters of database piles were identified as a concern, for both bridge and offshore structure foundations, by the Marine Foundations Committee of DFI, who solicited a paper into exploring possible scale effects. This paper, written by Lam (2009), was subsequently published in the Marine Foundations Manual. Lam (2009) suggested that the potential conservatism of the API approach was due, in part, to ignoring additional sources of soil resistance that were mobilised during failure. Rotational resistance not considered in the p-y methodology could lead to increased capacity and stiffness. Lam (2009) suggested using p multipliers to incorporate such scale effects in design – however, there remains a lack of calibration data to validate such procedures. There is a clear deviation between industry design practice and the design tools that have been developed. Differences between predicted and measured first mode periods of vibration indicate that the stiffness of the foundation is not well predicted using present techniques. Large scale instrumented lateral load tests on large diameter piles at well characterised research sites are needed to further broaden this database and confirm the current industry assumptions. It is important to add that the structural components of flexibility of the WTG monopiles and their tubular transition pieces need to be included in these tests to remove uncertainties from the results. !

!Figure 6: p-y spring approach

!Figure 7: Empirical factor for predicting the ultimate capacity of piles in sand (After API, 2007)

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!Figure 8: Comparison of the 1984 API lateral loading database with industry practice (after Doherty and Gavin 2011) !!TC5: Time Effects / Setup Around the globe there are a number of offshore platforms that have been subjected to hurricane and storm loading well above the 100 year events that were considered when calculating the pile geometries and penetrations. Despite these extreme loading events there have been very few case histories of pile failures. This indicates that there is an additional source of conservatism built into the design process. One possible explanation for this conservatism is that the pile capacity increases with time, termed pile ageing. The mechanisms underlying the change in capacity are poorly understood and therefore are not routinely incorporated into design. !!!!The capacity of piles is well known to exhibit time dependent behaviour, which most commonly presents as an increase in resistance, termed set-up or freeze. However, a small number of cases report a decrease in capacity, known as relaxation. This time-capacity dependency is not a new concept, with Wendel (1900) describing timber piles, which exhibited strength gains for two to three weeks after driving. Another common example of set-up is observed during pile installation where significant pauses in driving can lead to substantially higher blow counts when driving recommences (e.g. Fox et al, 1976, and Howard et al 2002). A number of investigations have been conducted into these effects, with changes in capacity quantified through comparison of multiple static load tests or multiple dynamic tests or in most instances a combination of both test methods. An example of one such research programme is depicted in Figure 9, which presents a series of load tests conducted on a 250 mm square concrete pile at Alborg, Denmark, which was reported by Skov and Denver (1988). This pile, which is 22m long and founded in a primarily cohesive deposit, clearly exhibits a consistent capacity increase with time. In addition, due to budgetary constraints load testing typically occurs over the days and weeks following installation and as a result the number of long term load tests conducted months and years later are limited. The relative influences of pore pressure dissipation and true ageing phenomena are therefore hard to separate from existing tests, leaving some doubt as to the reliability of extrapolative design procedures for predicting future long term capacities

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! !

Figure 9: Pile Capacity changes with time (data from Skov & Denver, 1988) A number of researchers have attempted to incorporate set-up into the design process by suggesting empirical correlations based on load test data at specific sites. For example Skov & Denver (1988) suggested that the pile capacity increased in a logarithmic trend with time based on a small number of case histories such as that illustrated in Figure 9. This empirical correlation takes the form outlined in Equation 1, where Qt is the capacity at time t after driving, Q0 is the reference capacity at time t0 and A is a constant dependent on soil type.

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!!!!The values of t0 and A recommended by Skov & Denver (1988), were 1 day and 0.6, respectively. As mentioned previously the data used to establish this correlation combines both static and dynamic testing procedures and includes the short term installation resistances in the regression analysis. The short term nature of these load tests reduces confidence in extrapolating such a procedure outside the load test time range where the mechanisms may change and the established trend may not be valid. Other researchers have also attempted to incorporate set-up into the design process, however the suggested design processes largely suffer from the same caveats as Skov & Denver (1988). A rational design approach, which considers set-up needs to consider the short term consolidation, induced capacity increases, separately from the long term aging phenomenon.!!!!!!As a pile is installed it penetrates the ground causing displacement of the soil in a fashion that resembles spherical cavity expansion ahead of the pile tip and radial expansion adjacent to the shaft. This expansion causes an increase in mean stress, which in the case of undrained cohesive soils is largely observed as an increase in mean pore pressure (!um). In addition, the shearing action of the pile shaft can induce additional changes in the pore pressure regime (!ush). The total excess pore pressure (!u) created by the installation process is the sum of these components. Following installation, the excess pore pressures dissipate and as a result the effective stresses increase causing an associated increase in shaft capacity. This behaviour is illustrated in Figure 10, which depicts the inverse relationship between excess pore pressures and shaft capacity observed by Konrad and Roy (1987), with the shaft capacity continuously increasing in response to the dissipation of excess pore pressures. Of particular interest for these pile tests conducted in the soft clay at St. Albans is the clearly non-linear variation of capacity with the log of time, in response to the non-linear pore pressure dissipation behaviour. This contrasts with the underlying assumption of the Skov & Denver (1988) model, highlighting the limitations of considering early stage and long term pile capacities within the same approach.

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!!

!Figure 10: Shaft Capacity compared to pore pressure dissipation at St Albans (Adapted from Konrad & Roy, 1987) !

!!!Similar field measurements on full scale piles, such as the Eugene Island Pile load tests, have measured pile capacity increases over considerable lengths of time, The first section of the 24-in.-diameter pile was driven to 125-ft penetration with a Vulcan 016 hammer. This section had a constant wall thickness of 1.5 inches. Blow counts ranged from 4 to 34 bpf. After making the add-on, the pile was then driven to 242-ft penetration with a Vulcan 020 hammer. Blow counts ranged from 18 to 54 bpf. Pile load tests were performed 54, 133 and 912 days after driving. After a 2.5-year set-up period, blow counts with a Vulcan 020 hammer were 625 and 115 bpf for the first two feet of driving. Blow counts then decreased to 75 bpf and gradually increased to 100 bpf at the final penetration of 316 ft. Pile load tests were performed 47, 104 and 140 days after redriving. Soil conditions consist of soft to stiff clay, with the exception of a sandy silt stratum encountered from 40- to 50-ft penetration, and a silty sand stratum encountered from 150- to 160-ft penetration. Additional details are given in Stevens (1988) and Vesic (1977). These increases in pile capacity are observed in a range of soil types from normally consolidated clays to overconsolidated sands indicating that there could be widespread benefits to industry. For example, Chow (1997) measured long term pile capacities at the Dunkirk dense sand research site, which were 250% of the initial test values, as shown in Figure 11. This time-dependent increase may be due to a chemical process, particularly corrosion, cementation, or creep, which reduces the arching effects of hoop stresses generated in the soil during pile installation. !

!!Figure 11: Load-displacement relationship for open-ended steel piles at Dunkirk, France (Chow, 1997)

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) 250% increase in capacity in less than 1 year

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By developing a more robust pile ageing model that explicitly considers the controlling mechanisms, it will reduce the uncertainty in predicting the long term pile resistance and will give more confidence to industry to adopt more economical designs. !TC6: Cyclic loading Existing design tools have been mostly developed to predict the ultimate resistance of foundations. However, the serviceability limit state is often the controlling design criterion, most significantly under cyclic loading (i.e., loading from waves). To date, detailed evaluations of load-unload-reload behaviour are typically limited to special value project, although the topic has been addressed in practice and research over the years. Poulos (1982) provided some of the first summaries of available data and proposed a method for evaluation behaviour of a pile subjected to variable cyclic axial loading. For lateral loading, API recommendations have early on included impact of cyclic loading by providing adjustment factors for soil p-y springs. A number of full size and model tests were also performed in recent years to study to cyclic loading effects, as well as analyses performed utilizing advanced numerical modelling programs. !!!!Evaluation of pile behaviour under cyclic loading combines multiple soil behaviour and soil-structure interaction aspects, which can be favourable or unfavourable. For example, clays can experience loss of strength and stiffness under cyclic loading, while at same time experience strength gain as a result of the strain rate effects (i.e., a more rapid loading compared to static loading at which the strength is typically measured). Large sustained tensile loads may, through creep, result in large movements of the pile and in a reduction in the tensile capacity. Cyclic loads may further degrade the tensile capacity. During axial loading, soil around long and slender piles loaded in cyclic tension can experience stress reversal and more rapid capacity degradation typically associated with tension-compression loading pattern. Tests performed on small diameter piles presented by Cox et al (1979) indicated that the rate effect associated with cyclic loading may more than offset this degradation, but full scale tests are needed. This problem could be exacerbated for jacket supported wind turbines where due to the low structural dead-weight the critical design case will be the cyclic tension loads in opposing leg piles. Finally, combination of axial and lateral cyclic loading can exacerbate rate of degradation, necessitating coupled evaluation of axial and lateral loading mechanisms. !!!! In addition to pile deflections and ultimate capacity, soil behaviour under cyclic loading affects the structural loading on the pile as well. With the soil degradation, bending moment distribution can shift along the pile length as measured on the example of the pile load test results shown in Figure 12 reported by Caliendo et al. (1999). This can further complicate optimization of pile design and pile sizing. !!

! Figure 12: Change in moment distribution for pile under lateral cyclic loading with increasing number of cycles (Caliendo et al. 1999). ! Overall, summarizing and clarifying available data in a well-developed framework for analysis of piles under cyclic loading, both in axial and lateral direction, could greatly benefit the practice. However, the analysis of behaviour under cyclic loading will be inherently challenged by the uncertainties in pile capacity assessment under static loads, as discussed earlier.

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Seismic loading, although cyclic in nature, is addressed differently from cyclic loading due to difference in loading cycle frequency and duration (i.e., number of cycles). While cyclic loading can measure thousands of cycles, seismic loading typically involves up to about 10 to 20 equivalent cycles at significantly higher frequency. While this limits some of the effects caused by cyclic loading (progressive degradation with high number of loading cycles), seismic loading has a global impact on the soil surrounding the pile, which in turn can impact pile capacity. Design aspects such as loss of axial and lateral capacity due to liquefaction of soil around the pile, soil softening and degradation due to high shear strains, and loading on pile from lateral soil movement need to be evaluate when susceptible conditions are present. While practice in localities with very high seismic activity typically integrated these aspects in the design, a development of universal framework and analysis methodology could assist in design optimization.!!Conclusions A number of technical concerns were identified and discussed. The implications of these challenges for industry were addressed individually, including:

• Pile axial capacity can be determined can be predicted by a number of different methods, including recently derived CPT based approaches, however the results can show vastly different pile penetrations, which indicates experienced geotechnical designers are needed when applying these methods.

• By trying to reduce capital expenditure, the offshore wind sector is pushing for more efficient and innovative substructure concepts. However, specific design tools that can accurately predict the behaviour of these foundations are also necessary.

• Predicting pile driveability is very sensitive to the assumed SRD approach, with considerable differences suggested by various industry accepted design methods.

• The design of lateral loaded piles (particularly large diameter monopiles) urgently requires research to investigate the impact of upscaling on the design stiffness and capacity.

• There is potential room for more efficient pile design by considering time dependent pile capacity increases in the design process.

• Design tools for considering the impact of cyclic loading are not as robust as those for static loading. !"#$#%#&'#(!Aldridge, T. R. (2010), Carrington, T.M., Jardine, R.J. , Little, R. , Evans T.G. & Finnie I. BP Clair phase 1 – Pile driveability and capacity in extremely hard till “Proc. of the International Symposium on Frontiers on Offshore Geotechnics” API (1969) "Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms, API RP2A, 1st Edition, American Petroleum Institute, Washington, D.C."

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