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The piled embankment represents one of the solutions for the realization of a soil body on a compressible subsoil where extended settlement or insufficient stability threatens the serviceability of related structures. Widely adopted analytical design procedures were analyzed: Marston’s formula and Hewlett and Randolph method contained in the British standard BS 8006-1, the German regulation EBGEO and the Dutch regulation CUR 226. Using these recommendations, the theoretical values of the individual parts of the load acting in the embankment and, subsequently, the values of the axial strain or tensile forces in the reinforcement were determined and compared with experimental data obtained from the tests in the large-scale physical model. For the presented case, without subsoil support, CUR 226 with the inverse load, which is recommended in the case of subsoil with low bearing capacity, shows better coincidence with the measured data. Overall, EBGEO and CUR 226 can be considered to be close to the real behavior of the piled embankment. Because of the frequent utilization of geosynthetic reinforcement and possible changes of subsoil parameters during the service life of the piled embankment, a rheological process of its elements should be investigated during the design process.

Reduced climate impact is a worldwide strive today. The foundation engineering industry is continuously searching for more sustainable solutions to reduce resource usage and pollutions directly or indirectly. One such solution is timber piling, as an alternative to the commonly used concrete and steel piles. Geosynthetic-reinforced pile-supported embankment (GRPSE) is a common foundation method for settlement reduction of both roads and railways on soft subsoil. Pile-supported embankments rely on arch formation within the embankment material, which transfers the traffic and embankment load onto the piles. Reinforcing of the embankment with geosynthetics further increases this load transfer, whilst also stabilising the formed arches. Substituting concrete and steel piles with timber piles allows for a GRPSE solution with lower carbon footprint, especially if the timber piles are untreated and the (concrete) pile caps are excluded. The lower strength of timber piles and exclusion of pile caps require narrower pile spacing and/or more extensive geosynthetic reinforcement to maintain a stable arch formation in the embankment. Unstable arches can cause unwanted differential settlements in the upper structure of the embankment. Although timber piling is being practiced in countries like United States, Canada, Australia, and the Netherlands, only Sweden has a dedicated code for GRPSE using untreated timber piles. However, the Swedish code is deliberately conservative with narrow required pile spacing and two layers of GR. The aim of the thesis is to improve the resource efficiency and sustainability of geosynthetic-reinforced timber pile-supported embankments, by optimizing the required number of piles and amount of geosynthetic reinforcement (GR) based on the Swedish code. First, a numerical study was performed to evaluate the Swedish code with a focus on the pile group. The code states that the timber piles should be installed with a centre-to-centre distance of 0.8–1.2 m in a triangular arrangement instead of the more common square arrangement. Finite element (FE) modelling setups—with square and triangular pile arrangements with varying centre-to-centre distance—were used based on a geosynthetic-reinforced timber pile-supported road embankment to evaluate the design criteria. As part of the evaluation, a state-of-the-art study was done on international design guidelines and analytical models. From the FE simulations, no evident difference of mechanical behaviour is found between the triangular and the square piling pattern. The maximum allowed centre-to-centre distance between timber piles can be increased from 1.2 to 1.4 m, decreasing the number of timber piles by as much as one-third. Second, a field study was carried out. A reconstructed road embankment with geosynthetic reinforcement and timber pile support was instrumented in 2020, and the first two years of post-construction data was analysed. Monitoring of the embankment included settlements, pile deflection, pore water pressure, load on piles and subsoil, and strains in the GR. Only small strains were observed in the GR because of minor GR deflections. Partial arch formation was found from the measured load distribution, as less than half of the total load of embankment and traffic was carried by the piles. The pile loads increased in winter as the frost front penetrated the embankment and stiffened the embankment fill. The field study data provides a detailed reference for this thesis and further research on timber pile-supported embankments. Third, a numerical study was performed on the effect of geosynthetic reinforcement in timber-piled embankments. Three different cases were studied: two layers of GR (“beam” theory load transfer), one layer of GR (“catenary” load transfer) and unreinforced. The hypothesis is that there exists a range of embankment heights and centre-to-centre pile spacings for which one of the cases is preferrable to the other two in terms of resource efficiency. FE simulations, calibrated using the data from the field, were performed of the three cases with varying pile centre-to-centre spacing, embankment height and subsoil material. Based on the results of the FE simulations and literature on arch formation, limits for the three cases are suggested as a guideline for stable arch formation in timber-piled embankments. The limits allow for a more object-specific design than the Swedish code, improving the resource efficiency both in terms of required number of piles and amount of GR. Based on the findings from the three sections presented in this thesis, recommendations are given on timber pile-supported embankments. The recommendations are based on theoretical calculations and field test data. As an outlook, the thesis outlines the design of a physical laboratory test setup to verify the theoretical results and establish implementable recommendations. Though the primary application of timber pile-supported embankments, the physical test results can be extrapolated to pile-supported designs in general. Minskad klimatpåverkan är idag en världsomspännande strävan. Grundläggningsbranschen söker kontinuerligt efter mer hållbara lösningar för att minska resursanvändning och utsläpp direkt eller indirekt. En sådan lösning är träpålning, som ett alternativ till de vanligt förekommande betong- och stålpålarna. Armerad bankpålning är en vanlig sättningsreducerande grundläggningsmetod för både vägar och järnvägar på mjuk undergrund. Bankpålning nyttjar valvbildning i bankfyllningen, som överför trafik- och banklasten på pålarna. Jordarmering av geosynteter i banken ökar denna lastöverföring ytterligare, samtidigt som den stabiliserar de formade valven. Ersättning av betong- och stålpålar med träpålar möjliggör armerade och pålade bankar med lägre koldioxidavtryck, speciellt om träpålarna är obehandlade och pålplattorna (i betong) exkluderas. Träpålarnas lägre hållfasthet och exkludering av pålplattor kräver dock ett mindre pålavstånd och/eller mer omfattande geosyntetisk förstärkning för att upprätthålla en stabil valvbildning i banken. Instabila valv kan orsaka oönskade differentialsättningar i bankens övre del. Även om träpålning praktiseras i länder som USA, Kanada, Australien och Nederländerna, är det bara Sverige som har en specifik standard för armerad bankpålning med obehandlade träpålar (TRVINFRA-00230, tidigare TK Geo 13). Den svenska standarden är dock medvetet konservativ på grund av osäkerheter. Syftet med avhandlingen är att förbättra resurseffektiviteten och hållbarheten för armerad bankpålning med (obehandlade) träpålar, genom att optimera erforderligt antal pålar och mängden jordarmering baserat på den svenska standarden. Detta gjordes genom tre studier. I den första studien gjordes en numerisk analys för att utvärdera den svenska standarden med fokus på pålgruppen. Enligt standarden ska träpålarna installeras med ett centrumavstånd på 0,8–1,2 m i ett triangulärt pålningsmönster i stället för det vanligare kvadratiska pålningsmönstret. Finita element (FE) analyser av ett flertal uppställningar—med kvadratiska och triangulära pålningsmönster för varierande pålavstånd—genomfördes baserat på en geosyntetförstärkt vägbank på träpålar för att utvärdera designkriterierna. Som en del av utvärderingen gjordes en litteraturstudie av internationella dimensioneringsriktlinjer och analytiska modeller. Från FE-simuleringarna erhölls ingen tydlig skillnad i det mekaniska beteendet mellan triangulärt och kvadratiskt pålningsmönster. Det maximalt tillåtna centrumavståndet för träpålar kan ökas från 1,2 till 1,4 m, vilket minskar antalet träpålar med så mycket som en tredjedel. I den andra studien upprättades en fältlokal. En ombyggd vägbank, förstärkt med träpålar och jordarmering, instrumenterades 2020 och mätdata från de två första åren efter ombyggnationen analyserades. Mätningarna i fältlokalen omfattade sättningar, pålutböjning, porvattentryck, belastning på pålar och undergrund samt töjningar i jordarmeringen. Små töjningar observerades i jordarmeringen på grund av liten nedböjning. Ofullständig valvverkan konstaterades utifrån den uppmätta lastfördelningen, då mindre än hälften av den totala lasten av vägbank och trafik bars upp av pålarna. Pållasten ökade vintertid då tjälen trängde ner i banken och förstyvade bankfyllningen. Data från fältstudien utgör ett detaljerat referensobjekt för denna avhandling och den fortsatta forskningen inom bankpålning med träpålar. I den tredje studien gjordes en numerisk analys av effekten av jordarmering i träpålade bankar. Tre olika fall testades: två lager armering (balkteorin), ett lager armering (kedjeteorin) och oförstärkt. Hypotesen är att det finns ett intervall av bankhöjder och pålavstånd där ett av fallen är att föredra framför de två andra vad gäller resurseffektivitet. FE-simuleringar, kalibrerade med hjälp av data från fältstudien, utfördes för de tre fallen med varierande pålavstånd, bankhöjd och undergrundsmaterial. Baserat på resultaten från FE-analysen och litteratur om valvbildning föreslås gränser för de tre fallen som riktlinje för stabil valvbildning i träpålade bankar. Gränserna möjliggör en mer objektsanpassad dimensionering än den nuvarande svenska standarden, vilket förbättrar resurseffektiviteten både vad gäller erforderligt antal pålar och mängden jordarmering. Baserat på slutsatserna från de tre studierna som presenteras i denna avhandling, ges rekommendationer för träpålade bankar. Rekommendationerna baseras på teoretiska beräkningar och en fallstudie. Som en framåtblick beskriver avhandlingen utformningen av en fysisk laboratorieuppställning för att verifiera de teoretiska resultaten och upprätta implementerbara rekommendationer. Även om den primära tillämpningen är träpålade bankar, kan de fysiska testresultaten tillämpas på generell bankpålning.

Timber piling allows for a solution with lower carbon footprint than concrete or steel piling, yet there exist few well-documented cases of modern timber piled embankments. In this paper, field measurements on a geosynthetic-reinforced timber pile-supported road embankment are reported and evaluated. The monitored road embankment is a section of a newly reconstructed semi-motorway in northern Sweden. The embankment was constructed on 8 m long untreated timber piles with 1.1 m spacing in a triangular pattern, without pile caps. On top of that, a 1.7 m high embankment was constructed, reinforced by two layers of biaxial geogrids. A long-term monitoring program is being carried out from when the semi-motorway was reconstructed. This study presents results from the first year of monitoring. The measurements include the load on the pile heads and subsoil, geogrid strain, pore water pressures, and settlements. The measurements show the development of arching over time, the interlocking of geogrid and embankment material, the subsoil consolidation, etc. The results of the monitoring are compared with results of analytical models from recommendations and codes. Funder: Branschsamverkan i grunden (BIG)

Geogrid-reinforced piled embankments (GRPEs) provide an economical and effective way to construct highways and railways on soft soil foundations. This paper proposed a new design method for GRPEs. The method was based on the soil arching and tensioned-membrane effects, the bearing capacity of the subsoil was considered as well. The originality of the proposed method lies in considering the stress history of the subsoil, and different over-consolidation ratios (OCRs) were used in calculating the settlement of subsoil. This design method, initially, established the vertical equilibrium of the unit body between the pile caps immediately above the subsoil. After that, the design charts were produced by solving the overall equilibrium equation from which engineers can intuitively obtain the maximum strain of reinforcement, and the tensile force can be used in the ultimate limit state analyses. The design method was then validated by three case studies, which showed good reliability with the maximum error being less than 18%. Parameter study results indicated that the maximum strain of reinforcement for the under-consolidated soil was 80–120% larger than that for normally consolidated soil and more than four times greater than that for over-consolidated soil.

A series of twenty-eight centrifuge tests was performed on piled embankments with basal geosynthetic reinforcement to assess the influence of pile spacing, embankment height, pile cap size and geosynthetic stiffness on the load transfer mechanism and surface settlements. The measurements of the forces on the piles made it possible to assess the load transfer mechanisms, and 100% efficiency was achieved for all tests performed. The results showed that for the thicker mattress and/or closer piles, the surface settlements were smaller or negligible. Geosynthetic maximum deflections were also examined experimentally and analytically, the latter based on BS8006 (2010) and its further corrigendum in 2012. Close agreement in the predictions of the maximum reinforcement deflection was reached with BS8006 (2012) by adopting a slight modification in the ratio of diagonal and orthogonal maximum deflection (yd/y = √2).

ISBN för värdpublikation: 978-91-7790-542-4, 978-91-7790-543-1

In recent years, the employment of deep foundations as settlement reducers has become increasingly popular in the design of earth embankments over soft soil strata. To further improve the system response, geosynthetic layers are often positioned at the embankment base. Owing to the presence of both piles and geosynthetics, complex interaction mechanisms, transferring stresses towards the piles and reducing those on the soft foundation soil, take place. Although these mechanisms are governed by the relative stiffness of the various elements constituting this system (piles, foundation soil, embankment and georeinforcements), the approaches commonly adopted to design these “geostructures” do not explicitly take into consideration the stiffness of the single element as design parameters. Moreover, the effect of the stepwise embankment construction process is often disregarded. As a consequence, the settlements at the top of the embankment cannot quantitatively be estimated.

There are currently several national standards or regulations for the design of the piled embankment, providing suitable solutions for foundation of transport structure on soft, high compressible subsoil, [1]. The most widely used and the best-known standard is British Standard BS8006 [2], which was confronted with another analytical design methodologies (Ebgeo, CUR). Today’s popularity and versatility of FEM numerical models brings many advantages, which analytical methods cannot achieved, but must be verified by proposed scaled physical model, which was currently being developed by Department of Geotechnics, University of Žilina.

Embankment piling is a widely used foundation method in geotechnical engineering and the design is mostly based on empirical studies found on experimental or field evidents. Several studies have been conducted to capture the behaviour of embankment piles by means of numerical models. One of the issues is to simulate the arching effect between the piles. This has raised the question of the optimum pile arrangement. Along the northern coast of Sweden, where soft soil types and dense woodlands are common, timber piles have often been used in piled road and railway embankments. The pile group is designed as a semi-floating deep foundation in soft soil, where the piles are resting on top of the locally common firm glacial till bottom layer, naming the method light embankment piling. The Swedish design guidelines recently changed from a square to a triangular pile arrangement. However, the motivation of the changed lacked research which raised the question of optimal pile arrangement and centre-to-centre pile distance. This project aims to optimise the light embankment piling method through numerical analysis, verified by field and laboratory experiments. The optimisation will mainly focus on pile arrangement and centre-to-centre pile distance. Optimal träpålning

International audience; The seismic stability analysis of an embankment lying over a soft foundation soil reinforced by a group of vertical piles is performed within the framework of the upper bound kinematic approach of yield design. The analysis is based on a previously developed 'multiphase' model of the reinforced ground, which explicitly accounts for the shear and bending resistances of the piles. Making use of appropriate failure mechanisms involving shear zones across which the reinforcements are continuously deforming, along with 'plastic hinge' surfaces, upper bound estimates to the critical seismic coefficient of the structure are derived. The results, which are confirmed by the simulations obtained from a finite element elastoplastic code, give clear evidence of the key role played by the bending strength capacities of the piles in ensuring the stability of the pile reinforced embankment.

Several analytical and numerical models have been introduced and tested to assess the arching effect on piled embankment. Given the prevalence and popularity of this case, it is not easy to select and recommend best design methodology. Used materials and types of reinforcement play an important role when it comes to designing high-speed railway lines in difficult geological conditions. Our numerical approach by 3D FEM analysis was focused on the influence of geometry to the arching creation in basal reinforcement layer, especially to distribution of internal forces in high tensile geogrids.

2D and 3D finite element simulations were performed to investigate the mechanical behaviour of the light embankmentpiling method as a pre-study to further develop a new Swedish design guideline. The light embankment piling method is used forsulphide soils and utilises timber piles as its key feature. The Swedish Transport Administration (Trafikverket) recently changed thenational standard of the light embankment piling method from the use of a square to a triangular pile arrangement, based on a theory thata triangular arrangement creates more stable arches in between the piles. The objective with the present study is to evaluate the currentstandard by modelling setups with square and triangular pile arrangements with varying centre-to-centre distance. Both, completelyfloating and semi-floating pile groups were modelled here. The evaluation mainly focused on comparing embankment settlements as wellas axial forces in the piles. No evident difference in the mechanical behaviour between the triangular and the square piling pattern wasfound. The maximum allowed centre-to-centre distance can potentially be increased from 1.2 to 1.5m; resulting in approximately 30% fewer piles used. Alternativ titel: Analyse numérique du comportement mécanique des remblais sur pieux. Optimal träpålning