Phragmites australis L., a widespread vegetation in riparian zones such as rivers and canals, is extensively studied for its ecological benefits such as nutrient removal and hydraulic retention. However, its direct contribution to bank stability through root reinforcement, a key factor for its use in soil bioengineering techniques, has received limited attention. This study investigated the root reinforcement provided by P. australis and its root traits at a soil bioengineering test site on a canal bank in the Province of North-Holland in the Netherlands. Direct measurements of root-soil composite strength were performed using a corkscrew extraction technique at two distinct distances from the canal. Concurrently, root distribution parameters, including Root Area Ratio (RAR) and Root Length Density (RLD), were quantified from extracted soil plugs. Root reinforcement was also indirectly estimated using biomechanical models, incorporating measured root tensile strength and root distribution parameters as inputs. A total of 12 excavations, each 0.25 m2, were conducted for comprehensive root trait analysis at both locations. Direct measurements revealed substantial root reinforcement (max 36 kPa; avg 6–19 kPa). RAR showed effective stabilization values between 0.03 and 0.65 %, peaking at 0.65 % in the area close to canal. Root systems were dominated by fine roots (<0.5 mm diameter), comprising >80 % of total root length and creating dense reinforcing networks. Corkscrew measurements yielded conservative values. Modeled estimates significantly exceeded these field measurements, which is consistent with conventional shear testing. The extensive root surface area (>3.9 m2 m−2) further demonstrates the species' soil-binding capacity, with higher values occurring in hydrologically favorable zones. While the ecological implications of using this widespread species must be contextually considered, its pronounced mechanical reinforcement makes it a highly effective biotechnical tool, particularly in managed environments like canals. ... Read more

Large parts of banks of canals in the Netherlands are protected by azobé timber sheet piles. Many kilometers of sheet piles in the province of Noord-Holland, are planned to be replaced or to undergo maintenance. Yet, there is insufficient knowledge on the current state of the azobé sheet piles and their residual service life. Based on this, a series of investigations on azobé sheet piles after 57 years of service were performed. Visual inspections showed surface deterioration on the water-exposed side for all boards. Nondestructive testing using micro drilling technique showed no signs of internal deterioration. A maximum reduction in thickness of 17% and an average thickness reduction of 6.7% of original thickness were observed. CT scanning showed that the remaining cross sections of the azobé boards were intact and had comparable density of new azobé boards. An exponential damage accumulation model was used to predict the residuals service life of the timber sheet piles subjected to earth stress. Conservative estimates based on physical measurements and residual bending strength indicate that the sheet piles have an additional service life of 22–43 years from the current state. ... Read more

Hundreds of kilometers of timber sheet piles protecting the banks of canals of the Netherlands are nearing the end of their service life and need to be replaced. Even though azobé wood is very widely used, little is known about the current state of the azobé timber sheet piles that have been in service for many decades. More information about the current strength is necessary for planning any intervention, maintenance or reuse of the recovered sheet pile boards. This study aims to characterize the current state of azobé sheet piles using destructive techniques such as four point bending tests, compression tests and non-destructive techniques such as micro-drilling and visual assessment. To achieve this objective sheet piles that were in service for over 57 years were pulled out and tested in the laboratory. The non-destructive tests results indicate that the deterioration of azobé sheet piles is concentrated on the superficial layers of the boards. Visual classification and micro drilling techniques did not yield results that support the findings from the destructive tests. The bending strength and modulus of elasticity of in service sheet piles used in current study was found to be lower by about 25 % and 30 % respectively when compared to new azobé sheet piles reported in literature. Based on average values of measured dimensions, density of the sheet piles in this study was in general, lower compared to new sheet piles. Thus, the lower strength could be due to deterioration, lower intrinsic quality of the recovered sheet piles, or simply fall within the natural scatter of the material. In addition an exercise to classify the samples to a strength class is shown for practicing engineers. ... Read more

Traditional "hard" protection systems, such as hardwood timber sheet pile walls, are often used to protect banks of canals and streams, but the tropical hardwood they require is not always locally available. This has led to increasing interest in nature-based, bio-engineered solutions that combine locally sourced wood with vegetation to protect the soil. To assess the behaviour of locally available softwood timber sheet pile walls, a full-scale surcharge loading test was performed under realistic conditions. The test applied a 30 kPa surcharge load, representing the weight of a heavy agriculture machinery, while monitoring the wall's horizontal and vertical displacement, along with its rotation at the top, mid-height, and base of the retained soil. This resulted in a displacement of approximately 1.9% of the one meter retaining height. The potential onset of a failure wedge was observed after an extended loading period. Nonlinear tilt measurements showed peak curvature at mid-depth (0.66° top, 0.71° mid, 0.69° bottom), indicating dominant flexural bending. Additionally, the measured horizontal displacement exceeded the rotational contribution estimated from the tilt. The material properties of the softwood sheet piles were determined through four-point bending tests. A numerical model, calibrated with experimental data, was then developed to simulate the long-term performance (10 years) of decayed sheet piles with both bare and vegetated backfill. The results indicate that vegetated backfills significantly reduce displacement and the bending moment on the wooden sheet pile compared to bare soil. ... Read more

There is an increasing need for using nature based solutions in protecting canal and stream embankments in the Netherlands and delta areas in general. Vegetation provides additional reinforcement and forms an integral part of many nature-based solutions. However, quantifying this reinforcement in-situ is challenging. This study aims to quantify the root reinforcement of three species prevalent along canal embankments – Salix fragilis L. (SF), Salix purpurea L. (SP), and Crataegus laevigata DC. (CL) – using the corkscrew extraction technique. Furthermore, canal bank stability was analyzed under different bank conditions regarding protection (unprotected, protected by vegetation), bank geometry, and hydraulic conditions.

Quantity of roots and Root Area Ratio (RAR) generally decreased with depth for all species. While root breakage was observed in most samples, all species exhibited increased ductility with higher root densities, except for CL at two depths. SF showed higher root reinforcement at shallower depths (≤ 250 mm), while SP demonstrated greater reinforcement at deeper depths. Results demonstrate that the corkscrew extraction technique is a quick and minimally invasive method for measuring root reinforcement in riparian environments.

Bank stability simulations revealed that vegetation significantly increases the stability of canal banks. Notably, when considering measured root reinforcement, the factor of safety improved dramatically from 1.08 to 2.46, even under analyzed worst case conditions. However, the analysis suggests a limiting root reinforcement beyond which further increases in root reinforcement have minimal impact on stability. Monitoring using the corkscrew apparatus and future design approaches could aim to achieve this minimum reinforcement. ... Read more

Enhancing urban tree stability is critical for public safety and infrastructure protection. This study evaluates a nature-based method for improving tree stability using inosculations to form interconnected tree systems. These systems establish biomechanical connections through inosculation, offering both biological and mechanical support. The research focused on lime trees (Tilia Cordata Mill.), comparing parallel and cross connected tree systems with the single tree to evaluate their mechanical performance. The mechanical performance of the interconnected tree systems was evaluated by pulling tests in different directions to simulate wind loads. The study spanned a two-year growth period to investigate the effects of growth on mechanical behavior, with the analysis supported by finite element modeling. The results showed that growth-induced changes increased the overall rigidity of the tree systems and reduced deformation, rotation, and local elongation. Cross connected trees exhibited notable bracing effects in the connected plane, which improved lateral resistance. In a parallel connected tree system, the basal stiffness increased due to the connection between the lower region. Compared to the single tree, interconnecting tree systems can provide additional support and reduce deformation caused by lateral loads, making it a promising strategy to improve tree stability under horizontal loads. ... Read more

The Netherlands has an extensive network of rivers and canals systems serving purposes like irrigation, transportation and water removal. The banks along the canals are either protected by earth retaining structures such as sheet pile walls or left unprotected. A bulk of the engineered sheet piles used to protect the canal banks in the Netherlands are made of timber. Tropical hardwoods such as Azobé (Lophira alata) are used to make these timber sheet piles durable, owing to its high biological resistance to decay. Pine from North-west Europe is also used, but need to be treated chemically for sufficient durability. Even though roots of vegetation are known to increase the shear strength of soil, the positive effects of vegetation are not quantified in depth. Vegetation roots in a root-soil composite primarily act in tension when subjected to load, thus acting similar to steel in reinforced concrete. This thesis summarizes the efforts to study a bio-engineered earth retaining structure made of non-durable locally available wood species and vegetation to protect the canal banks as an alternative to the currently used bank protection structures. Such a retaining structure would not only reduce the need for durable hardwoods, but are also more environmentally friendly than the ‘hard’ retaining structures currently in use. Two vegetation types, Humulus lupulus L. and Salix fragilis L. were chosen for investigation based on their potential to reinforce canal banks, nativity, root characteristics and growth conditions such as presence of high ground water table. An extensive laboratory campaign was planned and conducted to characterize the strength of roots, root-soil composite and to study the behavior of the timber sheet pile-vegetation combination as a system. The experimental results were further extended to develop approaches in the design of timber sheet pile-vegetation system. To study the root-soil composite behavior in shear, a large scale direct shear apparatus was built. The apparatus was built in the view of conducting tests in dual loading modes, displacement controlled and load controlled shear. In order to simulate the canal bank conditions as closely as possible, the samples were tested in saturated conditions at low confining pressure. Bare soil, rooted samples of Humulus lupulus L. and Salix fragilis L. were tested in both displacement controlled and load controlled conditions. The roots were excavated after the test and analysis of root orientation, diameter and root biomass was conducted. Rooted samples showed a higher friction angle when compared to bare soil. Contractive behavior was shown by rooted samples and the peak stress ratio vs displacement trend of rooted samples were seen to diverge from the trend for bare soil. Burger model was seen to be able to capture the time dependent behavior under loading mode, when simulated over the experimental results. A tensile testing program was devised to test roots in tension in a displacement controlled and load controlled tests. Roots of Humulus lupulus L. and Salix fragilis L. were tested in both wet and dry conditions in displacement controlled tests. Load controlled tensile testing was conducted on samples of Humulus lupulus L. of two diameters. Power law was observed to estimate volume-effect and fit all the tensile strength-diameter variations. Comparison of tensile strength in dry and wet conditions revealed that significant difference in tensile strength was observed for Salix fragilis L. while no significant difference in tensile strength was observed for Humulus lupulus L. Further, time to failure of roots were studied using a power law model. A physical modelling approach was attempted to study the behavior of the timber sheet pile-vegetation system. A root system similar to Humulus lupulus L. was 3D printed using PLA material to be used in physical model. A comparison of unreinforced bank and bank reinforced with root analogues revealed that the presence of roots increase the volume of soil that needs to be mobilized for failure to occur. It was also observed that when root analogues were placed in the most efficient spatial pattern, among the conducted tests, are able to sustain twice the drawdown pressure. Subsequently, finite element modelling was conducted by including the effect of roots as an increased cohesion parameter. The results from modelling were seen to be able to capture the failure. Parametric analysis revealed that the influence of spatial distribution of the roots on forces acting on the sheet pile is higher, after a threshold value of additional cohesion is reached. That implies any additional cohesion after the threshold value might not provide any additional benefits to the stability. The results thus indicate that vegetation with more spatially distributed roots will be more suitable to be used in timber sheet pile-vegetation retaining system. Finally, two different perspectives in design approach of a timber sheet pile-vegetation system are investigated. The system approach is based on the concept that the mechanical reinforcement of the soil with growth of vegetation could result in a reduction of horizontal pressure against the sheet pile, bending moments and shear stresses acting on the sheet pile over time. This results in decreasing the duration of load effect in the timber and counteracting the effects of slow biological degradation of wood in air-water-soil conditions. Timber sheet pile components that are below the water level are less prone to decay. However, those components that are at the air water-soil interface are more susceptible to decay. In the discrete approach the vegetation is perceived as supporting the top parts of the stream bank (< 2meters) after timber decay has occurred. The effect of vegetation is analyzed as both increase in internal friction angle and increase in cohesion, on stream banks of retaining height of 2m and 3m. An increase in service life of nthe timber sheet pile-vegetation system is achieved in a system approach compared to when only timber sheet pile is present. In the discrete approach, it was observed that modification to the landscape by changing the slope angle of the bank might be necessary when the influence of vegetation is incorporated as an increase in friction angle of soil. In laboratory scale, as in this study, timber sheet pile-vegetation earth retaining systems show promises to be used for stream bank protection. Future studies need to focus on field scale system level studies and quantification of reinforcement of vegetation in presence of multiple species of vegetation. ... Read more

Tropical hardwood or treated wooden sheet piles have been used historically to protect stream banks in countries like the Netherlands. However, there is an imminent need for alternative as exploitable tropical hardwood forests available in the country is limited. Locally available softwoods are less resistant to decay and in extreme conditions like in a stream bank with high moisture content, softwoods show minimal durability. The section of the sheet pile at air-water-soil interface is the most prone to decay. Hence twin-wood sheet piles have been developed in industrial scale, which is a combination of hardwood and softwood. Hardwoods which have high resistance to decay is used above the water table while softwood is used in the section which is always below water. An alternative stream bank protection system using vegetation and softwood sheet pile was proposed recently. In such a system the effect of vegetation can be included as an increase in cohesion of the soil or an increase in internal friction angle. In this article two different approaches (System approach (SA) and Discrete approach (DA)) for the design of wooden sheet pile-vegetation system are examined. The SA shows that the presence of vegetation increased the time for complete damage of the wooden sheet pile-vegetation system to occur. In DA approach a change in slope of the face of the stream bank is required if the effect of vegetation is considered as an increase in friction angle of the soil. ... Read more

Timber sheet pile walls are widely used for the protection of stream banks in different parts of the world. However, there is a tendency to create more sustainable types of stream banks not only because exploitable wood is more difficult to obtain, but also because of disturbance to the natural habitat of plants and animals due to hard embankments. In the Netherlands alone, about 2500 km of engineered timber sheet pile wall embankments exist, primarily made with tropical hardwood, besides an even much larger amount of ‘non-engineered’ small-size timber-based embankments. As an alternative, the authors propose to use a mixed timber sheet pile-vegetation system, where locally available timber can be applied in combination with natural vegetation. Unlike the usual bioengineering scheme, vegetation is not seen as an element, which could replace the timber sheet piles. Instead, a new perspective is tested, where the vegetation is included as a ‘structural’ element which can even counteract the consequences of time-dependent biological degradation of the timber sheet pile. By doing so, both long-term durability as well as reliability of the stream bank is improved. A comprehensive design strategy was developed based on well-established sub-models from the literature on plant growth, root reinforcement as well as timber damage accumulation. The timber sheet pile wall-vegetation system is illustrated in an example case study. Preliminary analysis including only the mechanical reinforcement of vegetation shows that there is a decrease in moment and shear acting on the timber sheet pile with growth of the vegetation. Consequently, the damage accumulation due to load duration effects on the timber decreases and the service life of the system increases. Thus, using vegetation in combination with highly degradable timber could possibly negate the need for using hardwood timber, or more generally, save resources that are currently used for these structures. ... Read more

The modelling of the triggering mechanism of rainfall-induced landslides in slopes covered by pyroclastic soil (as the area surrounding Mount Vesuvius in Campania, Italy) requires the hydraulic characterization of soil in unsaturated conditions in order to analyse the slope response to rainfalls. In previous studies carried out on Campanian pyroclastic soils, the volumetric soil changes due to suction changes have been disregarded, being them negligible in soils characterized by low plasticity and low clay contents. However, a more accurate determination of the water retention curve (WRC) in terms of volumetric water content requires a correct estimation of the total soil volume, which is affected by the soil stress-state. The proper approach would require the estimation of both WRC in terms of gravimetric water content and the shrinkage curve (SC). In the present study, a relation between void ratio and suction was determined for a pyroclastic soil sampled at Mount Faito in Southern Italy. Therefore, a correction of the volumetric water content was carried out resulting in updated water retention curves. Here, the matric suction was the only factor affecting the stress-state of the soil. ... Read more

The geotechnical construction industry is a major component of the overall construction sector and is strategically important in infrastructure development (transportation, flood and landslide protection, building foundations, waste disposal). Although industry and research in the overall construction sector have been investing significantly in recent years to produce innovative low-carbon technologies, little innovation has been created in geotechnical construction industry, which is lagging behind other construction industry sectors. This paper discusses the interplay between low-carbon geotechnical engineering and unsaturated soil mechanics based on the research carried out within the project TERRE (Marie Skłodowska-Curie Innovative Training Networks funded by the European Commission, 2015-2019,H2020-MSCA-ITN-2015-675762). ... Read more

Timber sheet piles are widely used to protect canal and stream banks. Quite often, riparian vegetation also grows along these retaining structures. Roots of riparian vegetation mechanically reinforce the soil with their root systems. A timber sheetpile- vegetation model is developed taking into account the mechanical reinforcement of the vegetation roots. The model uses easy to obtain physical parameters, which makes it suitable to have a preliminary estimate of how the forces on the bio engineered structure would evolve. ... Read more

Timber sheet pile walls are widely used for the protection of stream banks in different parts of the world. However, there is tendency of creating more sustainable types of stream banks not only because exploitable wood is more difficult to obtain, but also because of disturbance to the natural habitat of plants and animals due to hard embankments. In The Netherlands alone, about 2500 km of engineered timber sheet pile wall embankments exist, primarily made with tropical hardwood, apart from an even much larger amount of ´non-engineered´ small size timber based embankments. As an alternative, the authors propose to use a mixed timber sheet pile-vegetation system, where locally available timber can be applied in combination with natural vegetation. Unlike the usual bioengineering scheme, vegetation is not seen as an element, which could replace the timber sheet piles. Instead a new perspective is tested, where the vegetation is included as a ´structural´ element which will reduce or even counteract the consequences time dependent biological degradation of the timber sheet pile. By doing so, both long term durability as well as reliability of the stream bank are improved. We have developed a comprehensive design model, based on well-established sub- models from the literature on plant growth as well as timber service life. The timber sheet pile wall-vegetation system is illustrated in an example case study. Preliminary analysis including only the mechanical reinforcement of vegetation shows that the durability of timber sheet piles is enhanced. Thus, using vegetation in combination with highly degradable timber could possibly negate the need for using hardwood timber, or more generally, save resources that are currently used for these structures. ... Read more