Toe Berm Research Articles

Confined-Crest Impact: The Influence of the Toe Berm on the Impulsive Forces

Confined-Crest Impact: The Influence of the Toe Berm on the Impulsive Forces

Experimental Study Of Parsian Port Breakwater Toe Stability

The stability performance of toe layer in structures with single-layer concrete armoures such as accropods and Xblocks is more crucial regarding their fast instability propagation and failure mechanism. This paper presents the case study concerning toe stability of Pasrsian port breakwater by experimental tests on physical model in Tarbiat modares university wave flume. Parsian industrial port is located in the south of Iran at Persian Gulf coastline. Its nineteen berths in final phase will be protected by approximately 1500 meter breakwater which at the most critical section, W5, reaches the sea bed level of -25 m.C.D. Figure 1. In concrete armored breakwaters, heterogeneous packing could increase the porosity in top rows and eventually result in instability. Therefore the number of rows in which the concrete units can be placed would be limited. According to the SUGRA guidance for Accropode units, this the maximum number of rows shall be 20, which leads designers to consider toe berm for breakwaters in deep water. Hence, it was vital to conduct a comprehensive physical model study.

EXPERIMENTAL EVALUATION OF THE WAVE LOAD APPLIED TO THE TOE BERM OF A RUBBLE MOUND STRUCTURE BUILT ON VERY STEEP SLOPES

It is sometimes difficult to design a rubble mound structure along the shorelines of lakes located in mountainous environments, due to the typical extreme steepness of the underwater profile. A stable rubble mound structure requires that the underlying bed slope is at least, say, 2H:3V, and in addition there must be some space to accommodate a small horizontal berm. The berm stability assumes an extreme importance, obviously. This research, funded by the Brevetti+ program, studies the behavior of a system that allows for the placement of rubble mound structures along steep profiles, since the stability of the toe berm is assured by tie rods.

Development and Assessment of a Hybrid Breakwater-Integrated Wave Energy Converter

Harnessing and using marine renewable energy at seaports is a promising solution to put these energy-intensive infrastructures on the right track to energy self-sufficiency and environmental sustainability, reducing their carbon footprint. This paper presents a summary of the main conclusions and achievements of a recently concluded R&D project that encompassed the experimental study of an innovative hybrid wave energy converter integrated into a case-study rubble-mound breakwater in the Port of Leixões, Portugal. It also describes the prospective studies planned in two ongoing projects, PORTOS – Ports Towards Energy Self-Sufficiency and WEC4Ports – A hybrid Wave Energy Converter for Ports, intended to further develop and assess this promising technology. It has been demonstrated that its wave-to-wire efficiency and annual energy production are 27.3% and 35.0 MWh/m per year, respectively, for the case-study location. Hence, a 240 m long device could provide more than half of the port’s electricity consumption, which vows for the device’s potential. Moreover, the impact of its integration into the case-study breakwater showed that it leads to a 50% reduction of overtopping discharges over the structure, and no significant effects on the structure’s wave reflection, although the stability of the toe berm blocks was negatively impacted. Overall, the conclusions obtained are favourable to the integration of this technology into rubble-mound breakwaters. Notwithstanding, further research is still needed, namely in terms of wave forces acting upon the structure, important for the assessment of the functional performance and lifecycle readiness of the technology, and the use of PTO control strategies. This is being addressed in PORTOS and WEC4Ports projects.

Hydraulic and Structural Assessment of a Rubble-Mound Breakwater with a Hybrid Wave Energy Converter

Seaports’ breakwaters serve as important infrastructures capable of sheltering ships, facilities, and harbour personnel from severe wave climate. Given their exposure to ocean waves and port authorities’ increasing awareness towards sustainability, it is important to develop and assess wave energy conversion technologies suitable of being integrated into seaport breakwaters. To fulfil this goal whilst ensuring adequate sheltering conditions, this paper describes the performance and stability analysis of the armour layer and toe berm of a 1/50 geometric scale model of the north breakwater extension project, intended for the Port of Leixões, with an integrated hybrid wave energy converter. This novel hybrid concept combines an oscillating water column and an overtopping device. The breakwater was also studied without the hybrid wave energy device as to enable a thorough comparison between both solutions regarding structural stability, safety, and overtopping performance. The results point towards a considerable reduction in the overtopping volumes through the integration of the hybrid technology by an average value of 50%, while the stability analysis suggests that the toe berm of the breakwater is not significantly affected by the hybrid device, leading to acceptable safety levels. Even so, some block displacements were observed, and the attained stability numbers were slightly above the recommended thresholds from the literature. It is also shown that traditional damage assessment parameters should be applied with care when non-conventional structures are analysed, such as rubble-mound breakwaters with integrated wave energy converters.

Influence of toe berm geometry on stability of reshaping berm breakwaters

Reshaping berm breakwaters have been mainly designed and built for water depth less than 20 m. In such conditions, a toe structure may be needed to reduce bottom settlements and increase geotechnical stability in the loose subsoil condition. Also, a toe berm can be deployed to reduce the berm recession and increase the stability of the Reshaping berm breakwater, especially in deep water. Although many studies have been conducted to study the effects of toe berms on stability of conventional breakwaters, investigations on the influence of a toe berm on stability of reshaping berm breakwaters are rare. This paper presents results of an experimental work that has been carried out to investigate the influence of a toe berm on the hydraulic stability of reshaping berm breakwaters. In a 2D physical test setup in a wave flume, a total of 207 tests were conducted to systematically examine the effects of toe berm configuration on the reshaping of berm breakwaters with three types of armor stones. The experimental program covers the influence of different geometrical parameters of the toe berm on the berm recession under various sea state conditions. Comparing results of berm recession of the cases having a toe structure with the cases without a toe berm shows that the amount of berm recession with toe berm is considerably less than the analogous amount for without toe structure. It is observed that an increasing toe berm width and thickness both have considerable influence on recession reduction. However, the toe depth has a relatively larger influence on recession than the toe width, and the influence is larger for higher stability numbers. It is concluded that a toe berm in front of reshaping berm breakwater not only does have influence on recession due to the depth influence and the changing wave conditions, but also the toe depth has a direct influence on the reshaped profile by preventing the displaced rocks to fall down to deeper parts. Using the test results, a new formula is developed for estimation of berm recession by taking into account the influence of the toe berm configuration. Given an acceptable stability number (Ho) as a design criterion for reshaping berm breakwaters, the present method predicts that the recession can be reduced up to 35% by the application of a toe berm. Thus, the main berm width can be shortened, resulting in reduction of the required armor stones volume. So, by using a smaller stone size for the toe berm, the stability of the structure is secured and also the total cost of the breakwater can be reduced.

Experimental Study of a Hybrid Wave Energy Converter Integrated in a Harbor Breakwater

Sea ports are infrastructures with substantial energy demands and often responsible for air pollution and other environmental problems, which may be minimized by using renewable energy, namely electricity harvested from ocean waves. In this regard, a wide variety of concepts to harvest wave energy are available and some shoreline technologies are already in an advanced development phase. The SE@PORTS project aims to assess the suitability and viability of existing wave energy conversion technologies to be integrated in harbor breakwaters, in order to take advantage of their high exposure to ocean waves. This paper describes the experimental study carried out to assess the performance of a hybrid wave energy converter (WEC) integrated in the rubble-mound structure that was proposed for the extension of the North breakwater of the Port of Leixões, Portugal. The hybrid concept combines the overtopping and the oscillating water column principles and was tested on a geometric scale of 1/50. This paper is focused on the assessment of the effects of the hybrid WEC integration on the case-study breakwater, both in terms of its stability and functionality. The 2D physical model included the reproduction of the seabed bathymetry in front of the breakwater and the generation of a wide range of irregular sea states, including extreme wave conditions. The experimental results shown that the integration of the hybrid WEC in the breakwater does not worsens the stability of its toe berm blocks and reduces the magnitude of the overtopping events. The conclusions obtained are therefore favorable to the integration of this type of devices on harbor breakwaters.

TOE STABILITY IN VERY SHALLOW WATER COMBINED WITH STEEP SEA BOTTOM

It is common to construct a rock toe berm of three to four rocks wide when concrete armor units are placed in the armor layer. This toe berm is a relevant element, especially in very shallow waters combined with steep sea bottoms, where waves directly attack the toe berm and the lowest part of the armor. Several formulas are available to estimate the damage to rock toe berms. In this paper, these formulas are compared for different design conditions within their range of application. Most of these formulas use the damage parameter Nod. However, there are often situations in which wider toe berms are required for a safe design, and the damage parameter Nod is not recommended. A methodology is thus proposed to design wider toe berms based on the damage to the nominal toe berm of three rocks wide (Nod*), considered as the most shoreward part of the toe berm which effectively supports the armor layer.

TOE BERM DESIGN FOR RUBBLE-MOUND BREAKWATERS: EXAMPLE OF THE SAFI POWER PLANT PROJECT

DAEWOO E&C (Engineering & Construction) is in charge constructing a new 1320 MW coal-fired power plant located approximately 15 km south-west of the city of Safi in Morocco. ARTELIA Eau & Environnement was appointed by the Contractor to perform the hydraulic design review of the rubble-mound breakwaters protecting the intake and outfall. The toe berm is a key design feature of rubble-mound breakwaters built in breaking conditions, since it helps to support the armour layer and protect the structure from potential scour-induced damage. The initial toe berm design was based on Van Der Meer’s empirical formula (1998). Due to the very shallow water conditions, the toe design was verified through physical model tests (2D and 3D) in ARTELIA’s hydraulics laboratory located in Pont-de-Claix, near Grenoble (France). The physical model tests demonstrated that the toe berm (6t rocks, 3:1 slope) was not stable at key singular locations, namely roundheads and roots, where direct impacts of breaking waves caused severe damage. Given the site conditions and the construction methods, the usual solutions consisting in increasing the rock size and/or placing the toe berm in a trench had to be ruled out. It was hence decided to reinforce the toe with artificial blocks and to use rectangular concrete blocks with holes. These blocks reduced the anti-stabilizing pressure difference between the top and bottom of the blocks (Tanimoto et al., 1996) and drag force due to the considerable current. They are more usually used at the toe of vertical caissons.

Hydraulic stability of nominal and sacrificial toe berms for mound breakwaters on steep sea bottoms

When mound breakwaters are placed on steep sea bottoms in combination with very shallow waters, the design of the toe berm becomes a relevant issue. Toe berms built close to the water surface on a steep sea bottom must withstand such high wave loads that their design may not be feasible with available quarrystones. In this study, a new design method was developed to reduce the rock size by increasing the toe berm width. The analysis involved specific 2D small-scale tests with toe berms of different rock sizes and widths, placed on a m=1/10 bottom slope with the water surface close to the toe berm crest. Two new concepts were introduced to better characterize damage to wide toe berms: (1) the most shoreward toe berm area which effectively supports the armor layer, in this study referred to as the primary or “nominal” toe berm and (2) the most seaward toe berm area which serves to protect the nominal toe berm, in this study called the secondary or the “sacrificial” toe berm. Damage to the nominal toe berm was used to describe hydraulic stability of wider toe berms. Given a standard toe berm of three rocks wide (nominal toe berm), an equivalent toe berm with damage similar to the nominal toe berm was defined by increasing the berm width and decreasing the rock size. The reduction in rock size showed an inverse 0.4-power relation with the relative berm width.

Toe berm design for very shallow waters on steep sea bottoms

The toe berm is a relevant design element when rubble mound breakwaters are built on steep sea bottoms in breaking conditions. Different design formulas can be found in the literature to predict the damage caused to submerged toe berms placed on gentle bottom slopes. However, these formulas are not valid for very shallow waters in combination with steep sea bottoms where toe berms receive the full force of breaking waves. To guarantee breakwater stability in these conditions, new design formulas are needed for toe berms. To this end, physical model tests were carried out and data were analyzed to characterize rock toe berm stability in very shallow water and with a bottom slope m=1/10. Based on test results, a new formula was developed with three parameters to estimate the nominal diameter (Dn50) of the toe berm rocks: water depth at the toe (hs), deep water significant wave height (Hs0), and deep water wave length (L0p).

INVESTIGATIONS ON QUARRY STONE TOE BERM STABILITY

Model test results from four experimental studies have been compiled in a data set with 687 test results. Three widely used or recently developed toe stability formulae have been validated against these model test results. It was found that all three formulae suffer from a lack of accuracy and general validity. This is caused by shortcomings of the underlying wave flume studies. More precisely, the wave height and the water depth above the toe berm are two main influence parameters for the toe berm stability and they are not independent in these studies. Testing the interdependence of parameters is therefore recommended for wave flume studies. An alternative toe berm stability formula was developed by a step-wise approach starting with a simple case with a minimum number of influencing factors and followed by more complex cases. The new formula is believed to provide a more meaningful description of the toe berm stability than existing formulae; this however requires further substantiation. The new approach is proposed as a working hypothesis for further studies and is recommended as benchmark for toe berm testing and design.

SINGLE-LAYER BREAKWATER ARMOURING: FEEDBACK ON THE ACCROPODETM TECHNOLOGY FROM SITE EXPERIENCE

The single-layer technique appeared at the beginning of the 1980s, with the ACCROPODETM unit, and is thus entering its third decade. At the time, this solution was a real innovation, reducing the amount of concrete and steepening armour facing slopes, hence reducing the volume of materials required. After three decades in use and more than 200 projects to date, it was important to summarize the lessons learned during this period and to inspect (above and below water) some of these structures in order to assess their behaviour and particularly to confirm the validity of the unit placing rules. In addition to the aspects related to armour stability, the focus has been given to the colonization by marine life of the structures, including the bedding layers, toe berms, underlayer, armour units. The purpose of this paper is to share the experience gained throughout the inspections undertaken since 2010 on structures built more than 10 years ago. A large panel of structures has been inspected, of different ages and at various locations worldwide.

A pragmatic approach to rock toe stability

A new stability formula is proposed for the toe protection of rubble mound structures. The new approach is based on a critical stability number that refers to the initial movement of armour stones on the toe berm. Damage progression is subsequently described by the ratio of actual and critical stability numbers. The ratio of toe berm submergence and incoming wave height has been identified as the governing parameter for the toe stability. Other parameters including sea bed gradient, wave steepness and toe berm geometry were found to be of minor importance. The new toe stability formula has been validated against model test results and has been compared with the widely used formula of van der Meer (1998). The new approach appears more accurate and more physically meaningful. The practical application of the new formula for toe berm design is briefly described.

Use of tyre bales as lightweight fill for the A421 improvements scheme near Bedford, UK

Abstract The A421 improvements scheme near Bedford in England requires construction of the new dual carriageway and a realigned side road over two former borrow pits that have been partially infilled with up to 20 m of soft clay. The side road was constructed using lightweight fill comprising ‘Maxit’ expanded clay aggregate and tyre bales manufactured from old vehicle tyres under a Standard that produces an engineering material from a former waste product. A deskstop study and a series of ground investigations were carried out to establish the history of the site and the current condition of the infill and to provide parameters for geotechnical design. The solution was chosen after consideration of various options against a number of constraints. The bulk of the embankment was formed from layers of tyre bales with alternate thin levelling layers of ‘Maxit’. A period of surcharge with wick drains was incorporated to reduce settlements in the long term and a temporary toe berm provided for stability during construction. The use of tyre bales on this project provided a number of environmental benefits as well as practical benefits including cost and ease of storage, handling and placing.

Monitoring and analysis of the bending behaviour of discrete piles used to stabilise a railway embankment

Discrete piles are used to stabilise infrastructure slopes, especially where there is insufficient additional land to allow construction of large toe berms or regrading of the slope. Compared with more conventional structures such as retaining walls, there are few field data on how discrete piles typically bend and displace under slope loading. This paper presents the results from monitoring a number of discrete piles used to stabilise a railway embankment at Hildenborough, Kent, UK. Bending deflections deduced from strain gauges are compared with the displacements and rotations measured by inclinometer tubes cast into the piles. Four years after pile installation, the piles were bending downslope over their lower halves, with little bending measured in the upper sections. Regrading of the rockfill piling platform shortly after pile construction caused some of the pile loading, with further loading caused by the continued tendency for slope movement. Analysis of the piles using a simple elastic analysis gives bending moments and displacements close to those measured.

Analysis and design of veneer cover soils

Cover soil sliding on slopes underlain by geosynthetics is obviously an unacceptable situation and, if the number of occurrences becomes excessive, can eventually reflect poorly on the entire technology. Steeply sloped leachate collection layers and final covers of landfills are situations where incidents of such sliding have occurred. Paradoxically, the analytic formulation of the situation is quite straightforward. This paper presents an analysis of the common problem of a veneer of cover soil (0.3 to 1.0 m thick) on a geosynthetic material at a given slope angle and length. The paper then presents different scenarios that create lower FS (factor of safety) -values than the gravitational stresses of the above situation, e.g. equipment loads, seepage forces and seismic loads. As a counterpoint, different scenarios that create higher FS-values also are presented, e.g. toe berms, tapered thicknesses and veneer reinforcement. In this latter category, a subdivision is made into intentional reinforcement (using geogrids or high-strength geotextiles) and non-intentional reinforcement (cases where geosynthetics overlay a weak interface within a multilined slope). A standard numeric example is used in each of the above situations to illustrate the various influences on the resulting FS-value. In many cases, design curves are also formulated. Suggested minimum FS-values are presented for final closures of landfills, waste piles, leach pads, etc., which are the situations where veneer slides of this type are the most serious. Hopefully, the paper will serve as a vehicle to bring a greater awareness to this situation so as to avert such slides from occurring in the future. Note: This paper was initially published as the Giroud Lecture in the Proceedings of the Sixth International Geosynthetics Conference held in Atlanta, USA, in 1998.

Analysis and design of veneer cover soils

Cover soil sliding on slopes underlain by geosynthetics is obviously an unacceptable situation and, if the number of occurrences becomes excessive, can eventually reflect poorly on the entire technology. Steeply sloped leachate collection layers and final covers of landfills are situations where incidents of such sliding have occurred. Paradoxically, the analytic formulation of the situation is quite straightforward. This paper presents an analysis of the common problem of a veneer of cover soil (0.3 to 1.0 m thick) on a geosynthetic material at a given slope angle and length. The paper then presents different scenarios that create lower FS (factor of safety) -values than the gravitational stresses of the above situation, e.g. equipment loads, seepage forces and seismic loads. As a counterpoint, different scenarios that create higher FS-values also are presented, e.g. toe berms, tapered thicknesses and veneer reinforcement. In this latter category, a subdivision is made into intentional reinforcement (using geogrids or high-strength geotextiles) and non-intentional reinforcement (cases where geosynthetics overlay a weak interface within a multilined slope). A standard numeric example is used in each of the above situations to illustrate the various influences on the resulting FS-value. In many cases, design curves are also formulated. Suggested minimum FS-values are presented for final closures of landfills, waste piles, leach pads, etc., which are the situations where veneer slides of this type are the most serious. Hopefully, the paper will serve as a vehicle to bring a greater awareness to this situation so as to avert such slides from occurring in the future. Note: This paper was initially published as the Giroud Lecture in the Proceedings of the Sixth International Geosynthetics Conference held in Atlanta, USA, in 1998.

Slope Rehabilitation at the Baltimore-Washington Parkway with Rammed Aggregate Piers

The Baltimore-Washington Parkway is a major traffic artery traveled by daily commuters in suburban Washington, D.C. Much of the parkway roadbed lies on a raised embankment constructed to allow for grade separation between the parkway and surface streets, facilitating access to the parkway. After a series of rainstorms in the fall of 2002, the fill embankment at the recently reconstructed Route 197 interchange failed, with resulting settlement and lateral movement of a retaining wall that supported the southbound lanes of the parkway. The slide encroached on the traffic lanes and jeopardized the integrity and safety of the parkway. After analyzing a variety of possible solutions, FHWA and the National Park Service opted to stabilize the slide by using Geopier rammed aggregate piers, acting in concert with a toe berm stabilized with Tensar highstrength structural geogrid. The rammed aggregate piers improved the stability of the slope by providing significant increases in the composite shear resistance because of their high angle of internal friction (44° to 52°). A case history is presented of the use of rammed aggregate piers to stabilize a landslide. The analytical methods used in the design solution are presented along with a description of the construction sequence. This study is significant because it describes how a simple and cost-effective solution may be implemented to stabilize landslides.

Embankment reliability versus factor of safety: Before and after slide repair

AbstractProbabilistic slope stability analyses incorporate more information regarding the input variables than do traditional deterministic analyses. Consequently, more information is contained in the results. A probabilistic method for slope stability analysis for very general problems was presented by Wolff and Harr.1 The method allows analysis of a broad range of slope stability problems. In this Application Brief, the method is reviewed and used to illustrate the significant change in reliability of a dam slope from the time of an actual slide to the time the slide was repaired by the construction of a rock toe berm.