Conference Agenda

The paper presents the results of two methods applied to the impact of fast landslides against an artificial barrier. The first approach consists in simulating either the fast-moving landslide or the movable barrier through a single mathematical model based on inelastic collision formulation. In a second approach, the maximum expected impact pressure is estimated from the geometric and kinematic features of the impacting landslide mass by using literature impact formulations, and hence a FEM (Finite Element Method) dynamic analysis is performed to assess how the barrier is damaged and/or displaced during the impact. In such a case, a very detailed geometry of the barrier is considered and there is the chance of simulating the local/internal yielding of the structure. The latter is made of granular soil reinforced through geogrids wrapped around the facing, and it is free to move along the contact with the base soil. The landslide is constituted by a rectangular-like shaped volume of saturated soil moving at some meters per second at the impact stage. However, the height of the impact material is necessarily assumed as constant during the landslide-structure impact interaction. Globally, the two approaches provide consistent results as it concerns the Landslide-Structure Interaction (LSI) problem, with some discrepancies depending on the initial landslide velocity and type/geometry of the protection barrier structure.

An important role in landslide risk mitigation is played by the design of artificial embank-ments. In the case of very rapid or extremely rapid phenomena, such as debris flows, these structures are commonly protection works used to reduce the runout distance and velocity and the peak impact force. Protection works represent artificial obstacles that dis-sipate the flowing mass energy and they are set along the potential flowing path where the debris is transported and deposited. Such structures can be constituted of granular material reinforced by geosynthetics. For the design of the latter, the evaluation of the impact forc-es exerted by debris flow on the work as well as the characteristics of the obstacle are cru-cial issues. To this regard, the paper focuses on determining the impact forces transmitted over time to a geosynthetics reinforced structure and on analyzing the response of the structure to the impact in terms of horizontal displacement. The impact forces have been evaluated adding the static component, which depends on the flowing mass height, to the dynamic component, that depends on both the flowing mass height and its velocity. In the analyses of the geosynthetics reinforced structure response, two cases have been analyzed in the paper referring the soil that constitutes the embankment: (I) soil with constant shear strength angle and (II) soil with variable shear strength angle along the height of embank-ment. The obtained results have been discussed in terms of horizontal displacement.

<p>Rockfall hazard in mountainous areas is often mitigated by placing reinforced soil embankments at the bottom of the slopes. These structures combine the excellent shock absorbing capacity of soil with the tensile strength of the reinforcements. Despite the important research performed on this domain, the design of this type of structures remains empirical and highly conservative. Typically, reinforced soil embankments have trapezoidal cross section, which occupies an important construction area and the real role of the reinforcements remains uncertain.</p>

<p>Terre Armée company invested in an important research project concerning rockfall protection embankments. The goal was to test an innovative reinforced soil structure, which would have vertical facings and thus a reduced foundation footprint. The latter could be very practical, when dealt with steep slopes as it minimizes rock excavation. High strength geosynthetic reinforcements would assure its stability when subjected to impacts.</p>

<p>During this project, real scale tests were performed on a 15 long embankment, which was constructed with vertical facings and a foundation width that was almost half of its height. The structure successfully resisted the impacts from concrete blocks travelling with 5 MJ and 2 MJ kinetic energies. The deformed shape of the embankment showed that the internal geogrid reinforcements efficiently mobilized a large soil volume far from the impact location. A good mobilization of the geogrids’ strength was observed through the strain gauges installed on them, which confirmed the validity of the design.</p>

<p>After this experience, Terre Armée company is able to design such rockfall protection structures, which offer several advantages compared to existing solutions. For example, less construction space requirements, faster erection, and relatively low maintenance costs.   </p>

Back-to-back mechanically stabilized earth (MSE) walls are commonly used for ramp ways and bridge approaches. The behavior of back-to-back MSE walls is significantly influenced by the geometric configuration, especially the horizontal distance (i.e., width-to-height ratio) of the wall. This paper presents a numerical study on the behavior of back-to-back MSE walls to investigate the influence of horizontal distance on the interaction. Numerical simulations were conducted with different width-to-height ratios. Results indicate the horizontal distance has significant influences on the static behavior of back-to-back MSE walls. The lateral soil thrust behind the reinforced soil zone, the required reinforcement strength, and the vertical toe load generally increase with increasing horizontal distance up to a critical value. The Fed-eral Highway Administration (FHWA) simplified method significantly overestimates the re-quired reinforcement tensile strength, but underestimates the lateral soil thrust for the range of horizontal distances involving interaction between the back-to-back MSE walls.

<p>Rockfall is a widespread natural threat to human lives and property irrespective of seasons and altitudes. To deviate or arrest the falling rocks, Rockfall protection embankments (RPE) can be built at the toe of the hill to mitigate the risk. However, the design of such RPEs are not yet standardised. This could be due to a limited understanding of the interaction of the falling rocks with the RPEs. To investigate this dynamic interaction, three-dimensional finite element analyses were carried out for a 6.5 m high geogrid-reinforced embankment under rockfall impact. The dimensions of the RPE were determined using kinetic energy-based design criteria for protection embankment under a reported rockfall event on the Manali-Leh highway in the Himalayan region of Northern India.</p>

<p>The RPE investigated in the present study is built with granular fill material and reinforced with geogrids which are arranged in layers and wrapped around the facing. The kinetic energy of falling rock is ranging from 100 kJ to 1000 kJ. The impact response of the RPE was assessed by evaluating the stresses and deformations induced in the structural components for varying magnitudes of the impact kinetic energy. Results show that the geogrid layers in the region of direct impact undergo translation in the direction parallel to the impact direction, while the region above the impact zone heaves. This indicates that the internal stability of the embankments has reduced significantly in the zone of impact and the neighboring zones. The variation of response of the RPE with varying magnitudes of the impact force is mapped and presented in this paper.</p>

<p>Back-to-Back Mechanically Stabilized Earth (MSE) walls can sustain significant loadings and deformations due to the interaction mechanisms which occur between the backfill material and the reinforcement elements. These walls are commonly used in embankments approaching bridges, ramps and railways. The performance of a reinforced wall depends on numerous parameters, including the ones defining the soil, the reinforcement and the soil/reinforcement interaction behavior. The focus of this study is to investigate numerically the behavior of back-to-back mechanically stabilized earth walls considering synthetic and metallic strips. A two-dimensional finite difference numerical modeling is considered. The role of the soil friction angle, the soil material quality and the wall width to the height ratio are investigated in a parametric study. Their effects on the soil/strip shear displacements and tensile forces on the reinforcements are presented. The behavior of the reinforcement strips in back-to-back reinforced walls strongly depends on the distance between walls and on the soil parameters.</p>

Reinforced soil structures and their applications currently represent a key point in

the field of the geotechnical engineering because they make it possible to reinforce the soil structure while respecting the natural environmental aspects of the landscape. Though widely used, this conventional type of reinforced soil structure hides some drawbacks. One of them is the risk of soil pollution due to the degradation and corrosion of the metallic material making up the conventional formwork currently in use. This also applies to the zinc-coated facing elements which nonetheless release harmful particles of rust into the soil, though later than the classic black facing elements (without zinc-coating). The present invention, covered by a patent, concerns a structure for reinforcing soils made up of a formwork made of bio-degradable natural material, which can be combined both with planar reinforcing elements in metal mesh and with synthetic geogrids.

<p>In the last decades, geosynthetic reinforcement has been widely used in geotechnical applications. Recently, geogrid has also been used to back-anchor sheet pile walls. However, this system has not received sufficient attention neither in research nor in construction. Due to the complex interactions between soil, geogrid and sheet pile wall, the applicability of common design guidelines for conventionally back-anchored walls to this particular system has to be proven. To develop a fundamental understanding about the influence of various components of the system on its behaviour, numerical investigations have been conducted within this study. In this paper the influence of geogrid inclination, design of geogrid-sheet pile connection including prestressing and geogrid position on the earth pressure distribution and wall deformation is discussed. The numerical results revealed that the position of geogrid and design of geogrid-sheet pile connection significantly affect the earth pressure distribution. The wall deformations are mainly influenced by the geogrid position.</p>

<p>The High Speed Two (HS2) is the new high-speed railway for Britain. Thame Valley Viaduct is part of HS2 Phase One from London to West Midlands. The Thame Valley Viaduct is located in the Northern Vale to the north-west of Aylesbury. It travels across the River Thame within a broad, shallow floodplain.</p>

<p>In order to cross the Thame River and adjacent flood plain area, The Thame Valley Viaduct made up of 36 spans an overall length of 880 metres it is proposed. The foundation for the viaduct involves large diameter bored piles. The temporary works to construct the causeway over very soft soils consist of 18no. piling platforms with approximate 15,000m² and an adjacent haul road connecting the platforms with an overall length of 1,000 metres.</p>

<p>The ground conditions along the causeway route generally consists of very soft to soft alluvium/ head deposits which are encountered up to 4.9m bgl. SPT N values of alluvium are typically 0-8 with undrained shear strengths of 0kPa-36kPa. The alluvium is underlain by firm to stiff clay of the Ampthill Formation.</p>

<p>It is proposed to support the perimeter of the piling platforms and the western side of the haul road with a 45° reinforced soil slopes using layers of Biaxial geogrids wrapped around the face with permanent erosion control blanket pinned to the slope face. The eastern side of the haul road is former via a 70° reinforced soil wall using topsoil filled geotextile bags with horizontal layers of Uniaxial geogrids wraparound in the face. The maximum slope height is 2.5m.</p>

<p>A design is also required for the 18no. piling platforms using layers of Biaxial geogrids for reinforcement and Non-Woven Geotextile for separation. The piling rigs proposed were 182 tons with applied pressures between 227 to 497 kPa and loadings of approxiamte1500 kN.</p>

<p>Using geogrids to anchor sheet pile walls (SPWs) is a relative new application of geogrids. A series of medium scale experiments were conducted at the Deltares laboratory to show how this structure works and how this system relates to the Dutch design guidelines for sheet pile walls and geogrid-reinforced retaining MSE walls. The experiments were simulated using 2D Plaxis. From the experiments and the numerical analyses, the following is concluded. Two slip surfaces develop, starting at the edges of the strip footing. These slip surfaces divide the reinforced fill behind the SPW into three zones: the active zone, the zone behind the active zone and the zone between SPW and active zone. The paper analyses the contributions of each of these zones to the bearing capacity of the structure. Furthermore, it is concluded that the location of the strip footing surcharge load, the length of the geogrids and the number of geogrid anchors affect the bearing capacity of the structure most dominantly. </p>