Program ReSSA+ is an interactive, graphically rich program, allowing the designer to analyze reinforced walls and slopes including embankments with basal reinforcement. It also can be used for assessing the stability of unreinforced slopes.
New Features for assessing Internal Stability – The Baseline Solution:
- For a given problem (i.e., tiered slope/wall, different soil layers, complex layout of reinforcement, facing units, surcharge loadings, pullout interaction parameters, seismicity, seeping water, and user-specified rear-end reinforcement resistance) and a given target factor of safety, ReSSA+ calculates the maximum tensile resistance in the reinforcement, Tmax, its location, and the connection load, To, for each layer. This baseline solution provides a rational basis for selecting geosynthetic and facing considering long-term strengths values. The baseline solution defines the required reinforcement/connection load at any relevant location so as to produce a user-prescribed limit state. Note that the baseline solution is conducted using Bishop’s rotational failure considering failures emerging at the face and toe. Since other modes of failure, such as foundation or sliding instability, are possible, it is critically important to also assess global stability available in ReSSA+. However, this should be done after adequately selecting geosynthetic and connectors considering also the baseline solution.
- ReSSA+ enables visualization of the distribution of the required reinforcement resistance along each layer.
- ReSSA+ enables visualization of the location at which each Tmax is acting.
- ReSSA+ displays the Tension Map using a color-coded presentation to assess where reinforcement is hardly stressed or where it is overstressed. This diagnostic tool is critical towards optimizing the layout of the reinforcement system. See FHWA-HIF-17-004 or Leshchinsky et al. (2017).
- ReSSA+ allows for non-uniform reinforcement lengths. Such a feature is important if intermediate reinforcement layers are used. One can then include its impact on design, especially on connection loads.
- ReSSA+ enables visualization of the distribution of pullout resistance as calculated for both the connection and rear of the reinforcement.
- ReSSA+ presents tabulated numerical results for Tmax and To at each layer. The user can export this table to Excel.
- Upon switching to Global Stability mode in ReSSA+, the user can (and should) specify adequate reinforcement and connectors to ensure global stability considering various failure modes.
- Each dialog in ReSSA+ has its own extensive help.
- The user can specify up to 5 different types of interfaces for reinforcement for assessment of pullout resistance.
- In ReSSA+ live and/or dead surcharge loads can be considered. Also, impact of shear strength between stacked facing units can be explored in both Stage I and Stage II analysis.
- In the baseline mode (‘internal stability’), the actual minimum factor of safety of front and rear pullout is calculated and presented graphically and numerically in a tabulated format. Unlike conventional analysis where this factor is a function of Tmax, in ReSSA+ it is a function of the computed distribution of load T(x) in the reinforcement.
- Horizontal displacement at the face of the slope can now be estimated in the baseline solution. This approximate displacement is based on simplified calculations resulting from the elongation (stretching) of each reinforcement layer considering its tensile modulus. This approximation is a consequence of the limit state methodology used to assess the reinforcement tensile load distribution in internal stability.
New Features for assessing Global Stability:
- Rear or back-end pullout can be enhanced by specifying a starting resistance at the end that is greater than zero. Such resistance can be generated by elements such as deadmen or anchors. ReSSA+does not calculate this passive or anchorage resistance but rather enables the user to input these values for each layer. ReSSA+ considers this boundary value in calculating the pullout resistance away from the end. This feature could be useful in shortening upper layers especially considering seepage or seismic loading.
- Facing units, such as small or large blocks or gabions, can be specified for simple or complex slope geometry. The shear between inter-units or unit-foundation is considered in rotational stability (Bishop) as well as in direct sliding (2-part Spencer). Large units may result in shorter and/or weaker reinforcement.
- Some design standards require assessment of bearing capacity. In Direct Sliding run (available only in Global Stability), ReSSA+ also runs 2-part wedge using Spencer along the base of the reinforced soil mass as defined by the length of the bottom layer. Using Spencer’s results (which includes the normal stress distribution along the base as well as the inclination of the interslice force), the following is calculated: a. Eccentricity of the resultant vertical force acting on the base, b. Magnitude of this resultant, and c. The average bearing load acting over an eccentrically loaded ‘footing’ (B-2e). This enables one to calculate the implied bearing load corresponding to, for example, Meyerhof approach. Subsequently, the user can assess or rationally estimate the bearing capacity factor of safety as implied through an approximate solution. It is always wise to assess stability considering foundation failure; e.g., use Bishop and Spencer using common slope stability analysis considering foundation failures.
- In seismic Global Stability, 2-part wedge, the user can compute the yield acceleration at the elevation of each reinforcement layer. Using this acceleration, the user can follow given references to calculate the seismic displacement.
Other Special Features:
ReSSA+ can analyze stability of reinforced and unreinforced slopes and embankments considering circular failure surfaces (Bishop method) and two- and three-part wedge failure surfaces (Spencer method).
While in Results, using the right-click function on your mouse, the soil properties or reinforcement details can be changed so as to facilitate design.
An exclusion zone for slip surfaces can be specified to limit a search to ‘internal stability’, ‘external stability’, and more.
Data files generated by MSEW(3.0) or ReSlope(4.0) can be imported to ReSSA+ for in-depth global stability analysis. Elements such as connection strength generated in MSEW can be considered in ReSSA+.
User can select any color for soils and reinforcements.
ReSSA+ can display color coded safety map, a powerful diagnostic tool for assessing the stability of the slope and for optimizing the reinforcement. It follows the procedure introduced by Baker and Leshchinsky, “Spatial distributions of Safety Factors,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 127 (2), 2001, 135-145.
Each dialog in ReSSA has its own help.
Units can be SI or English.
User can specify up to 25 layers of soil, each possessing different strength properties.
Complex geometry can be input using mouse functions.
Slopes are divided into three types: simplified, tiered and general. Simplified slope allows for quick input data but is limited to three soils. Tiered geometry allows for quick input of up to ten-tiered slopes, each defined by its height, inclination and offset. The number of soils in tiered geometry is limited to three. General geometry allows for input of detailed geometry and soil profile.
The effects of elements such as tension crack (depth specified), water pressure (either phreatic surface or piezometric heads), vertical and inclined surcharge loads, and seismicity can be considered.
Total stress, effective stress and mixed type analysis can be conducted. Total stress corresponds to undrained shear conditions; effective to drained; and mixed analysis corresponds to alternating layers of soil where some layers are likely to exhibit drained conditions at failure (e.g., gravel) while other layers are likely to fail under undrained conditions (e.g., clay).
ReSSA+ was specifically developed for convenient implementation of horizontal reinforcement layers. Either geosynthetic or metallic can be specified in global stability.
ReSSA+ allows for reduction factors (construction damage, aging and creep) associated with polymeric reinforcement. ReSSA+ can assess the corrosion of metallic reinforcement over the life span of the slope and implement its effects in calculations.
Up to five types of reinforcement in a section can be specified for a wall or slope. Parameters such as strength, reduction factors (polymer), coverage ratio, and cross-sectional area (metal) characterize each ‘type’ of reinforcement.
User can create unlimited databases, each with up to 100 different types of reinforcement, each saved under a different name. The database includes designated name of reinforcement, its strength and the relevant reduction factors. One can easily retrieve and modify this database. Data retrieved from the database can be overridden.
ReSSA+ calculates the pullout resistance along the rear- and front-end of each reinforcement layer automatically. Front-end pullout signifies a case in which the soil moves outward relative to the embedded reinforcement layer. As a result, ReSSA+ can assess potential surficial instability. ReSSA incorporates in the analysis the available strength at each intersection as dictated by the pullout resistance or the reinforcement long-term strength, whichever is smaller.
User can input the reinforcement resistance at its front-end. This parameter can signify the strength of connection to a facing unit. It cannot exceed the long-term strength of the reinforcement layer. Moving away from the face, ReSSA increases the specified reinforcement resistance according to the overburden pressure and the reinforcement-soil interaction parameters.
User can specify the interaction parameters, signifying the pullout resistance or interfacial resistance to sliding. Effects of adhesion can be; such an option is useful in forensic studies. If a layer is embedded in more than one soil, its pullout resistance will be calculated cumulatively along its length considering the relevant interaction parameters at each location.
ReSSA+ checks along each reinforcement layer for resistance to direct sliding using 2-part wedge combined with Spencer. If the active wedge intersects layers of reinforcement, their available strength is incorporated in the analysis. Consequently, trapezoidal layout with shorter reinforcement layers at the bottom can rationally be evaluated.
Tiered slopes can be analyzed using the general structure scheme. Rotational (circles) and translational (2- and 3-part) through and away from the reinforcement can be considered.
For circular surfaces, the user tangibly specifies the points of entry and exit of all circles. ReSSA+ checks many circles for each combination of entry and exit point until it finds the critical circle. The user can display all the specified circles to be analyzed and judge whether a reasonable search domain was specified.
For 2-part wedge surfaces, the user can specify points along the reinforcement and foundation interface defining the boundary between the passive wedge and the active one. ReSSA+ checks many possible surfaces for each point to render the critical surface. The user can display all the specified wedges.
For 3-part wedge surfaces, the user specifies a grid of points for the boundary between the passive and the central wedge and for the boundary between the active and the central wedge. ReSSA+ checks all possible surfaces to capture the critical surface defined by the search domain.
In the results of Bishop, the user can view the distribution of Fs along the specified entry and exit points. Such review implies whether the problem has several minima or whether the absolute minimum has been captured. In case it has not, rerun using modified search domain needs to be conducted. The normal stress and porewater distribution over the critical slip surface can be displayed. Also, the detailed results of Fs corresponding to circles can be viewed. All analyzed circles can be displayed as well. The displayed information as a whole is important for judgment on ‘reasonableness’ of results.
Results are presented in tabulated as well as graphical fashion. Detailed results can be viewed by clicking on the desired parameter in the drop menu.
In Spencer results, the user can view the distribution of Fs along each reinforcement layer, the corresponding critical surface, the associated normal stress and porewater distribution, and the line-of-thrust for this surface. Also, the detailed results, such as the imbalance of the limit equilibrium equations and the inclination of the interslice resultant force, can be viewed. All analyzed slip surfaces can be displayed as well. The displayed information as a whole is important for judgment on ‘reasonableness’ of results.
Results can be printed as a report. The user can select from a print menu the desired material to be printed. The user can also print quantities. All selected material for printing can be previewed before printing.
All graphical results can be captured. Most graphic programs and word-processors can retrieve these files.
Graphical results can be saved as a DXF (Drawing Exchange Format) file. Such files can be read by AutoCAD® for further processing and implementation in design blueprints.
It is noted that ReSSA+ is an outgrowth of the long-running ReSSA(3.0). It includes all of the features of ReSSA(3.0) ReSSA+ can received exported data files generated in by program MSEW(3.0), excluding metallic reinforcement, for further in-depth global stability analysis. Note that ReSSA+, like most limit equilibrium analysis, is in the realm of ASD. It means that designs generated using MSEW(3.0) in LRFD mode will be analyzed in ASD.
Users of ReSSA(3.0): ReSSA+, can read and convert files generated with ReSSA(3.0).