Support

End-Anchored (Deadman) Bolts

This is the most simple support model in Slide. The user defines the anchor capacity, the out-of-plane spacing and whether the force is active or passive. If the slip surface intersects the anchor, a force is applied at the point of intersection equal to the anchor capacity magnitude divided by the out-of-plane spacing. The force direction is parallel to the bolt.

Refer to the Slide online help in the demo for more information.

Data input for end-anchored bolts



Grouted Tiebacks

The Grouted Tieback support type can be used to model support such as grouted tiebacks and ground anchors, which may have a variable bonded (grouted) length. The model requires parameters such as the grouted length, tensile capacity, plate capacity and bond strength. The bond strength is the shear force per unit length of the grout preventing pullout.

Parameter input for a Grouted Tieback. Notice the pullout strength is defined as a single bond strength per unit length of the grout



Bond strength may also be defined in terms of a Mohr-Coulomb strength criterion. Thus the bond has both an adhesion (or bond stress) and frictional component of strength. In this case, the bond strength is a function of the depth of the anchor.

Parameter input for a Grouted Tieback using frictional strength. Notice the pullout strength requires an adhesion AND a friction angle



Whether the bond strength is in terms of a single adhesion or includes friction, you may also specify the strength parameters according to the materials which are traversed by the anchors. The program will automatically use the values associated with the specified materials.

Strength parameters for Grouted Tiebacks can also be defined for each soil unit the anchor crosses



Failure Modes

At any point along the length of the tieback, there are 3 failure modes which are considered:

   1. Pullout (force required to pull the length past the failure surface out of the slope)
   2. Tensile Failure (maximum axial capacity of the tieback tendon)
   3. Stripping (slope failure occurs but tieback remains embedded in slope)

The failure mode is determined by which one of the above three produces the least restraining force. This force is then applied to the base of the slip surface at the point of intersection in a direction parallel to the direction of the anchor.

Refer to the Slide online help in the demo for more information.

Soil Nails

Soil nails can be modeled in Slide using the Soil Nail support type. The model requires parameters such as the tensile capacity, plate capacity and bond strength. The bond strength is the shear force per unit length preventing pullout. You may also define customized values of bond strength for different soil units. The program will automatically use the values associated with the soils the nail crosses.

Parameter input for a Soil Nail



Failure Modes

At any point along the length of the soil nail, there are 3 failure modes which are considered:

   1. Pullout (force required to pull the length past the failure surface out of the slope)
   2. Tensile Failure (maximum axial capacity of the soil nail)
   3. Stripping (slope failure occurs but nail remains embedded in slope)

The failure mode is determined by which one of the above three produces the least restraining force. This force is then applied to the base of the slip surface at the point of intersection in a direction parallel to the direction of the nail.

A common method in describing the behavior of the model is through a nail force diagram. The nail force diagram for the soil nail algorithm in Slide is as follows:



Refer to the Slide online help in the demo for more information.

GeoTextiles

The GeoTextile support type can be used to model the various types of slope reinforcement which are used in the form of meshes, grids, strips, etc. There are a wide variety of such support systems, referred to as geo-textiles or geo-fabrics, geo-grids, geo-synthetics etc.

Although the term "GeoTextile" actually refers to a specific category of reinforcing materials, for the purpose of the following discussion, the term GeoTextile will be used to refer to all forms of flexible planar reinforcement, which are used in the form of fabrics, meshes, grids, strips, membranes, etc, to reinforce slopes. This includes both synthetic (polymer) and metal (eg. steel strip) reinforcement.

Parameter input for a GeoTextile



Strip Coverage

When a GeoTextile is used to reinforce a slope, the material is placed in strips of a certain width. The Strip Coverage refers to the spacing of these strips, in the Out of Plane direction (ie. along the slope). If the strips are laid continuously beside each other, with no gaps between adjacent strips, then the Strip Coverage = 100%. If the strips are not laid continuously (ie. there are spaces between adjacent strips), then the Strip Coverage will be less than 100%. For example, if 4 meter wide strips were laid with a 2 meter spacing between each strip, then the Strip Coverage would equal 67% (ie. 4 / (4 + 2))

Tensile Strength

The Tensile Strength refers to the Tensile Strength (maximum load capacity in force units) per meter width of strip. This is also referred to as the Tear Strength when dealing with geo-fabrics for example.

Force Application

See the Active vs. Passive Force Application topic for a discussion of the significance of Active and Passive support force application in Slide.

Force Orientation

Because of the flexible nature of a GeoTextile type of reinforcement, the orientation of the force which is applied to the slip surface is NOT always assumed to be parallel to the reinforcement. When the support begins to take on load, due to displacements within the slope, the direction of the applied support force can be assumed to be:

   · Tangent to Slip Surface
   · Bisector of Parallel and Tangent (ie. at an angle which bisects the tangent to slip
      surface orientation, and the parallel to reinforcement orientation)
   · Parallel to Reinforcement
   · User-Defined Angle (ie. the user may specify an angle, measured from the
      positive horizontal direction)

Anchorage

When GeoTextile support is used, the support strips are normally anchored to the slope face, using some type of anchoring system. In addition, the embedded end of the GeoTextile may be anchored within the slope. The Anchorage option allows the user to select whether none, both or either ends of the GeoTextile are anchored (ie. considered fixed and immovable).

Pullout Strength

The parameters entered here (Adhesion and Friction Angle) will govern the pullout and / or stripping force, which can be generated by the support. The strength is a function of the normal stress on the GeoTextile which is calculated based on the overburden pressure. Thus the model automatically accounts for the increase in pullout strength with depth.

Shear Strength Model

Slide contains two shear strength models, Mohr-Coulomb and Hyperbolic, for the strength behavior of the soil-GeoTextile interface.

Implementation of Geotextile Support in Slide

At any point along the length of the geotextile, there are 3 failure modes which could be considered:

   1. Pullout (force required to pull the length past the failure surface out of the slope)
   2. Tensile Failure (maximum tearing capacity of the GeoTextile)
   3. Stripping (slope failure occurs but GeoTextile remains embedded in slope)

The stripping and pullout failure modes are used only for certain types of anchorage. For example, if the GeoTextile is anchored at the slope face then the stripping failure mode is not considered.

The actual failure mode is determined by which one of the valid modes produces the least restraining force. This force is then applied to the base of the slip surface at the point of intersection. The direction of the force is determined by the force orientation parameter.

Refer to the Slide online help in the demo for more information.

Piles and MicroPiles

The Micro Pile support type can be used to model a micro pile or pile type of support system. Due to the nature of pile support, this type of support behaves differently from other types of support in Slide, since resistance to movement is assumed to be provided by loading transverse to the support direction rather than parallel to the support direction. Note:

   · Like all other support types in Slide, it is necessary for a slip surface to intersect
      a Micro Pile in order for the support to have an effect on the safety factor of the
      slip surface.

   · However, unlike other support types in Slide, the only failure mode for a Micro
      Pile is by Shear, transversely through the pile.

   · Tension or pullout are NOT considered as failure modes for a Micro Pile in Slide,
      regardless of the pile orientation. Only transverse shear through the pile
      is considered.

Data input for piles and micropiles



Pile Shear Strength

The Pile Shear Strength, is the shear FORCE necessary to cause shear failure THROUGH the pile.

Out of Plane Spacing

The spacing between piles in the out-of-plane direction.

Force Application

See the Active vs. Passive Force Application topic for a discussion of the significance of Active and Passive support force application in Slide.

Force Orientation

When a slip surface intersects a Micro Pile, the orientation of the applied force (ie. the Pile Shear Strength) is always TANGENTIAL to the slip surface.

Implementation of Pile Support in Slide

As discussed above, a Pile will apply a constant force to a slip surface, regardless of where it is intersected by a slip surface. The applied load, PER UNIT WIDTH OF SLOPE, is simply equal to the Pile Shear Strength divided by the Out of Plane Spacing.

Refer to the Slide online help in the demo for more information.

User-Defined Support Model

It is important to realize that all of the pre-defined support types in Slide, ie.

   · End Anchored
   · GeoTextile
   · Grouted Tieback
   · Micro Pile
   · Soil Nail

are simply used to generate a Force Diagram for the support (ie. a plot of FORCE versus DISTANCE along the support). The way in which this graph is generated differs for each type of support and depends on the support parameters which are entered. However, as far as the Slide COMPUTE engine is concerned, the properties of an anchor are entirely determined by the Force Diagram (as well as the method of Force Application and the Force Orientation, which are independently specified).

If the properties of the pre-defined support types in Slide are not sufficient to generate a desired Force Diagram, then the user can simply define their own support type by creating an explicit Force Diagram. This means that any support type can be easily modeled with Slide.

Data input for user-defined support models



Once the force diagram is defined the user can also define whether the load is active or passive and the orientation of the support force.



Refer to the Slide online help in the demo or the model reference manual for more information

Active vs. Passive Anchors

In general terms, the Factor of Safety is defined as the ratio of the forces resisting motion to the driving forces. Driving forces include the mass of each slice accelerated through gravity, seismic forces and water in a tension crack. Resisting forces arise from the cohesion and frictional strength of the slip surface.

Active Support is assumed to act in such a manner as to DECREASE the DRIVING FORCE in the Factor of Safety calculation. Grouted Tiebacks, tensioned cables or rockbolts, which exert a force on the sliding mass before any movement has taken place, could be considered as Active support.

By this definition, Passive Support is assumed to INCREASE the RESISTING FORCE provided by shear restraint in the Factor of Safety equation.

Soil nails or geo-textiles, which only develop a resisting force after some movement within the slope has taken place, could be considered as Passive support.

Since the exact sequence of loading and movement in a slope is never known in advance, the choice of Active or Passive Force Application is somewhat arbitrary. The user may decide which of the two methods is more appropriate for the slope and support system being analyzed. In general, Passive support will always give a lower Factor of Safety than Active support.

Defining the Geometry of Anchors

Anchors may be entered individually or by defining a pattern. When entered individually, the user defines the two end points of the anchor. These points are entered graphically or by typing in their coordinates.

A pattern of anchors can also be easily entered by the user. This facilitates the entry of a number of anchors on a user-defined spacing. The pattern is defined by the spacing of the anchors in-plane and the length of the anchors. The orientation of the anchors in relation to the slope (eg. normal to the slope) must also be specified. The user may also define the pattern by specifying the number of anchors between any two points instead of the spacing.

Data input for pattern anchors



A pattern of six 5 meter long anchors spaced every 1 meter in-plane along the slope