Pile / Micro Pile

The Pile / 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 pile, in order for the support to have an effect on the safety factor of the slip surface.

• Three failure modes are considered for this support: shear, Ito & Matsui, and EFW. In all cases, the failure occurs transversely through the pile.

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

NOTE: make sure that you have read the Overview of Support Implementation topic, for important information which applies to all support models in Slide.

Shear Failure

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

Shear failure mode for micro pile

NOTE that the Pile Shear Strength is entered as a FORCE. The user must calculate the total shear force capacity of a pile, based on the pile cross-sectional dimensions, and the pile shear strength per unit area of cross-section.

Out of Plane Spacing

The spacing between piles in the out-of-plane direction (i.e. along the slope).

Force Application

By default, if Support Type = Micro Pile, the method of Force Application = Passive. See the 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 default orientation of the applied force (i.e. the Pile Shear Strength) is TANGENTIAL to the slip surface. This is illustrated below. This force orientation is consistent with the assumed failure mode of a Micro Pile. You can also choose Perpendicular to pile as the force orientation if you want the applied force to be exactly perpendicular to the pile regardless of the slip surface orientation.

Applied force orientation for micro pile, tangential to surface

Implementation of Pile Support in Slide: Failure Mode = Shear

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. i.e.

F = P / S

where:

P = Pile Shear Strength (force units)

S = Out of Plane Spacing

The Force Diagram for a Pile support type, is therefore a horizontal line, representing a constant value of applied force, regardless of where a slip surface intersects the pile.

Force diagram for Pile support

Ito & Matsui

The Ito & Matsui mode pertains to piles that are surrounded by plastically deforming ground. This is modeled after Ito & Matsui (1975).

Pile Diameter

The diameter of the pile.

Force Orientation

The same options available in the Shear failure option are available here. Since the ground deformation is resulting in the force application, Parallel to surface is recommended for this pile.

Implementation of Pile Support in Slide: Failure Mode = Ito & Matsui

The Ito & Matsui pile applies a force to the slip surface based on the method outlined by Ito & Matsui (1975), PER UNIT WIDTH OF SLOPE. It is dependent on the soil cohesion and friction angle (or equivalent values), the diameter of the pile, and the normal stress. As such the force increases linearly with depth, as shown.

Force diagram for Pile support with Failure Mode = Ito & Matsui

Equivalent Fluid Weight (EFW)

Pile designs are often provided to the structural engineer as an equivalent fluid weight. This option allows the user to input this value directly. This results in a trapezoidal distribution of the total force along the pile length, with the default resultant force applied at the centroid.

This support option can also be used to model the analysis of a failure beneath a retaining wall. The wall can be modeled as a pile with a triangular EFW. Slip surfaces that pass beneath the pile would not include the EFW.

Pressure diagram of pile. c = constant pressure input. P = total force.

Constant Pressure

The value of the pressure at the top of the pile. c in the figure.

Fluid Unit Weight

The equivalent fluid unit weight of the pile, or the slope in the pressure diagram figure.

Location of Force

The force can be specified at the slip surface intersection point as with other support options in Slide. It can also be specified as the centroid of the trapezoidal pressure diagram shown in the figure.

Force Orientation

The same options available in the Shear and Ito & Matsui failure options are available here, with the additional “Horizontal” option.

Implementation of Pile Support in Slide: Failure Mode = EFW

The applied force, PER UNIT WIDTH OF SLOPE, is calculated as follows

F = P / S

where:

P = Area of trapezoid shown = (Constant Pressure)*(distance between top of pile and slip surface intersection point) + (Fluid Unit Weight)*(distance between top of pile and slip surface intersection point)2/2.0

= c*H + efw*H2/2.0

S = Out of Plane Spacing. (NOTE: if fluid unit weight provided for a group of piles, spacing is 1.0. If provided for a single pile, the spacing is the pile spacing.)

As such the force increases linearly with depth, as shown.

Force diagram for Pile support with Failure Mode = EFW