Slope stabilization

A row of piles installed through a slowly creeping slope adds a resisting force across the slip surface, raising the slope's factor of safety. Rather than search for the critical surface itself, PileCalc imposes a free-field soil movement on the pile and computes the resistance the pile develops in response. It does this by combining its two core engines — the p-y analysis for bending and the t-z load transfer for axial drag — the same approach RSPile uses for pile-stabilized slopes. Try it on the slope tool.

How stabilizing piles work

The slope soil above the slip surface moves downslope; the pile, anchored in the stable ground below, resists. The imposed free-field movement δ is directed down the slope, at the slip angle to horizontal, and PileCalc resolves it into two components the pile carries through different mechanisms.

δ_lateral = δ · cos(angle), δ_axial = δ · sin(angle)

Resolving the imposed movement into pile-relative components

The lateral (horizontal) component is solved by the p-y analysis — the pile bends across the slip surface, mobilizing soil reaction on each side. The axial (along-pile) component is solved by t-z load transfer — the moving soil drags the pile along its length. By the convention above, flatter slips (smaller angle) load the pile more laterally, while steeper slips shift load into the axial direction.

The slip surface could form at any depth, so the analysis sweeps the slip-surface depth from the surface down to the maximum depth over a number of steps, and finds the depth at which the pile gives the most resistance. At each depth the two components are combined into a resultant:

R = √(lateral² + axial²)

Resultant resistance at a given slip depth

For the pile to anchor into stable ground, it must extend below the slip surface — a pile that stops short of the surface simply rides along with the moving mass.

The stabilizing pile

A single pile is described by its geometry and the two stiffnesses the combined analysis needs: a bending stiffness for the lateral component and an axial modulus for the load transfer. The length and diameter are shared by both engines.

LLengthlength

The embedded pile length crossing the slip surface.

Why it matters. The pile must extend below the slip surface to anchor against the sliding mass. A pile that does not reach into stable ground develops no net resistance.

bDiameterlength

The pile diameter.

Why it matters. Larger piles mobilize more lateral and axial resistance per pile, so diameter scales the stabilizing force each pile contributes.

EIStiffnessforce · length²

The pile's flexural rigidity — the lateral component of the analysis.

Why it matters. It sets how the imposed soil movement turns into pile shear at the slip surface in the p-y solution.

EModulusforce / length²

The pile's elastic modulus — the axial component of the analysis.

Why it matters. It drives the t-z load transfer that resolves the inclined soil movement into axial drag.

Imposed soil movement

These four inputs define the demand on the pile and the range of slip surfaces evaluated.

δDisplacementlength

The magnitude of the free-field soil movement, tangent to the slip surface.

Why it matters. This is the design-tolerance movement that loads the pile. It is capped at ~0.3 m for mobilization, following the RSPile convention.

—Slip angledegrees

The inclination of the slip surface — and of the movement — to the horizontal.

Why it matters. It resolves the movement into a lateral (horizontal) and an axial (along-pile) component. Flatter slips load the pile more laterally.

—Max slip depthlength

The deepest slip surface to evaluate.

Why it matters. The analysis sweeps slip depth up to this value to find where the pile gives the most resistance.

—Pointscount

The number of slip depths swept between the surface and the max depth.

Why it matters. More points trace the resistance-vs-depth curve more finely, at a little extra compute.

Soil profiles

Because the analysis combines the lateral and axial engines, it carries two layered soil profiles, edited with the same layer editors used elsewhere in PileCalc. The lateral (p-y) profile supplies the nonlinear springs for the bending response; see Lateral piles for its soil models and parameters. The axial (t-z) profile supplies the side-friction load transfer; see Axial capacity for its soil types and groundwater inputs.

Define both profiles to span the full pile length so every slip depth in the sweep sits within a defined layer. The two profiles describe the same ground through the lens of each engine, so keep their layer boundaries and strengths consistent with one another.

Reading the results

The tool reports the governing resistance and how it varies with slip depth.

Summary quantities

Max resultant and its slip depth — the largest resisting force the pile provides across the swept slip depths, and the governing depth at which it occurs.
Lateral @ max — the lateral (shear) component at the governing slip depth, from the p-y analysis: the pile bending across the slip surface.
Axial @ max — the axial (along-pile) component at the governing slip depth, from the t-z load transfer of the inclined movement.

Profiles & table

Resultant, lateral, and axial resistance are each plotted against slip depth, so you can see how the pile's contribution builds and where it peaks. The resistance table lists every swept slip depth with its lateral, axial, and resultant values for export.

This is the pile contribution, not the global FoS

The reported resistance is the stabilizing force the pile adds — feed it into a separate slope-stability (factor-of-safety) calculation as an extra resisting force. PileCalc gives the pile contribution, not the slope's overall factor of safety.