Tel: 1-206-279-3300

Free Webinar, Sep/18/18

Construction design and analysis for the 100ft soldier pile excavation for the 2nd and Broadway Station in Los Angeles by Pirooz Barar, P.E. of PB&A Inc.

Deep Excavation Software

DeepEX 2017 talk to it and design your deep excavation!

Excavations in 3D, VR, AR

Holographic model of a deep excavation with HoloDeepEX

Get the Free 3D Viewer


Free 45 min webinar

Deep Excavation Design

Fully booked!

Free Helical Pile Design Webinars:

To be announced

New Deep Foundation Software

DeepFND - Deep Foundation Software, caissons, CFA, drilled piles, driven piles, concrete, timber


New helical pile software HelixPile
Signup for a free trial and get our free pdf on the five most common errors in deep excavation design
Our Software Solutions
DeepFND 2017: Deep Foundation software (NEW!)
DeepEX 2018: Deep Excavation software
Soldier pile walls
Sheet pile walls
Secant pile Walls
Tangent piles
Diaphragm Walls
Soldier and Tremied Concrete
Soil Mix walls
Combined king pile sheet piles
Slope stability
Cost estimation for braced excavations
Waler-Strut Cofferdams
SnailPlus 2018: Soil nailing - soil nailing walls
SiteMaster: Inclinometer software (adopted by Geokon)
HelixPile: Helical Pile Software
RC-Solver: Concrete Design ACI-318, EC2, EC8
Steel-Beam: Steel beam column design, full equations, AISC, EC3

Soil nail wall safety factors, maybe not as safe as you think!

Soil nail wall analysis are typically performed with established traditional slope stability methods. In all methods a nail force is established at each intersecting slice. How this force is used next can affect the calculated safety factors. We will be illustrating the basic approaches below.

In the most basic approach many engineers would use a circular analysis with Bishop which only accounts for moment equilibrium. There are obvious limitations with this approach, and we will not be explaining them in full detail here!

If we wanted to be more theoretically consistent, we can select a method such as Spencer or Morgenstern-Price which considers both moment and force equilibrium. In typical calculations the nail forces would enter the full stability calculation at each slice so we can achieve static equilibrium. If very big forces are assumed then some strange numerical results can be obtained. In fact, the numerics would suggest that the soil mass movement would be happening in the opposite direction, so one needs to be considerate.

On the other hand, to avoid these issues, we could perform an available shear resistance method. In this case, we can compute the required shear required for a safety factor of one without any soil nails and also determine the available shear resistance by the soil. When we incorporate soil nails, we can calculate the shear resistance provided by the nails without consideration for changing the summation of forces on each slice. The new safety factor can then be defined as:

FS= (shear by nails + available shear by soil)/ (shear required for FS =1.0)

Take a look at this simple example below to see how the different methods compared when we evaluated them with our soil nailing software SnailPlus:

The Bishop method, produced a safety factor of 1.37, which would have been above the required for temporary short term conditions.

Bishop analysis for simple soil nail wall with SnailPlus software

Using Spencer's method with a circular surface and the traditional approach gives slighly smaller safety factors, close to 1.34:

Spencer analysis with traditional method for simple soil nail wall with SnailPlus software

Using the available shear method with Spencer gives a significantly smaller FS:

Spencer analysis with available shear method for simple soil nail wall with SnailPlus software

What method is finally selected requires engineering judgment, but we encourage that all methods are considered at-least at the preliminary analysis level. Obviously, when evaluating soil nail wall stability analysis we should be considering more complex sliding mechanisms and not only circular slip surfaces.

The available shear method is incorporated in our updated soil nailing software SnailPlus.


E-mail us for a live web demo at:

SnailPlus Pricing

$1100 for 1st license

FWHA LRFD and ASD available!

SnailPlus: $1,100.00
SnailPlus Soil nailing software