I am changing the type of wall, however, the maximum load stays the same. Why are the moments and design not changing when I change the wall properties?

SPW 911 uses a "classical" method in that the shears, moments, wall lengths, support loads, etc., are computed independent of any deflection of the sheeting. Since that deflection is dependent upon the material and cross-section of the wall, those moments, etc., will not change with changes in the cross-section or material.

For Active and Passive pressures please define the terms:  k, K_a,  K_ac, h, K_p & K_pc

•  h   Height of soil above a level
•  K_a  Active pressure coefficient for cohesionless soils
•  K_ac Active pressure coefficient for cohesive soils = 2 K_a ^1/2
•  K_p  Passive pressure coefficient for cohesionless soils
•  K_pc Passive pressure coefficient for cohesive soils = 2 K_p ^1/2

 All terms explained in "Notation" section of online Help file.

Does the Greek symbol delta represent the slope of the ground surface in degrees?

Angle of friction between soil and piles

What is the difference between the hinge method and area distribution and how should these be applied?

No difference in single propped walls. In multi-prop walls:

• Area Distribution: "The loads on frames above the lowest are found by load distribution. The top frame load is the total load between the top of the excavation and the midpoint between the top frame and the frame below. The load on other frames above the lowest is the total load between the midpoints between each frame and the frames immediately above and below. The load on the lowest frame is found by a combination of the area distribution method and the value obtained by treating the span below the lowest frame as a single propped wall and using the appropriate method for Free/Fixed Earth Support."
• Hinge Method: "The loads on frames above the lowest are found using the hinge method (Ref: BSC Handbook, 7th Edition, pg 5/12; BS 8002). A hinge is assumed to exist at each frame except the the top one, and the spans between frames above the lowest are considered as simply supported beams. The load on each frame is the sum of the loads found by analysing each of these simply supported beams. The load on the lowest frame is found by a combination of the hinge method and the value obtained by treating the span below the lowest frame as a single propped wall and using the appropriate method for Free/Fixed Earth Support (Ref: BSC Handbook; Ciria 95)."

What are the rules of thumb if this choice is selected?

"The toe predicted by SPW911 is mathematically correct. However, some rules of thumb based on experience may be applied to enhance the safety of a design. Assuming the depth of excavation is d, the rules applied are:

1. No toe should be less than d/10.
2. In firm clay (50 <= C < 75), the minimum toe should be 600mm or d/5, whichever is greater.
3. In stiff clay (75 <= C < 100), the minimum toe should be 300mm or d/10, whichever is greater.
4. In very stiff clay (C = 100), the calculated toe will be very small. It may be better to use a "no toe" design with a frame near the bottom of the excavation.
5. When water is present in granular soils, "no toe" designs should not be used.
6. When water is present in granular soils, the minimum toe should be 0.75 x depth of water above excavation depth.
7. A minimum of 2 frames are required for "no toe" designs.
8. In cantilever designs, the toe should not be less than d."

Since SPW 911 2.09 does not directly model sloped ground, explain the choice between modifying k values and using the BBPH Approximation.  Is this only for sloped ground?

Alternative methods for analysing sloped ground. Two methods of analysis are available for sloping ground:

•  "Modify K Values. The K values of each soil are recalculated using the given angle of slope. Note that the formula used demands that the angle of slope does not exceed the angle of soil friction. If such a condition exists, it is assumed that the angle of friction equals the angle of slope in any soil layer where it occurs, and a warning to this effect is displayed.
•  Use British Steel Piling Handbook Approximation. As described in BSPH (pg 4/12), it is assumed that soil pressure changes by 5% for each 5 degrees of slope. This avoids the problem outlined above."

I just ran the program on a cantilever wall.  The program's help file states that a 20% factor is on length OC is applied to compensate for the simplified design method.  Is this an additional factor of safety or what?

No.  The 20% factor is applied to compensate for the simplified design method.  Once this is applied, the toe should be extended until the program yields the factor of safety you need.

What formulas are used in the program to calculate the deflection of the retaining wall?  This includes the assessment of the coefficient of ground reaction and the passive resistance of the soil below the dredge line.

Deflection is calculated by solving the second order differential equation v_xx = M/EI.  Basically, this involves a double integration of the displayed bending moment graph.  Soil coefficients are calculated as outlined in the Help file, under "Calculation Methods  Soil Coefficients."

I changed the value of phi for my soil and yet the corresponding values of K_a and K_p don't change.  What can I do?

• Click the "Add Layer" button, then define the depth of the new layer and select a soil (depth/soil selection can be in any order).
• Click the "Apply Layer" button.
• Change phi, C, etc - the K values should be updated.

In a design with multiple walers, how is the required toe calculated in granular soils?

When more than one support is present, the toe and load on the lowest frame are found by considering the lowest span as a singly supported wall. Loads on the frames above the lowest are found using the current load model. The load model can be by a) Area Distribution (as described in Sheet Pile Design by Pile Buck) or b) the Hinge Method. The method used is selectd in the "Supports" page of the "Define" box.

We do not understand how the program calculates its pressure diagrams for cohesive materials. The program shows positive pressure starting at the top of the sheet and not the distance (unconfined comp strength - gamma effective). We calculate pressure at the top of sheet to be unconfined comp strength (a negative value or zero), unless surcharge overcomes the negative value.

This is due to the application of minimum fluid pressure. Refer to the "Design creation > Define Box: Soils page" section of the Help file for further details. You can effectively switch MFP off by setting the Minimum Fluid Density to 0 in the Excavation page of the Define box. In this case the minimum calculated pressure in cohesive soils is zero, as a negative pressure would suggest the soil is self-supporting.

"The Fine Print" for SPW 911

The information, including technical and engineering data, details, procedures and results, presented in this program have been prepared in accordance with recognized contracting and/or engineering  principles, and are for general information only.  While every effort has been made to insure its accuracy,  this information should not be used or relied upon for any specific application without independent  competent professional examination and verification of its accuracy, suitability and applicability, by a licensed professional engineer.  Anyone making use of this material does so at his/her own risk and assumes any and all liability resulting from such use.  The entire risk as to quality or usability of the data contained within is with the user.  In no event will Pile Buck^® International, Inc. be held liable for any damages including lost profits, lost savings or other incidental or consequential damages arising from the use of or inability to use the information contained within.  By continuing, the user of this program hereby agrees that the user has read the terms and conditions outlined in the attached "End User License Agreement" and "Disclaimer of Warranty and Limited Warranty" and agrees to be bound by the terms and conditions mentioned therein.  If the user is not in agreement with the terms and conditions outlined in the attached "End User License Agreement" and "Disclaimer of Warranty and Limited Warranty" in its entirety please return this program immediately for a complete refund of the original purchase price.

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