## Torsional Capacity of a Drilled Pier Foundation

SoilStructure Drilled Pier Software is one of the few programs where the engineer can investigate the Geotechnical & Structural Torsional Capacity of the Drilled Pier.  A recent study  by Oregon Department of Transportation, Final Report, SPR 304-701 was completed on May 2016. This report deals with Geotechnical Torsional Capacity.

The report can be downloaded from here:  and is titled “Evaluation of Torsional Load Transfer for Drilled Shaft Foundations” by Studlein, Barbosa & Li.

I have run the 3 examples shown in the Appendix of this report and compared it to results from Drilled Pier Software.  Below are my findings:

Case 1: Torsion of Drilled Pier embedded in Sandy Soil:

Case 2: Torsion of Drilled Pier embedded in Clayey Soil:

Case 3: Torsion of Drilled Pier embedded in Granular Fill over Clayey Soil:

CONCLUSION:

The comparison indicates that SoilStructure Drilled Pier Software predicts well when compared with other methods of Torsional Capacity in various soil conditions.

## How to Design a Cantilever Soldier Pile (Embedded Retaining Wall)

When excavation heights exceed 5 ft (1.5 m), we need to design a lateral support system. This is often a cantilever soldier pile or embedded retaining wall. A cantilever wall of this kind is permissible up to about 17 ft (5.2 m).

Backfill Earth Pressure

We can determine the backfill earth pressure using Rankine, Coulomb, Boussinesq, Muller-Breslau or Terzaghi & Peck method. Often, we need the soil’s friction angle and the soil unit weight. If pile-soil friction is to be ignored, then we can use Rankine Method. If pile-soil friction is taken as Phi/2, but the pile is vertical (not slanted), then Coulomb or Boussinesq method can be used for determining the backfill earth pressure. If you have a slanted pile, then we refer to Muller-Breslau equations. If you prefer a more general approach, the Terzaghi & Peck 5 soil classification method may be utilized.

Passive Earth Resistance

It is important to note the slope at the toe or at the dredge line. If it is descending, the passive resistance is much lower than if the toe is level. If soil’s friction angle is greater than 30 Degrees, it is best to limit the pile-soil friction angle to Phi/3, specially when using Coulomb method. In addition, we should factor the computed Ultimate Kp by 1.25 to 1.50 if we use wall friction. If Rankine method is used, it is already conservative & not necessary to use a Factor of Safety on Ultimate Kp.

This is often provided in the Geotechnical Engineering Report. For Cantilever Piles, it should be simple. It should give the Backfill Pressure per ft or meter depth and the Passive Resistance per ft or m depth, with a maximum passive resistance. For example, 45 pcf Backfill Pressure and 300 pcf Passive Resistance & a maximum of 3,000 pcf. This is a typical loading diagram:

Structural Beam Reinforcement

Once you have the loading diagram generated, you can pick an I-Beam and check first if it fits the drilled pier. As a minimum, we should provide a 2 inch (50 mm) diagonal clearance. See below:

Then you should plot Shear, Moment, Slope and Deflection with Depth. Also a check should be made on P-M interaction if we have axial loads. In addition a check should be made on Slenderness Ratio, kL/r and to stay within allowable lateral deflection @ the top of the Pile.

SoilStructure Software CANTILEVER SHORING performs all of the above checks. The program has SI units, English Units, AISC Beams, Australian Beams, European Beams & British Beams.

## Ascending Backfill & Equivalent Fluid Pressure in Retaining Walls

Definition:

Equivalent fluid pressure is a simplification of backfill earth pressure against retaining structures. If the backfill behind a cantilever retaining wall or restrained (basement) wall is level, we can use the equivalent pressure. For a cantilever retaining wall, level & drained condition (subdrain), the equivalent fluid pressure is simply the product of Ka * Gamma, where Ka is the Active earth pressure coefficient & Gamma is the moist unit weight of the backfill. For a restrained retaining wall, level & drained condition (subdrain), the equivalent fluid pressure is simply the product of Ko * Gamma, where Ko is the At Rest earth pressure coefficient & Gamma is the moist unit weight of the backfill.

History:

The first mention of the equivalent fluid pressure is in the 1953 publication, “American Association of State Highway Officials, Standard Specifications for Highway Bridges”, currently known as AASHTO Specifications. It said “Structures designed to retain fills should be proportioned to withstand pressure as given by Rankine’s formula; provided, however, that no structures should be designed for less than an equivalent fluid pressure of 30 lb per cu ft”.

Prior discouragement of the EFP method:

Professor Whitney Clark Huntington in his publication, “Earth Pressures and Retaining Walls”, said “The equivalent fluid method is commonly misused by applying it to walls which retain earth with a sloping ground surface..”. This statement was made in 1957, approx. 60 years ago. However, we still see the use of EFP method in today’s retaining wall designs.

Why is EFP not applicable to ascending slopes?

1. The EFP is increased arbitrarily to account for the ascending slope backfill angle while the vertical earth pressure component is sometimes not accounted for in design
2. The direction of the static earth pressure is level but in reality should be parallel to the ascending slope when using the Rankine’s method
3. The direction of the static earth pressure is level but in reality should be same as the soil-wall friction angle in Coulomb’s method
4. The sliding factor of safety computed by EFP will be larger than that calculated from Rankine, Coulomb or Muller-Breslau method (unconservative).
5. The overturning factor of safety computed by EFP will be smaller than that calculated from Rankine, Coulomb or Muller-Breslau method (overly conservative).
6. The Toe bearing pressure will be smaller with EFP method as compared to Rankine, Coulomb or Muller-Breslau method (under-predict induced wall settlement)
7. The Heel bearing pressure will show higher value with the EFP method as compared to Rankine, Coulomb or Muller-Breslau method (in reality eccentricity >B/6)
8. The Moment Demand, Mu, will be slightly larger with the EFP method as compared to Rankine, Coulomb or Muller-Breslau method (extra stem steel required)
9. The top of wall deflection will be slightly larger with the EFP method as compared to Rankine, Coulomb or Muller-Breslau method

Bearing Press.(EFP method)                  Bearing Press.(Phi/C /Gamma method)

Conclusion:

Geotechnical engineers should start to offer Phi, C, and Gamma values for cantilever and restrained retaining wall design & Wall designers should learn how to compute Active Ka, At Rest Ko, Passive resistance Kp, and Sliding friction coefficient. Professor Huntington discouraged it 70 years ago and SoilStructure Software discourages its use today. EFP is still in v3 of the SoilStructure Retaining Wall software as we hope this is a transitional time from EFP to Phi/C/Gamma.

## The 5 Limitations of the Pole Foundation Equation

It is common to see substructural engineers or foundation designers using 2012 IBC, Section 1807.3.2.1 to proportion drilled shaft (drilled pier) foundations.  If you have for example a flag pole or a light pole structure, this is understandable.  But when a structural engineer designs a deep foundation by just using the nonconstrained formula we need to be cautious.

 d = 0.5 A{1 + [1 + (4.36h/A)]1/2} (Equation 18-1)

1- If you will notice, Equation 18-1, does not contain the moment term. So if your lateral load has an arm to it, you are forced to abandon this equation.

2- Eqn. 18-1 is limited to a depth of 12 ft (3.66 m) and if your required embedment depth exceeds 12 ft, you are required to find another analysis method.

3- Most soils reports limit lateral soil pressure to something like 2,500 psf/ft. That means if your induced lateral pressure at one third embedment (S1) exceeds this value, the you are compelled to find new alternate approach.

4- We don’t know how much deflection takes place at the top of the drilled pier due to this lateral load by using Eqn 18-1.

5- Section 1806.3.4  allows doubling the given passive resistance if 0.5″(13 mm) lateral deflection does not adversely affect the foundation. There are three problems with this – (First, doubling the passive resistance is risky. We usually calculate this value ignoring wall friction & only factor the ultimate passive by 1.25 to 1.50 to arrive at the allowable passive resistance. By multiplying it by 2, means that you have just increased the geotechnical engineer’s values by 133% to 160%. Second, if we have a pole foundation near a descending slope, we can’t justify doubling the passive resistance as the passive wedge is interrupted. Third, if the geotechnical engineer used Coulomb’s method with say -phi/2 as wall friction angle and the angle of internal friction exceeds 32 Degrees, then the computed allowable passive resistance is already on the unconservative side and doubling this passive values is asking for a foundation distress).

We assumed your lateral force was a line load & not a distributed load & we also assumed the origin of this equation (use of steel piles) is similar to the performance of cast in place drilled piles.

In summary, if you have a bending moment, embedment depth more than 12 ft or just don’t want to risk exceeding the ultimate passive resistance of the soils, use a drilled pier software for complete lateral pier analysis.

## FAQ on Drilled Pier Software

1. We just want to use the skin friction values in the soils report- how do I enter that in the software?
Drilled Pier software (DP) asks you for “Ultimate” skin friction and then divides it by 2.0 factor of safety or the value of FS you enter to arrive at “Allowable” skin friction value most often shown on your soils report. Therefore enter two times the “allowable” value and set F.S. Skin Friction = 2 to match your soils report.

2. It is asking for many geotechnical inputs for the drilled shaft foundation design but I am a structural engineer- what do you suggest?
Drilled pier software gives you a range of suitable values for (a) lateral subgrade modulus & (b) ultimate skin friction. Just pick the soil type & consistency as shown on your soils report (like stiff Clay or medium dense Sand) and aim for the middle range of the displayed lateral SG modulus and Ult. skin friction values.

3. Passive wedge-can that be used for passive bearing?
Yes, that value is equal to 0.08*Phi and is multiplied to the isolated pier when determining lateral capacity. For example, if friction angle is 30 Degrees, Passive Wedge multiplier is 0.08*30 or 2.4.

4. Uplift Load, is that load along the pier surface from soil swelling?
Yes you can get uplift due to swelling or expansive soils on a drilled shaft foundation. You can also get uplift or tension from structural loads such as wind or seismic effects. The program will display geotechnical & structural tension capacities.

6. F.S. Torsional Moment is not a value given to us in the soil report.
Torsional moment can occur when you have a cantilever sign foundation or when the shear load applied to the drilled shaft foundation is off center. Based on calibration of DP software with instrumented drilled pier results, we recommend F.S. against Torsional moment in the range of 3.3 to 3.5.

7. What if the pier extends up several feet above the grade, can this situation be modeled in your software?

8. Can Drilled Pier software help me with depth to rebar cage?
Yes. On the results pages, you will see bending moment versus depth. For L/D (pier length/pier diameter) of 10 or less, we suggest rebar cage to be full length since you have a short rigid pier. However, for L/D >10, you may terminate rebar cage at 1 m or 3 ft below the depth where moment is equal to zero.

9. Where do I enter my load combinations for a drilled shaft design?
Under “Reinforcement” tab, you can override Pu, Mu, Vu and Tu (Axial, Bending Moment, Shear and Torsional Moment) values using the applicable load combination. DP software automatically factor mx. service level shear moment by 1.6 each, and axial downward load by 1.2.

10. Can the drilled pier software help me with bay mud?
Yes. When you are in compressible soils, the pier tends to settle faster then the surrounding soil. This causes negative skin friction which results in additional downward load which does not affect the allowable pier capacity. DP software computes the magnitude of this downdrag force.

11. How about vertical pier capacity & settlement calculations?
In some parts of the world only end bearing pressure is used, other regions only skin friction values are adopted while yet some other parts both end bearing & skin friction are utilized. Regardless of regional preferences, the initial load on a caisson or bored pile, causes elastic shortening of the pier. As you increase this downward load, skin friction picks up the load and yet as you increase the load, end bearing & tip resistance becomes engaged. Drilled Pier software will tell the capacity of a drilled shaft due to short term (Total Stress Analysis), long term capacity (effective Stress Analysis) and also the 3 components of vertical drilled pier settlement. These three components are settlement due to elastic pier shortening, settlement due to skin friction mobilization & settlement due to end bearing resistance.

12. I like your program- I can design several piers in a short time.
Thank you. We agree. The program performs axial, lateral & reinforcement design of a drilled shaft foundation under a single hood. Very few structures have just Axial only load or Lateral only load.

## Point load surcharges applied at a setback distance greater than 1H

A 15 ft Retaining Wall with a 16” thick footing (H=16.33’) has a point load surcharge of 4 kips at 21 ft setback (x=21’).  Most designers will say X > H, so there is no surcharge influence.

Looking at the chart above, you can see 20 psf surcharge. While this is small influence, it does show that surcharges setback greater than 1H still impose lateral stresses on the wall.  More surprising, if we were to look at the horizontal distribution, you can see 15 pound force at either side of the wall at 30 ft distance.  It only diminishes (say 2 lbf) at 45 ft perpendicular distance from point of application.  Look at the plan view below:

This means, you could have a surcharge 45 ft away from your wall, in another property line, but it still imposes lateral stresses.  Earth pressure does not recognize property line boundaries. With SoilStructure Retaining Wall Software, you can see this value on the Table below:

In this case it is only 2 lbf and can be neglected.  However, it is better to be aware of this phenomenon and quantify it.  SoilStructure Retaining Wall program tell you this value.

## Bearing Capacity of Shallow Foundations

When performing bearing capacity analysis, the experienced geotechnical engineer knows what range to expect and also know settlement controls.  But you need many years of experience to get there.  As a routine basis, why not run the bearing capacity and quick settlement calculation?  Shallow Foundation Software by SoilStructure simply helps you with this task.

From a convenience of one page, you can see the allowable bearing capacity based on shear strength of soils & the induced settlement with a given allowable bearing pressure.

More importantly – no matter which engineer writes the report, your analysis are standardized.