Battered Micropiles for Lateral Resistance

[JennyNakamura]

Hi Everyone,

I am working on a project that requires a significant amount of micropiles. It is a 150' long retaining wall with about 24' of retained soil and large lateral seismic loads and traffic loads. The geotech has specified micropiles for the project, noting that for lateral resistance the micropiles can be battered and to neglect lateral capacity of vertical piles. I don't understand this logic, since a pile battered at 10 or 20 or even 30 degrees will still see a large transverse load at the head when you break the lateral load down into axial and transverse components. Anyway, I took the geotech's report to mean that it doesn't matter what the orientation of the batter is (toward the backfill or away from the backfill), the lateral can be taken by piles in tension or compression. My boss says only battered piles acting in tension (tip of pile is angled toward the retained earth/load is pulling head of pile away from wall) can be used for lateral because of buckling issues.

What is the common rule of thumb on this issue? I am trying to avoid spending a day or two modeling this in RISA with soil springs etc. since it's not in our budget. Also, battered piles can be used to take sustained vertical loads and not only lateral, yes?

Our client is complaining about the huge amount of micropiles required and I am trying to minimize things without compromising safety.

Thanks in advance.

 

[BAretired]

I would regard all piles as primarily axially loaded members. Battered piles carry a horizontal component and a vertical component which is dependent on the slope of the pile. Battered compression piles are more effective than battered tension piles because they actually contribute to the support of gravity loads whereas tension piles act in the opposite direction, adding to the gravity load on the vertical piles.

The only way battered piles can resist sustained vertical load is when they act in opposition to each other, that is one in tension and one in compression. But your boss doesn't want to use compression piles. Perhaps he should review his decision as it would add substantially to the number of piles required. Vertical piles are usually in compression, so why not battered piles?

BA

 

[amec2004]

My reply as the followings

>>The geotech has specified micropiles for the project

Micropile may not be a good option unless there is site access or other restraints for other pile options.
We normally only use micropile in the existing plant where there is site access restraint such as for additions and alterations inside a building, the pile rig cannot move in due to the height or process equipment restraints, or close to the existing structures which are sensitive to the vibration caused by new construction of piles, etc.

Micropile is normally small diameter (less than 12.75” or 324mm diameter), as such micropile capacity is much less than concrete bored pile or steel pile. Due to the micropile reinforcement setup, with DYWIDAG threadbar or bundled threadbar positioned at the center of pile, it has zero moment resistance so normal vertical micropile is not suitable for lateral load resistance as the pile shaft section cannot take any moment caused by the lateral load.

In Fort McMurray region, Alberta, Canada, the cost of one micropile is over 20k, with its lower capacity at this cost the micropile is much more expansive than any of the other pile options, so only use micropile when the other pile options don’t work on the site.

>>noting that for lateral resistance the micropiles can be battered and to neglect lateral capacity of vertical piles.
>> I don't understand this logic,

What your soil consultant's recommendation is correct.

The logic behind this is that the vertical pile, which relies on the non-linear soil springs providing lateral resistance, has much weaker lateral resistance stiffness compared to the batter piles, which forms a triangle truss and the lateral load is converted to the AXIAL tension/compression force in the batter piles.

The load are always distributed based on stiffness when there are multiple structure systems working together.
One similar case is the steel frame's lateral load resistance system, the lateral resistance of one braced frame is better than combined 100 pieces of moment frames, lateral resistance of one shear wall is better than combined 10^6 pieces of moment frames. The same logic applies to vertical/batter pile case when they co-exist under one pile group. All lateral load will go to batter piles as they are much stiffer and the vertical piles will take zero lateral load.

>>Anyway, I took the geotech's report to mean that it doesn't matter what the orientation of the batter is (toward the >>backfill or away from the backfill), the lateral can be taken by piles in tension or compression. My boss says only >>battered piles acting in tension (tip of pile is angled toward the retained earth/load is pulling head of pile away >>from wall) can be used for lateral because of buckling issues.

I would recommend Reese book for your reference

Single Piles and Pile Groups Under Lateral Loading by Lymon C. Reese and Willem van Impe

See attached PDF file extracted from Reese book section 3.13 EFFECT OF BATTER and Fig 3.34

The effect of in-batter pile (negative batter angle and pile tip pointing towards lateral load coming direction) has better lateral resistance than the out-batter pile. That observation is more for single batter pile case and used as P-Y curve multiplier by the program LPile or Group.
In the real design case it’s more desirable to use the batter pile in a pair, either by pair of in-batter and out-batter pile, or by pair of batter and vertical pile. Getting pair of piles to form a triangle truss, mobilizing AXIAL tension and compression force in the pair of piles rather than resisting the lateral load through bending of vertical pile, is more important to us as the pair of pile will provide much stiffer lateral resistance. One pair of in-batter and out-batter pile forming a triangle truss, is much stiffer than using combined 1000 number vertical piles, which is similar to above mentioned lateral resistance of one shear wall is better than combined 10^6 pieces of moment frames case.

With regard to compression micropile lateral buckling issue, you only need to check it when there is weak soil, voids, or liquefiable soil conditions. Most of the time it won't be a concern due to the soil restraint, see page 4 and 5 of attached PDF file.

>>What is the common rule of thumb on this issue? I am trying to avoid spending a day or two modeling this in >>RISA with soil springs etc. since it's not in our budget. Also, battered piles can be used to take sustained vertical >>loads and not only lateral, yes?

My suggestions to you are

1. First option, if you have access to Ensoft software Group, use Group to do the analysis

2. Second option, if you don’t have access to Group, you can model the pile group as truss member together with retaining wall as solid elements + soil as springs restraint in RISA3D, plug in the soil spring using P-Y curve getting from your soil consultant, with the batter inclination degree multipliers to the P-Y curve. The P-Y curve is non-linear in the large displacement range but shall be good if you use them as linear in small displacement range, which will greatly simplify your analysis

3. Third option, refer to the an old book Foundation Design by Wayne C Teng

Use the Culmann's graphical method as shown step by step in that book shown as example of retaining wall plus batter pile design case
Culmann's graphical method is based on the following assumptions

1. Piles are grouped according to their slope
2. It assumes that all piles are subjected to axial load only
3. It assumes that piles in each group are subjected to equal axial load

There is a detail step-by-step retaining wall plus batter pile design case calculation on shown page 230~238

>>Our client is complaining about the huge amount of micropiles required and I am trying to minimize things without >> compromising safety.

I would suggest go for drilled bored pile or steel pile. As illustrated above, micropile is the most uneconomical and less capacity pile option. You should only use micropile when there is a site access issue or other restraints.

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[amec2004]

>>The effect of in-batter pile (negative batter angle and pile tip pointing towards lateral load coming direction) has better lateral resistance than the out-batter pile

To my surprise, the positive batter pile has zero lateral resistance ???

Lateral resistance from High to Low : Negative Batter pile > Vertical pile > Positive Batter pile

See attached PDF file, which also shows why the batter pile's truss model is much stiffer on the last page


anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[JennyNakamura]

amec2004 -

Thanks for the awesome posts! I can't thank you enough. I know you spent a ton of your personal time on this and I appreciate it. I will spend all day tomorrow studying and digesting all the info you provided before I hit the office on Monday. Rest assured your time has not been spent in vain...