Fully Bonded Free Stressing Length in Ground Anchors

[Obregon]

Hello everyone. I wonder if you can comment on the applications, advantages, disadvantages, of two-stage ground anchor grouting, leaving a fully bonded free length for corrosion and performance issues of retaining structures, besides mass structures, i.e., dams, etc., Your insights are well appreciated.

 

[Ingenuity]

Sorry, but you shouldn't dare to install a ground anchor under water or seepage conditions within the borehole.

Sure you can. But you need to now your soil/rock strata, drilling equipment/tools, and grout materials/admixtures.

I did a project (both design & construction) back in the late 90's with 100 each permanent vertical rock anchors (Williams 150 ksi x 2-¹/₄" dia bar) with 50 foot deep holes x 6" dia. Drilling 10' above sea level right on the coast for a 10' thick concrete sea wall protection. We hit an underground aquifer (karst limestone) at mid-depth and grout takes went from 10 bags to 400+ bags per hole. We had to use an anti-washout admixture to minimize tidal effects on washing out the cement grout, then re-drill each hole (cased holes were drilled using DTH and Stratex bits). Proof (and performance) tested all anchors to 320 kips. We also installed post-grouting tubes to the outer annulus in case we had to do secondary grouting.

To maintain corrosion integrity we pre-fabricated the tendon (bond length, greased-and-sheathed free length, HDPE corrugated sheath, spacers, end cap etc) and grouted the inner-annulus on a sloped fabrication bed, and once cured, crane lifted (slings and lifting beam) each tendon assembly into the vertical borehole.

Tough project, but ground anchors under water can be done.

 

[r13]

Wet situation is often encountered at tunneling and concrete dam stabilization projects. Every event is different, depending on the rock formation, water source and flow rate, the technics differ too. The selection of anchoring method/system and grout resin are to be evaluated in the case by case manner. Temporary mechanical anchors, grout, and area/local dewatering are technics been used to ensure the quality of the permanent anchorage installation. As addressed before, there are lots of headaches, and things to be concerned with.

 

[PEinc]

Obregon, yes, I mean it. If you remove the temporary casing from a partially grouted ground anchor, you risk drill hole collapse which can cause settlement of the ground surface and can prevent you from being able to grout the remaining anchor length later. I have never seen a project where voids were allowed behind a tiedback wall or under a tied-down footing due to collapse of the drill holes.

http://www.PeirceEngineering.com

 

[Obregon]

Good morning. I thank you both of you for sharing your experiences, it is well appreciated.

Ingenuity, do you do the greasing and sheathing of individual strands along the free length by hand and in the field, or you guys employ greased filled extruded high density polyethylene sheathing from a shop as a means to avoid leaving an air film within the sheath tube?.

Should this grease-and-sheath operation be done manually at all? How do you tight-extrude the sheathing over the strands if not with a proper equipment/tools in the factory to guarantee that proper corrosion protection has been achieved?

Thank you

 

[Obregon]

Good morning PEinc, please help me. How can we actually grout a ground anchor without simultaneously withdrawing the casing?. Why wait until the grout has set and the anchor has been tested?. Thank you.

 

[PEinc]

Obregon, if in collapsible soil, drill and case the hole to its full length or into the top of rock (if a rock anchor). Insert the tendon along with a full-length, removable grout tube. Tremie grout the full length of the drill hole until good, clean grout comes out of the casing pipe and extract the grout tube. Then, extract the casing pipe and, if necessary, top off the grout up to the face of the wall. Or, you can drill and case the hole, tremie grout its full length, pull the grout tube, and then insert the tendon and extract the casing pipe. Extraction must be done immediately after the anchor is grouted and before the grout sets. Remember that the unbonded length must be sheathed or greased and sheathed with a plastic bond breaker because you are not doing two-stage grouting.

http://www.PeirceEngineering.com

 

[Obregon]

Thank you PEinc: "Remember that the unbonded length must be sheathed or greased and sheathed with a plastic bond breaker because you are not doing two-stage grouting". If you allow me, I ask the same question to you as I did to Ingenuity.

Should this grease-and-sheath operation be done in the field and manually at all? How do you tight-extrude the sheathing over the strands if not with a proper machinery in the factory, in order to guarantee that adequate corrosion protection has been achieved, avoiding the presence of moisture/air within the HDPE sheath tube?

Thank you again.

Carlos R. Obregón

http://www.sueloanclado.com

 

[PEinc]

30 to 40 years ago, greased and sheathed strand tendons were produced by Lang Tendons (eventually bought by DSI) with a patented extrusion process that could apply grease and extrude a tight fitting plastic sheath. VSL manually greased and sheathed their strand anchors because they did not have Lang's extrusion process. Today, I think most strands have extruded sheath and grease. Although you could manually grease and sheath strands in the field. I have not seen this done in many years. Most US contractors buy the strands from suppliers like DSI, Williams Form Engineering, etc.

http://www.PeirceEngineering.com

 

[Obregon]

Got it. Thank you very much.

 

[Ingenuity]

Ingenuity, do you do the greasing and sheathing of individual strands along the free length by hand and in the field, or you guys employ greased filled extruded high density polyethylene sheathing from a shop as a means to avoid leaving an air film within the sheath tube?.

Should this grease-and-sheath operation be done manually at all? How do you tight-extrude the sheathing over the strands if not with a proper equipment/tools in the factory to guarantee that proper corrosion protection has been achieved?

For the project I mentioned above, I stated that the anchors used Williams threaded bar, not strand. Bar is relatively easy to field apply grease-and-sheath. We used PVC tubing and injected grease into the annular space.

As PEInc stated, for permanent strand anchors, projects in North America USA use extruded HDPE sheath to greased strand, manufactured under strict QA/QC procedures in accordance with PTI Certified Plant Program.