Grouted Tie-backs in Soil for a Sheet Pile Retaining Wall

[bridgegrl]

I am working on a project that uses grouted tie-backs in conjunction with a sheet pile retaining wall. The tie-backs are being installed in a material that has been classified as Poorly Graded Sand (Medium Dense) with the boring log showing sand and medium to fine gravel with cobbles and boulders noted. The tie-backs are being installed using a 7" diameter temporary casing. A few of the tie-backs have already been installed and much larger quantities of grout than anticipated were required during the intial grouting before a return was seen at the top of the casing. The worst case was 13 x the area of the casing/hole and the best case was 4 x the area of the casing.

My question consists of two parts:
1) Based on the soil information (before construction began), what would be a reasonable number used to estimate the amount of grout anticipated per hole?
2) Is there a better solution to implement when a much larger amount of grout is required than planned?

Any guidance is appreciated!

 

[dcarr82775]

The only time I ran into that much overage (13*) was when the contractor drilled through a sewer and just kept pumping grout. Are you grouting under pressure or just gravity grouting?

 

[PEinc]

Are you drilling with air or water for flushing the drill cuttings?

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[bridgegrl]

I was told by the Contractor it was due to void space in the gravelly soil which was unexpected. They are losing the majority of the material during the initial operation which is gravity fed. I believe the drilling only uses air.

At this point I'm trying to determine what a good estimate for the grout should have been to determine how much of an overage should be paid for as well as try to determine if another method should be used for the remaining anchors.

 

[BigHarvey]

You can reduce the influence diameter of the grouting by adding sodium silicate to your bentonite-cement grout. This will rigidify the grout and prevent it to flow to large distances from grouting point.

 

[dcarr82775]

If the contractor truly is running into voids of that nature I don't think there is a way to estimate grout quantities. On the remaining anchors try to thicken the grout a little via any number of methods. Of course you then run the risk of not getting grout everywhere you want it.

 

[PEinc]

Tieback anchors are most often installed using a performance specification. When the anchor contractor bids the job, he's taking the risk on making the anchors work. It's not unusual for some jobs to use excessive amounts of grout per anchor. This is more common in karst topography or where the contractor drills through a utility or other underground structure. If the tiedback wall has multiple tiers of anchors, less grout may be needed on the lower tiers as the upper tier grout fills lower voids. If it can be determined that grout is being lost only in the unbonded zone, perhaps a cheap casing pipe can be left in place to retain the grout in that zone.

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[casimmons]

I wonder if the holes are just becoming oversized because of the granular soils and the drilling method. Does it look like an excessive amount of cuttings are coming out of the holes? If that is happening, all this loss of ground could lead to subsidence at the ground surface, or of nearby structures.

 

[DarightQualityMan]

We are installing tie-backs at 20 Degree angle. My question is, is taking grout cube samples and breaking them in compression required? The grout is in tension so why would it be necessary to check the compressive strength of the grout? Our contract has no tie-back specification in it. The grout we are using has historical strength data on it but the Owner wants compressive strength on it but can't give us a requirement. Any thoughts or answers would be very much appreciated.

 

[PEinc]

DarightQualityMan, first, this should have been a new thread, rather than changing the subject of this thread. Second, this has been addressed before in other threads. You can search for other related threads.

On most permanent anchor projects, especially for DOT work, grout cube testing is required. However, what difference does the cube strength make if every anchor is successfully tested? If the anchor passes it load test, the grout must be OK. Also, results from grout testing are very dependent on factors other than the grout mix. Various size grout cubes or cylinder can affect the tested grout strength. Improper sample preparation, storinf, transportation, and curing all can have a negative effect on the test results. Also, by the time the grout cubes are tested, it may be too late to return to that tieback location to supplement or replace one or more anchors. What do you do if a series of anchors passes their load tests but their cubes come up short in strength? The importance of grout cube tests is overstated.

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