Retaining Wall Backfill

[surjaan]

The retaining wall is designed with 2000 psi grout strength at 28 days. The contractor wants to back fill in 7 days. I know the grout will gain about 70% strength in 7 days,should I allow back filling in 7 days or increase the grout strength.

 

[KBVT]

Allow the contractor to backfill, but make sure the wall is braced until it reaches the desired compressive strength.

 

[surjaan]

True, I agree with you, but contractor does not want to be in job site for 28 days.

 

[msquared48]

Then increase the grout strength, and ...

Specify hand tamping only - nothing heavier than a Bobcat within a lateral distance edqual to the height of the wall, and ...

If the contractor still complains, ask him how much his bond is.

His schedule is not your concern. Don't try to be Mr. Nice Guy here. As the engineer of record, you call the shots, not him.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BlastResistant]

Retaining walls are one of the most common structures under construction today. Some consultants (one or two man operations) can make a decent living doing nothing but retaining walls. All contractors know the typical drill regarding backfilling requirements, and if they don't they are not qualified to be building retaining walls. I am sure that in your notes you explicitly call out backfilling requirements regarding strength of material, time duration after grouting etc. Hopefully you did, and the contractor's bid should be based on the plans and specifications. Also in your notes, I hope you noted that shoring, bracing, etc. is the contractor's responsibility. As such, write a letter to the owner and the contractor explicitly stating your recommendations but also make it clear that you are not stopping work and that they may proceed at their own risk. However, if they choose to do so you will not sign off on the final inspection for the project.

 

[BlastResistant]

Also, I agree with Mike M. Sorry I got a little off track, but this nonsense with contractors and retaining walls is a very sore subject with me.

 

[cvg]

do not modify the grout strength or tell him when he can backfill early. That will in effect shift some of his liability for the cracks in the wall to you. He is responsible for building a wall without any defects, let him do it. He should have planned for that when he bid the job.

Bracing should always be done to protect against wind, vibrations or whatever for the first few days.

 

[Bobber1]

I carry a copy of a news clipping about a young guy that was killed here recently from an unbraced retaining wall falling on him. When the contractor starts to whine, I pick up a copy of the specs, and coincidentally, out falls that news clipping in front of the contractor. Usually works.

 

[concretemasonry]

What is the specified grout strength? (Assume this is a reinforced concrete masonry wall).

Very often engineers specify a grout that is too strong. It may be good the look at what is needed. The normal suggested practice is specify grout the is slightly higher than the hollow block strength. Some engineers put is a maximum allowed that is allowed for grout.

The principal is to have a balanced design since the main purpose to transfer the loads between the reinforcement and the masonry units. Strong grout can affect the distribution of the loads contrary to the basic assumptions mad in the design methods.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[tclat]

Anothoer option is to ask the contractor to improve the the quality of backfill such that the active pressure is less.

 

[Splitrings]

70% strength gain in 7 days seems high. I thought 50% was more typical. 70% in 14 days is what I thought
was more typical.

Run through the service level loading and compare to the estimated strength at the 7 or 14 day grout strength. I am guessing it won't be until 14 days is there adequate strength. We make them wait 14 days or use an accelerant.

Be careful about bracing a retaining wall. You can cause moments to develope in location in the stem that it was never designed for.

 

[DWHA]

I don't work with masonary walls. But for concrete walls we do not give a date, just the required compressive strength. That way the contractor can take samples & once those samples are tested and obtain the design compressive strength, they are okay to backfill.