Staging with boulder wall construction

[geopat69]

Hi all.
I have a residential subdivision which will require installing 2m high granite boulder walls at the boundary perimeter of each block( ie I have sloping blocks with cut and fill operations required) . My question is specifically in regard to Staging.

You see some of the blocks of land require compacted controlled fill across each block (vibrator compaction required) . Is it industry practice to cut the fill the site first... THEN INSTALL the boulder walls? Or should the boulder walls be placed first and the compacted fill be placed after?

I would have thought that the site is cut then filled and controlled compacted first... with boulder walls placed last.. But I'm not too sure.. I also suspect vibrato compaction near the base of a boulder wall is not right(ie destabilisation).

 

[oldestguy]

In my are of USA it all depends on what the project is as to any compaction and to what degree. Any "boulder walls" I am involved with requires the wall to be stable by itself, not dependent on any backfill compaction. Thus a boulder wall is a gravity wall where width is at least 1/3 the wall height. At less width, no doubt the compaction of backfill has to be done while it is built. Your geotecch can advise on this.

 

[geopat69]

Sorry - I should have been a bit more clearer. It has nothing to do with backfill behind the wall. Its all to do with the compaction of the fill in front of the wall...in particular when it is done versus when the boulders are installed. Note that the compacted fill is achieved by sheep foot roller. The attached should help!

 

[oldestguy]

My definition of "front" is that part of the wall you can see, exposed to air. In that case I have very rarely seen any need to compact the soil there, since it usually is natural ground, not fill. If it has been disturbed, yes you should have it compacted, at least to the same density as the natural ground which likely is no more than 85 percent of lab density.. Usually the lower part of a boulder wall may be half a boulder or so below the grade there in front. In severe frost areas one might go a little lower, but not necessarily to so called "frost depth" because any movement usually can be tolerated.

 

[geopat69]

Hi Oldest guy and thanks again. My concern is the block as a whole (ie not immediately in front of the wall). Attached is an example photo.

In the photo exmample from another job, the geotech confirmed only 200mm of compacted fill just in front of the wall. but again we were unsure at what stage the boulders are placed.

You will see a lot of land with compacted fill. So the question is really a "chicken and egg one". Would the fill(across the entire site) have been placed before or after the boulders installed.

i can see both arguments...but seems to me a vibrating sheep foot roller could destabilise boulders if travelling close to the base of the wall.

 

[geopat69]

photo here

 

[oldestguy]

You may be right with the effect of a heavy vibrator near the wall. Thus, I'd do all the earthwork in front first and build the wall later because one never knows if the compactor operator might get too close and cause damage. My experience is that clean sand in a low moisture condition can migrate out from under loads on it due to vibration in the area. In one case the source of vibration was some 50 to 70 feet from the column footing and the column settled due to sand migrating out from under. So, this risk all depends on the type of fill that is close to or under the wall site. Then comes the question of placing backfill behind the wall. Same concern.