Excavation/shoring in deep sand 2

[kwhitney]

We have a need to excavate up to 20 feet deep next to a building foundation. The geology consists mainly of uniform sand down to about 100 feet. Water table is about 10 feet below grade. The area to be excavated is approximately 100' x 150'.
Any options/suggestions for shoring the pit?

 

[emmgjld]

Flooding or 'jetting' backfill generally produces poor to very poor compaction. There are very few specifications that I am aware of that even allow flooding are some landscaping projects. This method of soil settlement is well addressed in the literature, regarding older earthen dams and other backfill situations which failed. I do not allow this type of soil settlement on any projects which will require subgrade or backfill soil stability. In my opinion, this is a very poor method of trench backfill and should not be used beneath buildings, slabs-on-grade or beneath road sections.

The only soils which I am aware of that respond reasonably well are some poorly to possibly moderately poorly graded sand and possibly gravels. Fine grained soils (silts and clays) or soils with fine grained components usually achieve 78% to 85% of ASTM D-698 maximum density if conditions are good. Good conditions require extremely good drainage characteristic of the flooding water into the adjoining soil. If the water cannot drain from the backfill soils, the material will be setup for future collapse. This is very similar to the mechanics exhibited in mudflows and debris flows, which oftentimes become metastable (collapsible) soil.

 

[emmgjld]

Sorry for the error, the previous should have been on an adjacent post.

 

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[DRC1]

Okay, First the excvation must be dewatered. the watertable is at 10 ft. below grade and the excvation extends 20 feet below grade. Although you could use sumps within the excavation, you will probably need wellpoints outside the excvation as well. The rub is that dewatering in sand may cause some settlment of the building, especially if the sand is loose. You may need to underpin. It would be good to have an engineer from a reputable dewatering contractor look at this aspect. You can use either steel sheet piles or soldier beams with wood lagging. For a 20 foot cut you will need one level of bracing. This can be accomplished by either tiebacks or rakers inside the excvation. A 100 x 150 ft excavtion is too large for cross lot bracing. The anchors are probably the more likely support system. Easements are required under adjacent property owners, but most states require the adjacent owner either grant a reasonable easment, or make provisions to support their structures. Installing the sheets or piles require impact equipment or viberatory equipment. These can cause settlement also, and it hard to predict the impact. It is best to keep the sheeting as far from the building as possible. In general, soldier piles induce less vibration than steel sheeting and impact hammes induce less vibration than vibratory drivers. However driving with an impact hammer is slower and more expensive than driving with a vibratory hammer. It is also very important to moniter the structures for movement before during and after sheeting instalation and excavation. Finally wether you do the design or specify the contractor provide a design, it should be prepared and sealed by a P.E. experienced in earth retention. Prior to doing the design an adequate site investigation should be performed in the project design phase. This should include a good topo survey and research into adjacent structures.
Good Luck with your project!
P.S. You may want to post this in the Earth Retention Forum for more comments.

 

[sgwpe1]

You didn't say the depth of the foundation next to which you are excavating - and whether or not the excavation could be done in smaller segments or if it has to all be opened at once. The effects of the adjacent foundation, depending on its depth, should also be considered and how close you will be to that foudation.
Most excavation I did near structures was to remove contaminated materials and we would do piece work to limit open areas and leave as much adjacent soils in place to keep load on the soils next to the foundation as we could. We used modular shoring boxes sometimes which are very rigid but mostly useful on smaller excavations. Sometimes tight sheeting was more useful especially where groundwater control was paramount. DRC1 gives you a great amount of information and good advise in his post. Best wishes on your project.

 

[jgro]

Without an understanding of the job, it sounds like you are limited to sheeting the entire area, toeing in the sheeting enough below subgrade for a water cutoff. It also seems as if you will need to interally brace the sheeting against each other, as opposed to using a whaler tie back system. After the sheeting is installed, place a series of well points in the cofferdam to lower the level of ground water sufficient enough to approx, 2' below bottom of excavation. Then begin excavation placing bracing for sheeting as necessary until excavation is complete.
This is just a rough idea that is typically used, good luck.

 

[ayfarm]

Tight interlock sheets driven and grouted/sealed outside to prevent seepage into the pit would work. Bracing is not necessary if sheets are sized for cantilever action.