Settlement in trench from raising ground water levels.
Settlement in trench from raising ground water levels.
(OP)
Wanted to see if any one has any idea the cause of the settlement seen in the trench zone. Background the trench is approx. 10 feet and 21 feet deep the ground water table is usually at 10-12 feet. dewatered every 50 feet to draw down the water table to 21 feet below grade, pipe 60" sewer pipe installed and encased in CLSM (2 feet above pipe) that that point was backfilled with Clayey Sand, 35percent passing #200 sieve, LL of 25 and PI of 11 Proctor around 121.8 @13.8, all fill was compacted to 90 percent or greater and 2to 5 percent above the optimum. Two feet of Type II compacted to 95 percent. and no problems were encountered.
When the dewatering was stopped and the groundwater levels raised up back to 10 to 12 feet below grade , 8 to 10 inches of settlement was observed in certain areas along the trench line?????
Any thoughts When we drilled borings the blow counts dropped of to 1 and 2 when we got close to the water and the fill above the water table ranged in 5 to 10 blows per foot (N60).
When the dewatering was stopped and the groundwater levels raised up back to 10 to 12 feet below grade , 8 to 10 inches of settlement was observed in certain areas along the trench line?????
Any thoughts When we drilled borings the blow counts dropped of to 1 and 2 when we got close to the water and the fill above the water table ranged in 5 to 10 blows per foot (N60).





RE: Settlement in trench from raising ground water levels.
RE: Settlement in trench from raising ground water levels.
On Monday after the settlement the asphalt felt like it was pumping.
RE: Settlement in trench from raising ground water levels.
Anther thing to try is compact samples at the percentage used for filling and also do the same at the percentage found later. Run a form of commissioner test, such as unconfined compression, CBR or other to see if only that moisture increase was the cause. Compute void percentage for each which might explain the increase in moisture permitted.
RE: Settlement in trench from raising ground water levels.
You say, "the water flowed down the trench". Did it flow in a coarse granular bedding layer? I have see cases where the backfill was dumped in and compaction started well above the spring line, and there was no soil beneath the haunches. Water flowed down that trench very well.
The behavior is what would be expected if the compaction in the lower part of the trench was very poor, like none. I would question whether the procedure described was actually followed in the areas that subsequently settled.
One more item: If the subsequent tests were taken by pushing tube samples with a drill rig, they were probably compacted in the sampling tube. I have seen results where every sample plotted on the ZAV line showing that the wet soil was compacted to 100% saturation during sampling.
RE: Settlement in trench from raising ground water levels.