GeoCOE
Geotechnical
- Jan 13, 2005
- 1
I currently have a major problem with the as-built condition of an Soil-Cement-Bentonite Cutoff Wall. Questions regarding contractor quality control prompted an investigation of 6,200 LF of constructed wall. Part of the investigation involved drilling 5 SPT holes through the center of the wall. All 5 holes revealed the bottom portion (several feet) of the wall does not contain the homogeneous mixture of S-C-B that should have been there, in addition to randomized layers of sand trapped in the upper portion of SCB backfill.
After futher investigations, the contractor is offering up a differing site condition with respect to groundwater containing high concentrations of salts (100 < sulfates < 1000 ppm), hardness (300 < Calcium < 10,000 ppm), and alkalinity up to 57,000 ppm, which he is claiming may be causing flocculation of bentonite thereby increasing its density, and retarding hydration of cement. The contractor is using Sodium-Bentonite powder and Type 1 Cement.
Our doubts regarding QC involve an interpretation, by the contractor, that the differences measured (ranging from 1-4 feet) between PM soundings and morning-after soundings were a result of "crudeness of sounding methods" and within rounding errors as opposed to (our view) being an indication of overnight settling of sand or trench walls sloughing. These differences were evident throughout entire job and not just limited to a few occurrences. Contractor has been wrestling with above average sand contents in bottom of trench since day 1.
Does anyone have any experience with SCB walls placed in groundwater exhibiting hardness and high sulfates? What materials were used? Has anyone had similar concerns over contractor quality control?
Any thoughts or referrals would be greatly appreciated.
After futher investigations, the contractor is offering up a differing site condition with respect to groundwater containing high concentrations of salts (100 < sulfates < 1000 ppm), hardness (300 < Calcium < 10,000 ppm), and alkalinity up to 57,000 ppm, which he is claiming may be causing flocculation of bentonite thereby increasing its density, and retarding hydration of cement. The contractor is using Sodium-Bentonite powder and Type 1 Cement.
Our doubts regarding QC involve an interpretation, by the contractor, that the differences measured (ranging from 1-4 feet) between PM soundings and morning-after soundings were a result of "crudeness of sounding methods" and within rounding errors as opposed to (our view) being an indication of overnight settling of sand or trench walls sloughing. These differences were evident throughout entire job and not just limited to a few occurrences. Contractor has been wrestling with above average sand contents in bottom of trench since day 1.
Does anyone have any experience with SCB walls placed in groundwater exhibiting hardness and high sulfates? What materials were used? Has anyone had similar concerns over contractor quality control?
Any thoughts or referrals would be greatly appreciated.
