Using the Grouting Intensity Number (GIN) to Assess Compaction Grouting Performance

The Grouting Intensity Number (GIN) concept has been previously used in consolidation grouting to standardize the level grouting effort conducted at each grouted hole. On this project the GIN concept was applied to compaction grouting to provide real-time quality control (QC) to ensure that the construction work was being successfully completed. This approach reduced the contractor's risk of failure to meet the design requirements for no increase in construction cost or schedule. QC for acceptance testing was provided by periodic test borings using conventional Standard Penetration Test (SPT) at four progress milestones.

The Consumers Energy (Consumers) owned and operated Croton Dam on the Muskegon River was constructed in 1906. Hydraulic fill placement methods used during construction created zones of loose saturated sand adjacent to the powerhouse and other structures. Analysis of the left earth embankment found potential for liquefaction during the design earthquake. Consumers retained Acres to determine the most economical means to rehabilitate the embankment. Acres selected ground modification as the best technical option. Based on the evaluation of competitive bids submitted by leading firms in the field, Denver Grouting was selected to conduct compaction grouting.

The contract's performance specification required the contractor to compaction grout the embankment to achieve a specified "N" value determined by the SPT with the means and methods to be selected by the contractor. This gave the contractor freedom to develop the most efficient grouting program while retaining responsibility for achieving the compaction criteria. Avoiding test failure and subsequent remedial grouting at split spaced centers was clearly in the contractor's best interest as well as the owners.

Based on the analysis of test panel grouting results, the GIN concept was applied to provide a real-time method to monitor the degree of compaction achieved for each 1 ft stage of each grouted hole. This method provided QC analogous to the slump test used for concrete placement (i.e. indicative and immediate rather than precise and after the fact). The conventional GIN approach was modified for compaction grouting as follows. The product of the peak injection pressure (psi) and the injected volume (cf/1f) divided by 10 times the hole separation distance was used to predicted the SPT "N" value taken at the center of the four surrounding holes. This approach treats each grouted hole a "test" hole. Testing for acceptance was based on the standard SPT "N" value measurements conducted by an independent testing firm.

The GIN concept was applied to 338 grouted holes totaling approximately 13,000 ft on an 8 ft grid. Ninety-four percent of the project area met the acceptance criteria on initial testing and the method correctly predicted that the other six percent would not meet the specified compaction criteria. The extent of the poorly compacted area was accurately predicted by the GIN method as well. The grouting program in several holes through very loose sand (N=WOR) was modified during the grouting process to achieve the required compaction thus avoiding the necessity for remedial grouting and re-testing. 12 pp., 11 figures, 2 references.