MOORE & TABER                     

 

 

Soil Grouting and Ground Improvement Specialists

 

         

The need for an accurate method of verifying emplaced quantities of compaction grout, also known as limited-mobility displacement (LMD) grout, has become evident through years of working with clients and engineers who have come to expect a high degree of quality control. Contract values are often a function of injected grout quantity and installed casing length, and this type of grout is commonly batched at the project site.

Clients have classically relied solely on the grouting contractor, rather than their consultant, to accurately report production grout quantities. Little more information is typically disclosed than that provided by the pump stroke counter. What most clients don’t realize is that typical industry-standard grout pumps seldom (if ever) operate at 100 percent efficiency. Consequently, the need for a fair and accurate method to verify LMD grout quantities is clear.

If pump stroke counters are to be used effectively, the tally must be reconciled with all constituents used in a grout batch. LMD grout is commonly batched at or near the point of injection. The following procedure has been used to successfully reconcile reported production grout quantities that were based on stroke count estimates, with the actual quantities of all grout constituents. It does not rely on cumbersome or expensive equipment, but is based on a simple material balance.

Verification involves maintaining complete records of all material delivered to the jobsite or otherwise used as a grout constituent. These records include all delivery tickets for cement, citing either sack count or certified weights for bulk deliveries; delivery tickets for all soil or aggregate, including certified weights; and similar records for any admixtures. The contractor then keeps track of the number of pump strokes required to complete each grout stage.

The grout’s unit weight is determined periodically by sampling into a 1.0-cubic-foot box. Determinations can be made periodically throughout a project’s duration, and can be averaged where appropriate. The contractor must keep a daily balance sheet of all delivered quantities, daily water meter readings (for water used as a constituent in the grout), and a daily estimate of grout pumped.

A simple balance calculation is then used to reconcile recorded material weight data with contractor’s production estimates. Discrepancies in grout balance allow the contractor to refine its production estimates, resulting in accurate reporting of the grouting process.  A simplified method of this procedure may be implemented if the engineer has determined the quantity of dry bulk-delivered aggregate (sand) required to batch one cubic foot of compaction grout material.  This method may be appropriate in instances where drill water is recycled, and water meter readings are difficult to track.  In this case, periodic moisture content determinations of bulk-delivered sand would be needed to determine the dry delivery weight.

Verification of installed casing length is accomplished by lowering a weighted tape through the casing, subtracting the portion above the ground’s surface. Casing length should be verified at each and every injection point.

As with any verification method, the margin for error will depend on the upkeep and condition of the contractor’s equipment, the observational dexterity of the foreman or inspector in charge, and the ethics with which the verification task is approached. Typically, total discrepancy at a project’s conclusion is on the order of 5 to 10 percent.