Subcontracted mixing: quality checks on someone else’s crew

The batch looked right. It had the colour, the consistency and the smell of a good batch, and the contractor’s supervisor signed the sheet. Two hours later the trailer was rejected at the gate for free liquid, and the site spent the afternoon explaining a failure that had happened on a pad they did not operate.

Subcontracting leachate mixing is normal on sites without permanent staff, and it works when the contract describes outputs rather than activities. Most contracts describe activities, which is why the site carries the consequence of someone else’s shortcut.

Write the output into the contract

The contract should state the acceptance standard, the sampling method, the record set and the consequence of failure. Not “polymer to be added at the agreed rate” but “each batch to pass a paint filter test before release, sample retained for 30 days, result recorded on the shared sheet, re-batch at contractor cost if the load is rejected”.

Add a stop-work clause. A contractor who cannot stop a failing batch without a phone call will keep going and hope. The authority to stop should sit with the person on the pad, not with a manager reachable in office hours.

Three checks you run yourself

First, weigh or count the polymer that goes into a batch. Contractors buy polymer on the site’s account or their own, and either way the mass per batch is the number that decides the cost. Watch one batch from bag to mixer without announcing it in advance.

Second, take your own sample from the discharge line, not from the mixer. A sample from the top of a mixed tank is not the same as the material that leaves it.

Third, compare the retained samples across a month. If every sample looks identical in a month when the leachate changed substantially, the sampling is not real.

Witness points and how to use them

Agree two or three points where the site must be present: dose calculation, first batch of the day, and any batch after a grade change. These are not inspections; they are short attendances that create a record. Fifteen minutes at the start of a shift prevents most of the arguments that follow.

Keep the attendance log separate from the contractor’s own records. Two independent records that agree are evidence. One record held by the party being paid is a claim.

Pay attention to handover. Most failures on a subcontracted pad happen in the twenty minutes between crews, when the tank is half full, the dose is half in, and both crews believe the other one finished it. A written handover line — volume, dose applied, dose outstanding — takes a minute and removes the ambiguity.

The diluted polymer

A site found its polymer consumption had risen 22 percent after moving to a contractor, with no change in leachate. The contractor was pre-mixing bags into an IBC at what they described as “working strength”, which turned out to be half the specified concentration, then dosing twice the volume. The arithmetic was roughly neutral on paper and consistently short in practice, because the diluted slurry settled in the lines between batches.

The site only found it by weighing the bags. The contractor’s records showed the correct kilogram figure because they counted the bags that left the store, not the polymer that reached the batch.

That is the pattern with subcontracting: the paperwork can be perfectly consistent and still describe a different operation from the one happening on the pad. Physical checks — bag counts, discharge samples, batch times — are the only ones that cannot be talked around.

If you are writing or renewing a mixing contract this year, attach the acceptance standard and the record set as schedules, and name the person with stop-work authority. The SAP powder data sheets give you the grade and lot information to specify, so there is no ambiguity about what was supposed to be in the bag.