Stacking Solidified Cake: Slope, Lift Height and the 30-Day Settlement

A site built its first stockpile of solidified cake to 6 m in three weeks, at a face slope close to 1:1.2, because that was the angle the loader could climb. Thirty days later the crest had dropped 240 mm, a tension crack 1.8 m deep had opened 12 m back from the face, and leachate was seeping from the toe into the drainage channel. The material had passed every acceptance test at the gate. It simply had not been designed as a structure.

Solidified cake is a weak, saturated, consolidating material, and stacking it like inert fill is the reason many sites develop seepage and slope problems within a season. This article covers what cake does under its own weight, how to set lift height and compaction, the slope and drainage that keep a face stable, and what to monitor for the first thirty days.

What Cake Does Under Its Own Weight

A gel at 60 to 70 percent moisture is close to saturated, and the polymer network holds much of that water under tension. When the material is stacked, the weight of the upper layers squeezes water out of the lower ones. This is consolidation, and it is unavoidable. Typical settlement in the first 30 days runs 3 to 6 percent of height, which on a 6 m pile is 180 to 360 mm.

The water that leaves has to go somewhere. In a well-built pile it drains to the toe and into a collection channel. In a poorly built one it pools inside the mass, raises pore pressure, and reduces the effective stress that gives the material its strength. That is the mechanism behind most stockpile slope failures: not a wet batch, but a pile that could not drain.

There is a second effect that matters on longer storage. The polymer network continues to rearrange under load, and after 60 to 90 days the material can lose 20 to 30 percent of its water-holding capacity, releasing it as free liquid. A pile that was compliant when built can produce a seep at the toe two months later without any change in the material that went in.

Lift Height and Compaction

Build in 300 mm lifts, not in 2 m pushes. A lift of 300 mm can be compacted by two passes of a tracked loader, which removes the large voids and gives the material enough shear strength to support the next layer. A 2 m push leaves a loose, heterogeneous mass with voids that later collapse unevenly.

Six metres is a sensible maximum for unlined temporary storage, and 4 m is safer where the pile sits on a clay cap rather than a lined area. Beyond 6 m the toe stress exceeds what most solidified cake can carry at 65 percent moisture, and the face needs engineering rather than a loader operator’s judgement.

Do not compact with a vibratory roller. Vibration liquefies a saturated gel locally and produces a weak layer that is invisible until the face slumps. Tracked plant at walking pace is the right tool.

Slope and Drainage That Keep a Face Stable

Build the face at 1:2.5, not 1:1.2. A 6 m pile at 1:2.5 occupies a much larger footprint, which is the real constraint on most sites, but it is the difference between a face that stands for a year and one that cracks in a month. Where space is short, reduce height rather than steepen the slope.

Shape the top with a 3 percent fall to the rear and cut a toe drain along the downslope edge, lined and connected to the leachate collection system rather than to surface water. Seep from a cake stockpile is leachate, and it belongs back in the sump.

Cover the pile if it will stand more than a month. A 1.5 mm HDPE cover or even a weighted geotextile reduces rainfall ingress, which is the largest single source of additional water in a stockpile, and it slows the consolidation seep. Sites that cover report toe seepage dropping by 60 to 80 percent within a fortnight.

Monitoring for the First 30 Days

Install four settlement pins at the crest on day one, surveyed weekly. A settlement rate above 2 percent per week after week two indicates the pile is still consolidating under load and should not be raised further. Map cracks weekly with a tape and a photograph, and treat any crack deeper than 300 mm as a signal to stop loading and investigate the drainage.

Check the toe daily for the first fortnight. A damp patch is normal for the first week and should decline. A flowing seep, or one that appears after week two, means the internal drainage is not working and the pile needs to be pulled back and rebuilt in thinner lifts.

Record all of it on one sheet per pile with the build dates, lift depths, moisture at placement and settlement readings. That sheet is what turns a stockpile from an accumulating liability into a managed structure, and it is what an inspector will ask for if a seep reaches a drain. The SAP grades we supply for leachate work are specified with a target moisture at placement, which is the input that most strongly predicts how much a pile will settle.