Theory and Method
Measuring one layer inside a bonded composite runs into a basic obstacle. A needle-punched or thermally bonded product holds its layers together by design, and any attempt to separate them stretches fibers, opens the structure, and gives a reading that no longer describes the material as manufactured. The method therefore leaves the composite intact and creates the measuring condition around it instead, compressing the product to the pressure at which thickness is defined and holding it there while the layer boundaries are examined.
Stacking five specimens serves a purpose beyond convenience. A single layer within a geocomposite may be well under a millimeter thick, which sits close to the noise floor of a mechanical measurement. Repeating the layer five times through the stack multiplies the dimension being measured, so the same instrument resolution translates into a far finer effective resolution per layer. The four intermediate plates keep adjacent specimens from nesting into one another, since a compressed nonwoven surface will otherwise interlock with the specimen above it and shorten the stack.
Plate behavior governs whether any of this holds together. The lower plate stays fixed while the upper plate travels down, and both must remain flat under load, because a plate that bows by more than 0.1 mm across the specimen area introduces an error comparable to the layer thickness itself. The upper plate's own mass is held low enough that its weight alone cannot push the specimen past the specified pressure before the test begins. Locking the upper plate at position preserves the compressed geometry, so the measurement reflects the layer under load rather than the layer as it rebounds.
