Roctest’s Structural Health Monitoring (SHM) layer is a suite of sensors embedded in and attached to the concrete during construction that tracks stress, strain, settlement, inclination, and lateral movement from the first concrete pour through 40 years of operation. This layer is particularly important because it covers the construction period and hydro-test phase, when concrete is still curing and the structure is being loaded for the first time where early anomalies are most likely to appear.
Layer 1: Embedded Strain and Temperature Sensing
Strain is the microscopic elongation or compression of concrete under load and is the most fundamental structural measurement.
The Vibrating Wire Strain Gauges is Roctest’s primary embedded concrete sensor. Rather than measuring electrical resistance (which drifts with temperature and age), it measures the resonant frequency of a tensioned steel wire. This is a signal that travels over long cables without degradation and requires no recalibration. The gauge is deliberately compliant so it doesn’t disturb the concrete it’s embedded in, and can be installed by tying to rebar, casting directly into wet concrete, or pre-casting into a concrete briquette.
A companion “no-stress” gauge is an identical sensor placed in an isolated box of the same concrete mix which sits adjacent to each main gauge. It captures strain from aging, shrinkage, and temperature rather than structural load, allowing engineers to separate the two and extract true load-driven readings.
Layer 2: Foundation Slab Settlement Monitoring
A large LNG tank holds over 160,000 cubic meters of liquid which is an enormous load on the foundation slab. Repeated fill and drain cycles over decades can cause differential settlement: uneven sinking that, if left undetected, can tilt the inner wall and create the annular space movements the TMMS monitors. The SHM layer catches this earlier, at the slab level.
Roctest uses its PROFIL Digital Inclinometer Probe inserted into horizontal casings pre-installed beneath the slab during construction. The probe travels horizontally through the slab and maps the slab’s vertical profile from end to end. Changes between survey campaigns reveal where and how much the slab has settled.
Where continuous automated data is needed, the GEOSTRING In-Place MEMS Inclinometer is a string of closely spaced sensor nodes at 0.5 m intervals that sits permanently in the same casings and feeds live settlement data to the control room around the clock, with no manual surveys required.
Layer 3: Outer Wall Inclination — Tiltmeters
If the foundation settles unevenly, the outer concrete wall may begin to lean. Roctest monitors this with precision tiltmeters mounted at multiple heights and orientations around the wall perimeter, distinguishing between uniform sinking, a directional lean, and localized wall deformation.
The standard instrument is the Uniaxial Tiltmeter, a gravity-referenced electrolytic sensor offering long-term stability in a rugged weatherproof enclosure. Where intrinsic safety is required closer to the tank, the Mono-Axial Tiltmeter provides equivalent performance in a fully fiber optic design, using two FBGs in a push-pull pendulum configuration that inherently compensates for temperature without additional correction.
Layer 4: Outer Wall Lateral Movement — Pendulum Systems
Tiltmeters measure angle but not absolute position. To track whether the outer wall is shifting horizontally, or whether its top is moving relative to its base, Roctest deploys pendulum systems. A wire suspended from the top of the wall passes through coordinate plates at multiple levels. Gravity keeps it vertical, and the wire’s position at each plate gives the wall’s lateral displacement at that elevation. The result is a complete picture of the wall’s deflected shape from foundation to roof, not just a single tilt angle.
How the Layers Work Together
The real value of the SHM system is in how the layers cross-check each other. Consider a routine cooldown in a cold winter: the TMMS flags increased differential movement between the walls. Is this a structural problem, or just normal thermal response? The tiltmeters show whether the outer wall is also moving. The inclinometers clarify whether the foundation is settling asymmetrically. The strain gauges confirm whether concrete stress is within normal bounds. With all four layers, the operator has a complete, unambiguous picture. Without any one of them, both false alarms and missed alarms become more likely.
All sensors are installed during construction when concrete access is unrestricted and incremental sensor cost is minimal relative to the structural assurance they deliver over a 40-year tank life. Retrofitting equivalent monitoring into an operating tank would be impractical, making the construction-phase installation not just cost-effective, but irreplaceable.
