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Chapter 9 — Field Inspection & Performance Monitoring

9.1 Preconstruction reviews

Before construction, review the plans, specifications, and submittals against the design intent: the reinforcement type and layout, the fill specification, the facing details, and the drainage system. Confirm that any supplier design has been approved and that the specified fill is actually available from the project borrow — a very common source of trouble is a specified fill that no local source can meet.

9.2 Prefabricated materials inspection

Inspect the reinforcement (type, strength, dimensions, coating, and freedom from damage), the facing units (panels or blocks, dimensions, and reinforcement connection points), the connectors, and the drainage and filter materials against the specification. Damaged or out-of-specification materials are rejected before they are built into the structure, where they cannot be seen again.

9.3 Construction control

The manual sets out the control points for each construction stage:

  • Foundation preparation — excavate to the design level, proof-roll, and verify the foundation soil and the levelling pad.
  • Facing erection — control the verticality, horizontal alignment, and joint widths of the facing; these are the tolerances most visible in the finished structure and the ones most often missed.
  • Reinforcement placement — verify the length, orientation, elevation, and spacing of every layer, that it is not damaged, that it is placed on compacted fill (not on loose fill or a slope), and that the connection to the facing is made correctly.
  • Backfill placement and compaction — place fill in lifts and compact it to the specified density. Within about 1 m of the face, use only lightweight compaction equipment so that the facing and the reinforcement connection are not disturbed (Figure 84); larger equipment is permitted farther from the face.
  • Tolerances — the manual gives out-of-tolerance conditions and their likely causes (Table 16), so the inspector can diagnose a problem rather than simply record it. Table 15 is the field inspection checklist.

9.4 Performance monitoring programs

Purpose of a monitoring program. Monitoring confirms that the structure behaves as the design predicted — that the facing deformation, reinforcement tension, and settlement stay within the assumed values — and gives early warning if they do not. It applies the Observational Method: predictions are made, measurements are compared with them, and pre-planned actions follow.

Limited monitoring program. For routine structures: visual inspection of the facing for cracking, bulging, or joint opening, plus survey of facing movement and settlement of the wall crest. Cheap, and enough for a well-understood wall on a competent foundation.

Comprehensive monitoring program. For tall walls, abutments, walls on soft foundations, or critical structures, the program adds instrumentation:

  • Inclinometers — lateral movement of the facing and the ground behind it.
  • Settlement platforms and survey points — vertical movement of the foundation and the reinforced mass.
  • Strain gauges or load cells on the reinforcement — the tension actually mobilized in selected layers, to compare with \(T_{max}\).
  • Pressure cells — contact stress at the base and behind the facing.
  • Piezometers — pore pressures in the reinforced fill and the foundation, which reveal whether drainage is working.

Table 17 lists the possible instruments and what each one measures.

Program implementation. Define the instrument locations, the reading frequencies (frequent during construction, reducing afterwards), the trigger values, and the actions to be taken if a trigger is exceeded — all before construction begins.

Data interpretation. Plot each measurement against time and against the construction stage, and compare it with the design prediction. A single reading means little; the trend and the rate are what matter — the same principle that governs dam and excavation monitoring elsewhere in this knowledge base.

9.5 Key takeaways

  • Inspect materials before they are buried — reinforcement and drainage cannot be inspected later.
  • The tolerances that matter most are facing verticality, alignment, and joint width, and reinforcement length and elevation.
  • Compact lightly within 1 m of the face; heavier equipment farther away.
  • Choose the monitoring program by risk: limited (visual + survey) for routine walls, comprehensive (instrumented) for tall, soft-ground, or critical structures.
  • Instrument the reinforcement itself if you want to know the actual mobilized tension — and set trigger values and actions in advance.

9.6 Where this fits

This chapter is the manual's bridge to the site's instrumentation core. For the instrument families and their working principles, see the FHWA Geotechnical Instrumentation Reference Manual and Dunnicliff; for how the readings are turned into decisions, see Monitoring Dam Performance and the ADAQS automated monitoring framework.