Standards reference

Horak (2008): FWD Deflection Bowl Parameters (D0, BLI, MLI, LLI)

Deflection bowl parameters localise pavement structural condition by differencing Falling Weight Deflectometer readings at set sensor offsets — BLI = D0 − D300 (base), MLI = D300 − D600 (middle), and LLI = D600 − D900 (lower layers), all expressed in micrometres (µm) (Horak, 2008).

Last reviewed: July 2026 · Verified against Horak (2008) benchmark methodology
Table 1. Deflection bowl parameters and the layer each reflects
ParameterDefinitionLayer reflected
D0Peak deflection under the load plateWhole pavement structure
BLID0 − D300Base / upper structural layers
MLID300 − D600Middle layers / subbase
LLID600 − D900Lower layers / subgrade
BLI = D0 − D300   ·   MLI = D300 − D600   ·   LLI = D600 − D900
where Dx = FWD surface deflection (µm) at x mm offset from the load centre, normalised to a standard 40 kN / 566 kPa drop.

Interactive: drag the four geophone deflections (D0, D300, D600, D900) or pick a preset to reshape the Falling-Weight-Deflectometer bowl and watch the Horak (2008) indices — BLI (base/upper), MLI (middle), LLI (lower/subgrade) — and the resulting layer diagnosis update live, showing how the bowl's shape, not just its depth, reveals which pavement layer is weak.

Figure 1. The Falling-Weight-Deflectometer deflection bowl and its Horak (2008) curvature indices. The Base Layer Index (BLI = D0 − D300), Middle Layer Index (MLI = D300 − D600) and Lower Layer Index (LLI = D600 − D900) partition the bowl into successively deeper structural responses: a steep near-load drop (high BLI) localises distress in the base and upper layers, whereas a broad, shallow decay with elevated far-offset deflection (high LLI) signals deep-seated subgrade weakness. Two bowls of comparable peak deflection D0 can therefore carry opposite diagnoses. Deflection bands are indicative and vary by pavement family.
Table 2. Indicative severity bands for bowl parameters
BandRange (µm)
Sound< 500
Warning500 – 750
Severe> 750

Exact thresholds differ per pavement family (granular, cementitious, bituminous) and per parameter — Horak (2008) tabulates each combination separately; the bands above are indicative screening values, not a substitute for the pavement-family-specific tables.

Worked example — reading one FWD test

  1. Normalised deflections: D0 = 620 µm, D300 = 380 µm, D600 = 210 µm, D900 = 120 µm.
  2. BLI = 620 − 380 = 240 µm
  3. MLI = 380 − 210 = 170 µm
  4. LLI = 210 − 120 = 90 µm
  5. D0 = 620 µm falls in the indicative Warning band (500 – 750 µm).
D0 in Warning with BLI the dominant index → structural weakness is concentrated in the base layers; targeted base rehabilitation outranks full-depth reconstruction.

How RoadSense processes FWD data

RoadSense ingests standard FWD survey output (.erd / .fwd files) and automatically computes D0, BLI, MLI and LLI for every test point along the surveyed route, applying the Horak (2008) deflection bowl parameter method without manual spreadsheet work. Results are banded per pavement family, plotted on a georeferenced map alongside other condition indicators, and exported in TMH 18 FWD exchange format for pavement management and rehabilitation-design workflows.

What is a deflection bowl parameter?

A deflection bowl parameter is a value derived from the shape of the FWD deflection curve — such as D0, BLI, MLI or LLI — that isolates structural condition information for a specific depth band of the pavement, rather than relying on a single peak deflection reading to describe the whole structure.

What does BLI indicate?

BLI, calculated as D0 minus the deflection at 300 mm offset, indicates the structural condition of the base and upper pavement layers; a high BLI relative to MLI and LLI points to distress concentrated near the surface rather than deep in the pavement structure.

What FWD deflection is considered severe?

Deflection bowl parameter values above roughly 750 micrometres (µm) are generally considered Severe and indicative of significant structural distress requiring rehabilitation, while values below roughly 500 µm are considered Sound; exact thresholds vary by pavement family (granular, cementitious, or bituminous) and by parameter, per Horak (2008).

References

  • Horak, E. (2008) 'Benchmarking the structural condition of flexible pavements with deflection bowl parameters', Journal of the South African Institution of Civil Engineering, 50(2), pp. 2–9.
  • Horak, E. and Emery, S.J. (2006) 'Falling weight deflectometer bowl parameters as analysis tool for pavement structural evaluations', Proceedings of the 22nd ARRB Conference, Canberra, Australia.
  • Committee of Transport Officials (COTO) (n.d.) TMH 18: Network-level road condition data specification (draft). Pretoria: Department of Transport.
  • South African National Roads Agency (SANRAL) (2014) South African Pavement Engineering Manual. 2nd edn. Pretoria: SANRAL.

For the full set of survey standards RoadSense reports against, see the standards hub, or contact us to discuss an FWD deflection survey for your network.