Tower Leakage Review

Tower Leakage Review

Review of the repetitive leakage report and the internal investigation: does the recurring leak at the tower change the groundwater and foundation safety of the twin-deck bridge, and what must the investigation establish?
100 m single span2 decks, box girder4 corner towersutilities inside box
Evidence

Leak event log from the report

Enter every recurrence recorded in the leakage report. The log totals the water released and can set the model inputs from it. Example rows are shown until you replace them; the log is saved only in this browser.
Internal investigation

What the investigation must establish

  • Where the water goes — which tower, and whether the discharge reaches the ground (at the base, through the cap, along the pile) or is caught by drainage. Only water reaching the ground at a tower is modelled here.
  • Rate, duration, dates — metered loss or pressure-drop estimate for every recurrence. The repair-to-recurrence interval decides whether the ground drains between events (compare it with the response time shown above).
  • Source — potable, fire main, sewer, irrigation or deck drainage. Sewer adds biogenic sulfuric-acid attack; pressurised mains add jet erosion of fines under the cap.
  • Root cause — joint failure, thermal movement at the expansion joint, corrosion from the groundwater chemistry, or a pipe already cracked by tower movement. A pipe broken by movement is itself evidence that the foundation is moving.
  • Field evidence to request — piezometer readings at all four towers before and after events, precise levelling of the cap corners, crack mapping on towers and joints, GPR or probing for voids under the cap, and whether sediment was seen in the leaked water (internal erosion).
  • Decision rule — the leak is a foundation issue when the local water rise exceeds the critical level, when cap corners differ by more than the differential limit, when collapse settlement is added, or when the leak source sits in its own groundwater mound (self-reinforcing).
Depth log

Safety margin vs groundwater depth

Each line is FS ÷ required FS for one mechanism (margin ratio). Left of the dashed line the selected code is not met; left of the dotted line the mechanism fails outright (FS = 1). Horizontal markers: the site water table, the leaking tower's local level, and the design level. Depth increases downward, as on a borehole log.
Failure mechanisms at the leaking tower
Benchmarks

Required factors of safety by code

Eurocode 7 and AASHTO LRFD use partial / resistance factors; the values below are the approximate global-FS equivalents used by this screening model, not code text. The selected column drives every pass / fail call on this page.
Method

How each number is computed

  • Leak mound — the leak is treated as recharge over its infiltration footprint (radius a = √(A/π)) into an unconfined aquifer of saturated thickness H. Steady-state Dupuit mound: hmax² = H² + (w·a²/k)·ln(R/a) + w·a²/(2k). A finite event of duration t reaches the fraction 1 − e−t/τ of that mound, with the aquifer response time τ = Sy·R²/(4·k·H); it is also capped by the pore space available (V / (Sy·Aeff)). Repeated events separated by an interval Δt accumulate by the factor 1 / (1 − e−Δt/τ). The larger of the repetitive peak and the long-term average mound is the local level used for the leaking tower.
  • Tower reactions — deck self-weight per m² by girder type (concrete box ≈ 16 kN/m², composite ≈ 9 kN/m²), superimposed dead load, internal utilities per m of deck, HL-93-equivalent live load, plus tower self-weight; shared equally by the four corner towers. Replace with the structural model's reactions when available.
  • Effective stress — σ′ = γz above the water table, γ·dw + (γsat − γw)(z − dw) below it; ponded water above ground adds equal total and pore pressure.
  • Pile group — bored piles; FHWA β-method shaft friction (β = 1.5 − 0.245√z, 0.25–1.2, reduced for N60 < 15) capped at 200 kPa; tip qp = 0.5·Nq·σ′v,tip ≤ 2.9 MPa; clay α = 0.55, qp = 9su. Downdrag from a collapsible layer wetted by the leak is added to the demand.
  • Bearing capacity — Vesic N-factors with Meyerhof shape and depth factors; water correction cases I–III. FS on net pressure.
  • Uplift (UPL) — buoyant force on the cap base vs its weight plus 70 % of pile shaft friction in tension when tied; without (construction) and with (service) the superstructure dead load.
  • Piping / heave (HYD) — Terzaghi exit gradient i = h / (h + 2·De) for a cut-off of embedment De; icrit = γ′/γw.
  • Liquefaction — simplified procedure (Seed & Idriss, NCEER / Youd 2001) with fines correction and MSF = 102.24/M2.56; minimum FS over 1–20 m below the water table.
  • Settlement — equivalent footing at ⅔ of the pile length (or the footing base), 2:1 stress spread, Janbu modulus Es = m·pa·(σ′/pa)0.5 with m = 10·N60, so a local rise in water softens the ground under that tower only. Collapse settlement = Ic × newly wetted thickness of the collapsible layer. Differential = leaking tower minus the others, against a 25 mm serviceability limit (AASHTO angular distortion 0.004 rad is far looser; a torsion-sensitive box girder on four corner supports needs the tighter value).
  • Dewatering — Dupuit–Thiem flow to an equivalent well of radius re = √(A/π) with Sichardt R = re + 3000·s·√k; computed at the site level and at the leaking tower's local level.
  • Chemistry — sulfate class S0–S3 per ACI 318-19 / SBC 304 Table 19.3.1.1, EN 206 XA class for comparison; chloride class with cover and w/c practice for Gulf conditions. The leak source adjusts the note, not the class.
What this is and is not. A screening and decision-support model for the leakage review, built on published Saudi and international benchmark parameters. It shows whether the reported leak is large enough and frequent enough to matter for the foundations and what evidence the investigation needs. It is not a design: final conclusions need the measured piezometer response at the towers, the structural reactions, a site-specific geotechnical investigation, and seepage / soil–structure analysis (e.g. PLAXIS, SEEP/W) reviewed by a licensed geotechnical engineer.
Sources for the benchmark values