Ground Works
Excavation Monitoring Trigger Levels (Alert–Action–Alarm)
Build a defensible Alert–Action–Alarm trigger register for inclinometers and piezometers on deep excavations — formula rules, stage-specific alarm values, reading validity and the PTW response at each level.
v2.1Updated 9 Aug 2026ADOSH-SF v4.1 (February 2026)Learning Objectives
- Explain why ADOSH-SF CoP 29.0 is risk-based and does not prescribe universal numerical movement or piezometric limits.
- Derive preliminary Alert (50%) and Action (80%) values from a designer-approved Alarm value.
- Apply the correct formula for internal head, external drawdown and rate-of-change parameters.
- State the monitoring and PTW response required at Normal, Alert, Action and Alarm.
- Specify baseline, datum, frequency and reviewer requirements that make a reading valid.
- Identify abnormal events that require immediate unscheduled readings.
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1. Scope and Critical Limitation
A trigger framework is not a substitute for the geotechnical, temporary-works and dewatering designs. ADOSH-SF CoP 29.0 requires the employer to consider ground and groundwater conditions, support systems, competent inspection and dewatering controls — it does not prescribe universal numbers. Never set one generic '25 mm' or '0.5 m' threshold across a project without design approval. The controlling Alarm value is always the most onerous applicable design, adjacent-asset or authority limit.
2. The Formula Rule
Set a separate Alarm value for each instrument AND each construction stage, then derive: cumulative movement Alert = 0.50 × M_AL, Action = 0.80 × M_AL. Movement rate Alert = 0.50 × R_AL, Action = 0.80 × R_AL. Internal piezometric head: Alert = H_T + 0.50(H_AL − H_T), Action = H_T + 0.80(H_AL − H_T), where H_T is the design target head. External drawdown: Alert = 0.50 × D_AL, Action = 0.80 × D_AL. External rise: Alert = H_L + 0.50(H_U − H_L). Rate of pressure change uses 0.50 and 0.80 of the designer's maximum daily rate. The designer may impose more conservative values.
3. Worked Example (Illustration Only)
Inclinometer I-04 with a designer-approved Alarm of 25 mm gives Alert 12.5 mm and Action 20 mm; a 2.0 mm/day rate alarm gives Alert 1.0 and Action 1.6 mm/day. Piezometer P-02 with target 10.0 mRL and alarm 11.0 mRL gives Alert 10.5 and Action 10.8 mRL. Piezometer P-07 with 0.50 m external drawdown alarm gives Alert 0.25 m and Action 0.40 m. Where a limit is expressed as a percentage of exposed height (e.g. 0.20% of 12 m = 24 mm), the temporary-works designer must convert it to an instrument-specific value. These are examples, not acceptance criteria.
4. Trigger Response Matrix
Normal/Green: continue at approved frequency, confirm instruments functional and data plausible; PTW may remain live. Alert: notify excavation and dewatering supervisors, the Temporary Works Coordinator and monitoring lead, re-read and validate instrument and benchmark, inspect shoring, ground, water level, surcharge and pumps, increase frequency; work continues only if the competent engineer confirms performance is within design assumptions. Action: stop affected excavation, piling, lifting and surcharge, exclude the zone, notify designers, implement the approved contingency; suspend the affected work under the PTW and restart only after documented review and PTW revalidation. Alarm/Emergency: make safe, evacuate where instability, uncontrolled inflow, piping or rapid movement is credible, activate the emergency plan, suspend all relevant PTWs pending full reassessment and new permits.
5. What Makes a Reading Valid
The monitoring plan must record baseline date, stable datum, instrument location and depth, construction stage, Alarm input, derived Alert/Action levels, rate limit, frequency, named reviewer and response authority. A reading is not valid simply because it appears in a spreadsheet. The monitoring lead must investigate unexplained movement, datum instability, damaged casing, sensor drift, loss of telemetry, or any reading inconsistent with adjacent instruments and the visual condition of the works.
6. Reading Frequency by Stage
Before sheet piling, dewatering and excavation: obtain stable baselines, verify survey datum, record groundwater condition. During sheet-pile driving or extraction: read after relevant driving stages and correlate movement with hammer/press-in operation and vibration. During each excavation or support stage: read before and after each lift, strut/anchor installation or preload, and after any surcharge or sequence change. During active dewatering: review internal and external pore-pressure trends including pump commissioning, changeover and any flow anomaly. After abnormal events: immediate readings after heavy rain, flooding, pump or generator failure, service strike, visible ground distress, vibration event or unexpected ground condition.
7. Roles and Controlled Records
The geotechnical/temporary-works designer approves instrument locations, baselines, stage-specific Alarm values, rate limits and contingency measures. The dewatering designer confirms pump sizing, duty/standby resilience, allowable drawdown and water-level triggers. The monitoring lead validates readings, identifies trends and escalates at each AAA level. Controlled records: trigger register, baseline and trend plots, calibration and test records, daily inspection log, PTW, water-level and flow logs, shift-handover log and corrective-action close-out.
Knowledge Assessment
1. Under ADOSH-SF CoP 29.0, numerical movement and piezometric limits are:
2. In the AAA framework, the Alert level is normally:
3. The Action level is normally:
4. For internal piezometric head with target H_T and alarm H_AL, the Action value is:
5. A designer-approved Alarm of 2.0 mm/day gives an Alert rate of:
6. At Action level, the correct PTW status is:
7. A movement limit of 0.20% of a 12 m exposed excavation height equals:
8. Which is NOT a valid reason to trigger an unscheduled reading?
9. The controlling Alarm value should be:
10. An unexplained reading inconsistent with adjacent instruments should be:
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