Pit Construction Safe Work Method Statement

WHS Regulations classify trenches and shafts exceeding 1.5 meters depth as high-risk construction work requiring specific control measures. However, excavations shallower than 1.5 meters still require protection if ground is unstable, water present, or other factors create collapse risk. The competent person assessing excavation must consider soil type, nearby loads, vibration, water, and other site-specific factors determining appropriate support requirements. Cohesive soils including clay may stand vertically for shallow excavations but require support at greater depths. Granular soils including sand typically require support from shallower depths. Common practice requires shoring for excavations exceeding 1.5 meters depth regardless of soil type unless excavation battered to safe angles. Safe batter angles vary with soil type typically 1:1 (45 degrees) for cohesive soils and flatter for granular soils. Space constraints on many construction sites prevent battering requiring shoring systems for most deep pit excavations. Australian Standard AS 2870 provides guidance on excavation support design considering soil properties and excavation geometry. The key principle is preventing ground collapse regardless of excavation depth requiring competent person assessment of all excavations before worker entry.

Confined spaces under AS 2865 require atmospheric testing before entry measuring oxygen content (acceptable range 19.5-23.5%), combustible gases (acceptable below 5% lower explosive limit), hydrogen sulfide (acceptable below 10 ppm for 8-hour exposure), and carbon monoxide (acceptable below 30 ppm for 8-hour exposure). Testing must be conducted using calibrated multi-gas detector with current calibration certificate typically due every 6 months. Lower detector into pit allowing minimum 2 minutes for atmosphere sampling at multiple levels as heavier gases accumulate at bottom while lighter gases accumulate at top. Record test results on confined space entry permit documenting oxygen percentage, combustible gas percentage, hydrogen sulfide concentration, carbon monoxide concentration, time of test, and tester name. If any parameter outside acceptable limits, provide forced ventilation and retest after ventilation period until acceptable atmosphere achieved. Continue atmospheric testing every 2 hours during occupancy or more frequently if work activities may affect atmosphere including welding, spray painting, or connecting to potentially contaminated services. Provide continuous atmospheric monitoring using personal gas detector worn by entrant providing immediate alarm if atmosphere deteriorates during work. Some jurisdictions or worksites require more stringent testing including additional contaminants or more frequent testing intervals based on site-specific hazard assessment.

Confined space entry requires standby person maintaining constant contact with entrants capable of initiating rescue without entering space per AS 2865. Standby person must be positioned where they can see or hear entrant at all times establishing communication protocol typically verbal communication for shallow pits or radio communication for deeper pits where direct conversation difficult. Standby person must remain at pit opening throughout entry period not conducting other work that would distract from monitoring duties. They must be trained in confined space rescue procedures, emergency notification, and use of retrieval equipment but must not enter space to conduct rescue as majority of confined space fatalities are would-be rescuers. Standby person maintains entry permit, atmospheric test results, and emergency contact information immediately accessible. They monitor entrant condition watching for signs of distress, impairment, or emergency situation. If entrant becomes unresponsive, standby person must initiate retrieval using mechanical retrieval system, activate emergency response calling triple zero (000), and prevent others entering space to rescue. One standby person can monitor up to two entrants if both are visible simultaneously but large or complex spaces may require multiple standby personnel. Standby person must understand atmospheric hazards, health effects, and recognition of impaired entrants enabling early intervention before serious emergency develops.

Groundwater management depends on site conditions, pit depth, and soil permeability. For pits excavated above water table, groundwater management may not be required. For pits penetrating water table, dewatering prevents water accumulation maintaining dry working conditions. Assessment of groundwater level requires test pit excavation or soil boring logs identifying water table depth and seasonal variations. If groundwater above excavation base level, install dewatering system before excavation commences. Simple dewatering uses submersible pumps in sump locations collecting seepage water for removal typically adequate for low to moderate groundwater inflow in fine-grained soils. Extensive dewatering requires wellpoint systems or deep well systems lowering water table before excavation eliminating inflow at source. Dewatering discharge requires EPA approval in some jurisdictions preventing environmental contamination. Discharge to sewer may require trade waste approval preventing sediment damage to sewer systems. Site discharge requires sediment control preventing turbid water entering waterways. Monitor groundwater levels throughout construction responding to increased inflow with additional pumping capacity. Water table rebound after heavy rain may require increased dewatering. Maintain dewatering continuously including overnight and weekends preventing water accumulation during unattended periods. For temporary pits in high groundwater, consider alternative construction methods including sheet piling creating water-tight excavation support, underwater concrete placement eliminating dry working requirement, or precast pit installation minimizing excavation time and exposure. Document dewatering quantities and discharge locations for environmental compliance reporting.

Confined space entry requires rescue equipment enabling extraction of incapacitated workers without rescue personnel entering space. Full-body harness with dorsal D-ring worn by all entrants provides connection point for retrieval system. Retrieval system comprises mechanical advantage system typically tripod with winch, rescue davit, or block and tackle system positioned over pit opening enabling vertical lifting. Retrieval line connects harness to mechanical system with adequate length reaching pit bottom with excess for surface handling. System must be rated for person weight typically minimum 150kg with safety factor. Test retrieval system before entry conducting test lift verifying smooth operation and adequate capacity. Some sites require self-retracting lifeline providing automatic tension and immediate locking if fall occurs. For particularly hazardous atmospheres, consider supplied-air respiratory equipment providing emergency egress air supply if atmosphere suddenly deteriorates. Communication equipment enables entrant to request assistance including two-way radio or voice communication. Emergency contact information must be immediately accessible including triple zero (000), site emergency contacts, and nearest hospital details. First aid equipment including resuscitation equipment must be available at site. Advanced rescue scenarios may require emergency services notification before entry ensuring rapid response if serious emergency occurs. Some high-risk sites require dedicated rescue team on standby throughout confined space entry operations particularly when multiple entrants or particularly hazardous conditions present.

Pit construction near existing services requires careful coordination preventing service damage and maintaining safe working conditions. Submit Dial Before You Dig enquiry providing accurate work location and obtaining service location plans from utility asset owners. Contact service owners directly when working near critical services including high-voltage electrical, high-pressure gas, or major water mains discussing work scope and obtaining approval. Some services require owner attendance during excavation providing technical advice and emergency response capability if damage occurs. Engage qualified service locator conducting non-invasive location verifying actual service positions and depths before excavation. Pothole verify services at key locations providing physical confirmation before mechanical excavation. Hand excavate within 500mm of services eliminating mechanical excavation strike risks. Support exposed services preventing damage from undermining or excessive spanning using timber supports, sandbag supports, or purpose-built service supports. Barricade exposed services preventing construction traffic contact. Maintain service access throughout construction coordinating with service owners regarding any required service shutdown or isolation. Some services have specific clearance requirements exceeding general 500mm clearances including high-voltage electrical requiring minimum 600mm clearance. Document service positions on as-built drawings showing relationship to completed pit enabling future maintenance without service strike risks. Notify service owners after completion particularly if service positions differ from plans or if any concerns identified regarding service condition. Construction bonds or insurance requirements may apply when working near critical infrastructure protecting service owners against damage claims.