Performing civil temporary works is a critical skill for civil engineering professionals in Kenya, particularly when executing excavation and trenching operations. Temporary works such as trench timbering ensure the safety of workers and the stability of adjacent structures during construction. Proper design, installation, and monitoring of these systems are essential to prevent accidents and maintain project timelines. This chapter focuses on trench timbering, a common method used to support trench walls during excavation for utilities, foundations, and pipelines.

2.1 Trench Timbering

Trench timbering involves the use of timber supports to shore up the sides of a trench to prevent collapse. In Kenya, where soil types vary from sandy to clayey and construction sites may be close to roads or buildings, careful assessment and execution of timbering are vital to safeguard workers and property. This section covers the entire process from site assessment to safe dismantling of timber supports.

A detailed cross-section of a timber trench shoring system (Source: Chudley)

2.1.1 Site Assessment

Site assessment is the foundation of safe trench timbering. It involves understanding the physical and environmental conditions that affect trench stability and worker safety. In Nairobi, for example, urban excavation often encounters mixed soils and underground utilities, requiring thorough evaluation before timbering design.

Soil Type and Condition

Identifying the soil type is crucial because it influences the trench wall’s stability and the timbering method required. Clay soils tend to hold together better than sandy or loose soils, which are prone to collapse. For instance, at a county hospital expansion site in Kisumu, engineers noted that sandy soils necessitated more robust timbering compared to the adjacent clay layers.

Groundwater Presence

Groundwater can weaken trench walls by saturating soil and increasing hydrostatic pressure. Assessing groundwater levels helps determine if dewatering or additional support is necessary. At a Nairobi Water Company pipeline installation, high water tables required continuous pumping and enhanced timbering to maintain trench integrity.

Proximity to Structures and Utilities

Trenches near buildings, roads, or underground utilities require extra caution to avoid structural damage or service disruption. Before timbering, engineers at a county government office project conducted utility surveys to map electrical and water lines, ensuring timber supports did not interfere with existing infrastructure.

Trench Depth and Dimensions

The depth and width of the trench directly affect the timbering design. Deeper trenches present greater risks and need stronger supports. For a hotel foundation in Mombasa, engineers designed timbering to accommodate a trench depth exceeding 3 meters, complying with safety regulations.

A detailed diagram of timber shoring for trench support (Source: gbc mod I-III (1))

2.1.2 Selection of Timbering Materials

Choosing the right timber materials influences the effectiveness and safety of trench timbering. Kenyan construction sites often balance material availability, cost, and durability when selecting timber.

Types of Timber Used

Hardwoods such as eucalyptus and mahogany are preferred for their strength and resistance to decay. Softwoods may be used temporarily but require treatment or replacement during prolonged projects. For a retail complex excavation in Nakuru, treated eucalyptus was selected to withstand humid conditions.

Timber Dimensions and Strength

The size and thickness of timber must match the trench load requirements. Larger cross-sectional dimensions provide greater support but increase costs. Engineers at a university campus construction specified timber planks of at least 50 mm thickness and struts sized to withstand anticipated soil pressures.

Treatment and Preservation

Timber exposed to soil moisture is vulnerable to rot and insect attack. Chemical treatments extend service life and maintain structural integrity. In a SACCO office trench excavation, chemically treated timber was mandated to prevent premature failure.

Alternative Materials

Where timber scarcity exists, steel or aluminum trench boxes may be used. However, timber remains cost-effective and widely accessible in many Kenyan regions, particularly rural sites. A farm irrigation project opted for timber due to budget constraints and local availability.

2.1.3 Construction of Trench Timbering

Constructing trench timbering requires precise assembly to ensure trench stability and worker safety. Proper sequencing and secure fitting of components are essential.

A typical timbering support system for loose or wet subsoil in constru (Source: Chudley)

Excavation Preparation

Before timbering installation, the trench must be excavated to the required depth with minimal disturbance to the walls. At a county hospital sewer line project, excavation proceeded in stages to allow timely placement of timber supports.

Installation of Wale Boards

Wale boards are horizontal timber members fixed against the trench walls to distribute soil pressure. They are positioned vertically at intervals along the trench length. For a bank branch foundation trench, wale boards were spaced at 1.5-meter intervals to maintain effective lateral support.

Placement of Struts and Rakers

Struts are horizontal braces that connect opposite wale boards, preventing inward collapse. Rakers provide angled support from the trench bottom to the wale boards. In a retail store underground utility trench, engineers installed struts every 2 meters and rakers at 45-degree angles for additional stability.

Securing Joints and Fixings

Nails, bolts, or screws must secure all timber members tightly to prevent movement under pressure. At a county government water project, engineers used galvanized nails to resist corrosion in damp soil conditions.

2.1.4 Installation of Protective Systems

Protective systems in trench timbering safeguard workers from soil collapse and falling debris. Regulatory compliance and best practices guide their implementation.

Guardrails and Edge Protection

Guardrails prevent personnel from falling into open trenches. These are installed along trench edges, especially in high-traffic areas such as construction sites near schools. For example, a hotel construction site in Eldoret installed wooden guardrails around all trenches exceeding 1.2 meters in depth.

Access and Egress Points

Safe entry and exit points such as ladders or ramps must be provided within 8 meters of workers in the trench. A university campus project ensured ladders were securely fixed and extended above the trench edge for safe climbing.

Warning Signage and Barricades

Clear signage and physical barricades alert site workers and visitors to trench hazards. At a Nairobi retail development, bright reflective signs and barriers were used to demarcate trench perimeters during night operations.

Emergency Rescue Provisions

Rescue equipment such as harnesses and first aid kits should be readily available on site. A county hospital expansion project trained staff on emergency response and positioned rescue tools adjacent to trench sites.

2.1.5 Monitoring and Inspection

Regular inspection and monitoring of trench timbering are critical to detect hazards early and maintain safety standards throughout the project.

Daily Visual Inspections

Site supervisors must conduct daily checks for signs of timber movement, soil displacement, or water infiltration. During a SACCO office foundation trench, daily logs documented inspection findings to track trench condition.

After Adverse Weather Checks

Heavy rains or flooding can weaken timber supports and soil stability. Following such events, inspections ensure the trench remains safe for workers. For example, after a heavy storm in Kisumu, a farm irrigation trench was re-assessed and reinforced before resuming work.

Load and Stress Assessments

Engineers periodically evaluate timber member loads using calculations or instrumentation to confirm they remain within safe limits. At a Nairobi Water Company project, strain gauges were installed on critical struts to monitor stress levels.

Documentation and Compliance

Inspection reports are maintained to comply with occupational safety regulations and to inform any required corrective actions. A retail business construction site used digital forms to streamline documentation and improve traceability.

2.1.6 Safe Dismantling Procedures

Removing trench timbering safely is as important as its installation. Proper dismantling prevents accidents and preserves timber for reuse.

Planning the Dismantling Sequence

Dismantling must follow a planned sequence starting from the top and moving downward to avoid sudden soil collapse. In a county government office project, engineers prepared detailed dismantling plans reviewed by safety officers.

Use of Appropriate Tools and Equipment

Tools such as crowbars, hammers, and lifting devices facilitate safe removal without damaging timber or injuring workers. At a hospital expansion site, mechanical lifting gear was used for heavy timber panels.

Ensuring Worker Safety

Personnel involved must wear protective equipment and avoid working directly under unsupported trench walls during dismantling. A university construction site enforced strict PPE use and restricted access during timber removal.

Timber Inspection and Storage

After removal, timber should be inspected for damage and stored in dry, ventilated areas to prolong usability. At a SACCO foundation project, timber was stacked off the ground and covered with tarpaulins to prevent deterioration.

Chapter Summary

This chapter covers the essential practices involved in performing civil temporary works, starting with trench timbering. It emphasizes the importance of thorough site assessment and careful selection of appropriate timbering materials to ensure stability and safety during excavation. The construction process includes proper installation of protective systems, continuous monitoring, and safe dismantling procedures to prevent accidents. Formwork or shuttering construction is explored through the identification of suitable materials, design considerations, integration of reinforcement, and adherence to safety protocols during concrete pouring and curing, followed by systematic dismantling. Scaffold erection and dismantling require strict compliance with job requirements, detailed design and planning, and regular inspection and maintenance to uphold worker safety. Finally, the chapter addresses the erection and dismantling of shores, highlighting the need for job-specific planning and safety precautions to support structural elements temporarily. Together, these topics provide a comprehensive understanding of managing temporary structures critical to successful civil engineering projects.