Temporary works are essential for ensuring safety and stability during excavation activities in carpentry and joinery projects. Trench timbering, a critical form of temporary works, provides support to excavation walls, preventing collapses that could cause injury or project delays. In the Kenyan construction context, where soil conditions and weather variability can pose significant risks, proper selection, preparation, and installation of trench timbering materials are paramount to safeguarding workers and maintaining project timelines.

2.1 Trench Timbering Materials

In carpentry and joinery, trench timbering materials must be carefully chosen and prepared to provide reliable support for excavation walls. The materials used must balance strength, durability, and ease of installation while complying with safety regulations in Kenya. Understanding the characteristics and proper handling of these materials is essential for carpenters to deliver safe and efficient excavation support.

2.1.1 Identify materials

Identifying appropriate materials for trench timbering involves recognizing the types of timber and supplementary components that provide adequate strength and resistance to soil pressure during excavation. Each material has a role: sheeting holds back the soil face, walers spread the pressure, struts resist it across the trench, and fasteners and wedges keep everything tight. A carpenter who can recognise each material and its condition on delivery prevents weak or unsuitable members reaching the trench. Example: on a sewer extension for a housing estate, the site carpenter sorted the delivered timber into poling boards, walings and struts and rejected two split pieces before any excavation started.

Timber Types Commonly Used in Trench Timbering

• Hardwood: Commonly used hardwoods like mahogany and camphor provide excellent strength and durability. They are preferred for longer-term projects due to their resistance to decay and high load-bearing capacity.
• Softwood: Pine and cypress are softwoods frequently used for temporary works. While less durable than hardwoods, they are lighter and easier to handle, making them suitable for short-term trenches.
• Plywood: Used mainly for sheeting in trench support, plywood distributes soil pressure evenly. Marine plywood is preferred for its resistance to moisture.

Close-up of plywood sheets (Source: Wikipedia (Plywood))

• Treated Timber: Timber treated with preservatives resists fungal and insect attacks, prolonging its service life in damp trench environments.
• Steel Components: Occasionally, steel props and struts supplement timber supports to enhance stability, especially in deeper or more complex excavations.

Supplementary Materials

• Nails and Bolts: High-quality galvanized nails and bolts secure timber members, preventing loosening under pressure.
• Wedges: Timber or metal wedges are used to tighten the fit between members, ensuring rigidity.
• Bracing Materials: Additional timber or steel braces provide lateral support to prevent deformation.
• Cleats and Lips: Short timber blocks nailed below struts and walings so they cannot slip down if they loosen.
• Packing Pieces: Thin timber packers fill gaps between the sheeting and an uneven soil face so the boards bear evenly.

2.1.2 Tools and equipment

Trench timbering requires tools for measuring, cutting, fixing and tightening timber members, and equipment for lowering them safely into the trench. Using the correct tools ensures members are cut to the right length, fixed tightly and installed without workers having to stand in an unsupported trench. All tools must be checked before use, and heavy members are lowered with ropes or lifting equipment rather than thrown in. Example: before timbering a 2.4 m deep trench for a water company's main, the gang checked their saws, sledgehammer and lowering ropes and kept them in a tool box at the trench edge.

Care of trench timbering tools

• Inspect before use: Check handles, heads and blades for looseness, cracks or blunt edges before each shift.
• Keep away from the edge: Store tools at least 1 m from the trench edge so they cannot fall on workers below.
• Clean after use: Remove mud and soil, and dry steel tools to prevent rust.
• Sharpen saws: Keep saws sharp so cuts are square and members bear fully against each other.
• Replace worn ropes: Discard ropes that are frayed, cut or stiff with dried mud.

2.1.3 Select temporary works materials

Selecting materials for trench timbering requires evaluating factors such as soil type, trench depth, load conditions, and environmental exposure. These criteria ensure the chosen materials meet safety and performance requirements. Example: for a deep drainage trench in waterlogged ground at a flower farm in Naivasha, the engineer specified treated hardwood walings and close boarding instead of the open poling used in dry ground.

Factors Influencing Material Selection

• Soil Characteristics: Clay soils exert different pressures compared to sandy soils. For instance, in Nairobi's clay-rich areas, sturdier timber like hardwood is necessary to resist moisture-induced swelling.
• Trench Depth: Deeper trenches demand stronger timber and additional bracing to counteract increased lateral earth pressure.
• Duration of Support: Temporary works for prolonged periods require treated or hardwood timber to prevent degradation.
• Environmental Conditions: In regions with frequent rainfall, treated timber or waterproof sheeting minimizes moisture damage.
• Availability and Cost: Local availability of timber species and budget constraints influence the final material choice, balancing performance with economic feasibility.

Material Quality Considerations

• Defect-Free Timber: Select timber free from knots, splits, or warping to maintain structural integrity.
• Straightness: Straight timber members ensure uniform load distribution and easier installation.
• Moisture Content: Timber with appropriate moisture content reduces the risk of shrinkage or swelling after installation.
• Correct Section Size: Members must be of the specified size, as an undersized strut or waling may buckle or bend under soil pressure.
• Grade: Structural timber should be graded and marked so its strength class is known.

2.1.4 Prepare temporary works members

Preparation of timber members involves cutting, shaping, treating, and assembling components to ensure they fit precisely and perform their role effectively during trench support. Preparing members at ground level, away from the trench, reduces the time workers spend in the excavation. Example: for a storm drain at a county referral hospital, the carpenters prepared a full cutting list and pre-cut all struts on the bench before the excavator opened the trench.

Cutting and Sizing

• Accurate Measurement: Timber must be cut to precise lengths based on trench dimensions and design specifications to avoid gaps or overlaps.
• End Treatment: Ends of timber members are often chamfered or rounded to prevent splitting during installation.
• Notching and Drilling: Notches and holes are prepared to accommodate braces, bolts, and nails, facilitating secure connections.
• Square Cutting: Strut ends are cut square so they bear fully on the walings without rocking.
• Pointing Boards: The lower ends of poling boards may be cut to a blunt point to ease driving into the trench bottom.

Surface Treatment

• Preservative Application: Applying wood preservatives before installation extends the lifespan of timber in moist trench environments.
• Moisture Sealing: Timber may be coated with water-repellent solutions to reduce water absorption.
• End Sealing: Cut ends are re-treated because preservative does not reach the inside of the timber.
• Drying Time: Treated members are left to dry before handling so the chemical is not washed off or absorbed through skin.
• Reuse Treatment: Timber recovered from earlier trenches is cleaned and re-treated before reuse.

Assembly of Components

• Pre-Assembly of Frames: In some cases, timber frames are partially assembled off-site to speed up installation.
• Marking Members: Clearly marking timber pieces according to their position in the trench supports correct placement on-site.
• Fixing Cleats: Cleats are nailed to walings in advance at strut positions.
• Stacking in Order: Prepared members are stacked near the trench in the order they will be installed.
• Checking Against the Cutting List: All members are counted against the cutting list before work starts.

Worked example: number of poling boards

Number of boards per side = trench length ÷ board width

A trench 12 m (12 000 mm) long is close-boarded with 225 mm wide boards.

Number of boards per side = 12 000 ÷ 225

Number of boards per side = 53.3, so 54 boards

Total boards = 54 × 2 = 108 boards

Worked example: strut length

Strut length = trench width − 2 × (board thickness + waling thickness)

The trench is 900 mm wide, boards are 38 mm thick and walings are 75 mm thick.

Strut length = 900 − 2 × (38 + 75)

Strut length = 900 − 2 × 113

Strut length = 900 − 226

Strut length = 674 mm

2.1.5 Install temporary work

The installation of trench timbering must follow a systematic process to ensure stability and worker safety throughout excavation. Proper sequencing and secure fixing are critical. Example: on a county water project laying a pipeline through Machakos town, timbering was installed in 3 m lengths immediately behind the excavator so no section of trench was left unsupported overnight.

Steps in Installation

1. Excavation to Required Depth: Begin by excavating the trench carefully, stopping periodically to install supports.
2. Positioning of Side Sheeting: Place timber sheets vertically against trench walls to hold soil in place.
3. Insertion of Walers: Horizontal timber members, called walers, are installed against the sheeting to distribute soil pressure.
4. Installing Struts or Braces: Diagonal or horizontal struts are fitted between walers or opposing trench walls to resist inward soil movement.
5. Securing Connections: Use nails, bolts, and wedges to fix members tightly, preventing displacement.
6. Progressive Installation: Continue excavation in stages, installing timbering progressively to maintain continuous support.

Site Coordination

Installation must be coordinated with excavation teams to minimize exposure of unsupported trench walls. Communication between carpenters and machine operators enhances safety.

2.1.6 Safety checks

Safety checks are vital throughout the trench timbering process to prevent accidents and ensure compliance with Kenyan occupational safety standards. Checks are made on the materials before installation, on the work as it proceeds and on the completed support before anyone works in the trench. Example: at a secondary school sewer connection, the supervisor stopped work when a strut was found loose during installation and had it re-wedged before the gang returned to the trench.

Pre-Installation Inspections

• Material Condition: Verify timber is free from defects and adequately treated.
• Equipment Readiness: Ensure tools and fasteners are in good condition and suitable for the task.
• Trench Edges: Check that spoil heaps and plant are kept back from the trench edge.
• Services: Confirm that underground cables and pipes have been located and marked.
• Access: Confirm a ladder is in place before anyone enters the trench.

During Installation

• Structural Stability: Monitor timber members for signs of movement or stress during installation.
• Worker Safety: Confirm that personnel use appropriate personal protective equipment (PPE) and follow safe working procedures.
• No Working Under Unsupported Faces: Ensure no one stands beyond the last installed frame.
• Lowering Members: Check that members are lowered by rope, not thrown or dropped.
• Supervision: A competent person must be present while timbering is installed.

Post-Installation

• Inspection of Supports: Conduct thorough checks to confirm all members are securely fixed and bracing is effective.
• Emergency Procedures: Establish clear protocols for rapid evacuation and support reinforcement if instability is detected.
• Edge Protection: Check that barriers are in place around the trench.
• Records: Record the inspection, its date and the inspector's name.
• Water Control: Check that water is not collecting behind the sheeting or at the trench bottom.

Chapter Summary

This chapter covered the essential aspects of installing trench timbering for excavation support, beginning with the identification and selection of suitable materials, tools, and equipment necessary for temporary works. It detailed the preparation and installation process of timber members while emphasizing the importance of conducting thorough safety checks at every stage. The discussion then focused on timber walling boards, poling boards, and struts, highlighting the criteria for selecting appropriate struts and their correct placement to ensure effective trench support. Securing and bracing techniques were explored with attention to the choice of braces, their installation, and the safety measures required. Safe access to the trench was addressed by identifying proper entry points, selecting suitable materials for these areas, and placing supports to maintain stability. The chapter also stressed the need for continuous inspection and maintenance of the timbering throughout excavation, identifying critical areas that require reinforcement and ongoing safety evaluations. Finally, good housekeeping practices on-site were underscored as vital for maintaining a safe and efficient working environment.