Underpinning of Existing Boundary Walling: Creating Building Space and Preventing Collapse During Excavation

In Kenya's rapidly urbanizing landscape, construction sites are increasingly characterized by tight spaces and properties built close to boundary lines. When new developments require deep excavations adjacent to existing boundary walls or neighboring structures, underpinning becomes a critical construction technique. This comprehensive guide examines underpinning practices in Kenya, focusing on how this method creates usable building space while stabilizing adjacent structures to prevent collapse during excavation work.

Understanding Underpinning in the Kenyan Context

Underpinning is a specialized construction technique that strengthens and stabilizes existing foundations by extending them to deeper, more stable soil layers or by improving their load-bearing capacity. In Kenya's construction sector, underpinning serves two primary purposes: creating additional building space adjacent to boundary walls and protecting neighboring structures from potential collapse during excavation activities.

According to recent incidents documented by the Architectural Association of Kenya (AAK), improper underpinning procedures have contributed to several construction failures. A May 2025 boundary wall collapse along Suswa Road in Parklands, Nairobi, highlighted critical issues including foundation works carried to the edge of boundary walls, inadequate root barriers, and improper underpinning of existing foundations. These failures underscore the importance of proper underpinning techniques in Kenya's construction industry.

Regulatory Framework Governing Underpinning in Kenya

The National Building Code 2024

Kenya's construction industry operates under the National Building Code 2024, which replaced the 1968 Building Code and represents a significant modernization of construction standards. The code addresses critical aspects of excavation and foundation work, including requirements for general stability during excavation and structural works supervision.

The National Building Code 2024 is mandatory for all new construction projects in Kenya, with enforcement overseen by county governments and the National Construction Authority. The code sets higher standards for structural design and materials, ensuring buildings are safer and of higher quality while reducing the risk of structural failures.

National Construction Authority (NCA) Requirements

The National Construction Authority, established under Act No. 41 of 2011, plays a central role in regulating construction projects involving underpinning work. All construction projects must be registered with NCA, which ensures compliance with quality standards, legal requirements, and proper professional handling.

For projects involving underpinning and excavation near existing structures, NCA requires:

Registration of all contractors, supervisors, and construction workers
Submission of approved architectural and structural designs
Evidence of qualified structural engineers overseeing the work
Compliance with health and safety regulations
Use of KEBS-certified construction materials

County Government Approvals

Before commencing underpinning work, developers must obtain approvals from county planning departments. Building plan approvals cover architectural and structural aspects including structural integrity, fire safety, sanitation, and compliance with zoning regulations. Counties also issue excavation permits for major earthworks that could affect adjacent properties.

When Underpinning is Necessary in Kenya

Scenarios Requiring Underpinning

Underpinning becomes essential in several common situations encountered in Kenyan construction:

Maximizing Building Space: In high-value urban areas like Nairobi's Westlands, Kilimani, and Kileleshwa, developers seek to maximize every available square meter. Underpinning existing boundary walls allows construction to extend closer to property lines, creating additional usable floor area without violating setback requirements.

Deep Basement Construction: Modern commercial and residential developments increasingly incorporate basements for parking and utilities. When excavation extends below the foundation level of adjacent structures, underpinning protects these buildings from settlement or collapse.

Soil Movement and Subsidence: Research on Nairobi's geological conditions has identified that certain subsurface materials are prone to collapse during excavation. Materials including tuffs, agglomeratic tuffs, silty clays, and red clays are liable to collapse in excavations, with distress common in structures supported on concealed fractures or moisture-sensitive soils.

Increased Load Requirements: When adding additional floors to existing buildings or when adjacent new construction imposes additional loads, underpinning strengthens foundations to handle the extra weight.

Poor Original Construction: In cases where original foundations were inadequately designed or constructed—a problem highlighted by the National Construction Authority's 2015 audit which found 58% of Nairobi buildings unfit for habitation—underpinning can rectify structural weaknesses.

Adjacent Property Protection

When excavation for new construction occurs near existing buildings, proper underpinning protects neighboring structures from:

Differential settlement caused by soil pressure changes
Loss of lateral soil support
Groundwater level changes affecting bearing capacity
Vibrations from construction equipment
Direct undermining of existing foundations

Underpinning Methods Used in Kenya

Mass Concrete Underpinning

Mass concrete underpinning represents the traditional and most widely used method in Kenya due to its cost-effectiveness and suitability for shallow foundations common in residential construction.

Process: The method involves excavating soil below the existing foundation in controlled stages (typically 1.2-1.5 meter sections called "pins"). Once stable soil is reached, the excavation is filled with concrete and allowed to cure before proceeding to the next section. The load from the old foundation transfers to the new foundation through dry sand-cement packs.

Applications in Kenya: This method works well for the standard strip foundations used in boundary wall construction and single-story residential buildings. It's particularly suitable for projects in areas with stable soil conditions and where depth requirements don't exceed 3-4 meters.

Cost Considerations: Mass concrete underpinning typically represents the most economical option for Kenyan developers, with materials (cement, aggregates, and reinforcement steel) readily available from local suppliers.

Beam and Base Method

The beam and base method, also known as the cantilever needle method, provides a more sophisticated approach suitable for complex underpinning scenarios.

Process: This technique involves constructing a reinforced concrete beam beneath the existing wall or foundation, supported at intervals by mass concrete bases extending to stable soil. The beam distributes loads to these strategically positioned bases, eliminating the need for continuous excavation.

Applications in Kenya: This method proves valuable when underpinning must extend to only one side of the structure—common when maximizing building space adjacent to boundary walls. It's also useful when interior structural elements (such as strong columns) can provide support points.

Needle and Pile Method

For projects requiring deeper support or in areas with weak bearing strata, the needle and pile method offers a robust solution.

Process: Steel or concrete piles are driven on adjacent sides of the wall or foundation. Needles (horizontal beams) penetrate through the wall and connect to these piles, which function as pile caps. This transfers building loads to deeper, more stable soil layers.

Applications in Kenya: This method suits waterlogged areas (common near Nairobi's Ngong River and other drainage courses), clayey soils, and locations where stable bearing strata lie at significant depth. Research indicates it's particularly appropriate for Nairobi's areas with variable subsurface conditions.

Mini-Piled Underpinning

Mini-piling represents an advanced technique increasingly used in Kenya's urban construction projects.

Process: Small-diameter piles (typically 150-300mm) are installed at close intervals beneath existing foundations using specialized drilling equipment. These piles transfer loads to deeper, stronger soil layers or bedrock.

Applications in Kenya: Mini-piling works well in confined urban spaces where access is limited, and where minimal vibration is critical to protect sensitive neighboring structures. The method has gained popularity in Nairobi's densely built areas where traditional underpinning methods prove impractical.

Step-by-Step Underpinning Process for Boundary Walls

Phase 1: Site Investigation and Assessment

Geotechnical Investigation: Before underpinning commences, thorough geotechnical investigation determines soil properties, bearing capacity, and groundwater conditions. For Nairobi projects, this investigation must account for the city's complex geology, including volcanic deposits, weathered rocks, and potentially collapsible soils.

Structural Survey: A detailed structural survey documents the existing condition of boundary walls and adjacent buildings. This baseline record protects all parties by establishing pre-construction conditions, crucial should disputes arise later.

Survey and Boundary Confirmation: A licensed surveyor should locate and mark exact property boundaries from the title deed by planting beacons, with the surveyor pegging plot corners and producing a sketch or certified plan. Resolving boundary disputes before construction prevents expensive complications.

Phase 2: Design and Approval

Structural Design: A qualified structural engineer registered with the Engineers Board of Kenya (EBK) must design the underpinning works. The design specifies foundation depths, concrete grades, reinforcement requirements, and construction sequence.

Regulatory Approvals: Obtain necessary approvals including:

County government building plan approval
NEMA Environmental Impact Assessment (for larger projects)
NCA project registration
Excavation permits where required

Neighbor Notification: While Kenya lacks comprehensive party wall legislation similar to the UK's Party Wall Act, good practice and emerging legal precedents suggest formally notifying adjacent property owners of intended underpinning work. This notification should describe the work scope, timeline, and protective measures.

Phase 3: Site Preparation and Protection

Installation of Monitoring Systems: Before excavation begins, establish monitoring systems including settlement markers on adjacent structures and regular survey readings. Real-time settlement meters may be warranted for sites with high-yielding wells or areas with potential for large settlements.

Temporary Works Design: Design and install temporary shoring systems to support excavation faces and protect workers. Shoring prevents soil collapse during underpinning operations and maintains stability of adjacent structures.

Root Barrier Installation: For boundary walls near mature trees, proper root barriers must be installed to prevent destabilization of the remaining wall section—a failure mode identified in the Parklands collapse.

Phase 4: Excavation and Underpinning Execution

Staged Excavation: Excavate in controlled stages, typically working on alternate pins to maintain structural stability. The general rule limits unsupported wall length to no more than 25% of the total length at any time.

Continuous Monitoring: Throughout excavation, continuously monitor adjacent structures for any signs of movement. Immediately halt work if unexpected settlement or cracking occurs.

Concrete Placement: Place concrete in excavated pits within the shortest practical time to minimize exposure of existing foundations. Use high-quality concrete meeting KEBS standards, typically class 25 or higher for structural underpinning.

Pinning and Load Transfer: Once new concrete achieves adequate strength (typically 7-14 days), execute proper pinning to transfer loads from existing foundations to new underpinning. This may involve dry-packing with sand-cement mortar or using hydraulic jacks to ensure full contact.

Phase 5: Completion and Verification

Final Inspections: Conduct final inspections by the structural engineer and NCA officials to verify compliance with approved designs and construction standards.

Documentation: Compile comprehensive as-built documentation including photographs, concrete test results, and final survey measurements. This documentation proves invaluable for future property transactions and resolves any subsequent disputes.

Post-Construction Monitoring: Continue monitoring adjacent structures for at least 30 days post-completion to detect any delayed settlement.

Shoring Requirements During Excavation

Purpose of Shoring

Shoring provides temporary support to excavation faces, preventing collapse that could endanger workers and destabilize adjacent structures. Shoring is used to support beams and floors in buildings while construction proceeds, with vertical supports serving as temporary replacements for building columns or walls.

Types of Shoring Systems in Kenya

Timber Shoring: The most basic and commonly used shoring in Kenyan construction sites, timber shoring employs horizontal walings and vertical struts to support excavation faces. While economical, timber shoring requires skilled installation and regular inspection.

Steel Shoring: Steel beams or hydraulic jacks provide stronger support for deeper excavations. Steel shoring offers advantages in reusability and faster installation compared to timber systems.

Soldier Pile and Lagging: This method involves drilling holes and inserting steel I-beams (soldier piles) at regular intervals, with timber or concrete lagging placed between piles to retain soil. This system suits excavations ranging from 4-16 feet deep in soils with low moisture content.

Sheet Pile Shoring: Interlocking steel sheet piles create a continuous wall to support excavation faces. This method proves particularly effective in areas with high water tables, helping control groundwater while supporting soil.

Safety Considerations

Recent building failures in Kenya emphasize the critical importance of proper shoring. Construction site safety requires:

Shoring systems designed by qualified engineers
Regular inspection of shoring installations
Proper worker training in excavation safety
Adequate access and egress points for workers
Clear signage and barriers around excavation areas
Emergency response procedures

The Occupational Safety and Health Act (OSHA) Kenya mandates specific safety measures for excavation work, including proper shoring to protect workers from cave-ins.

Common Challenges and Solutions

Challenge 1: Inadequate Site Investigation

Problem: Insufficient geotechnical investigation leads to unexpected soil conditions, groundwater issues, or encountering underground utilities.

Solution: Invest in comprehensive site investigation including multiple boreholes, laboratory testing of soil samples, and utility location surveys. The cost of thorough investigation represents a small fraction of expenses from addressing unforeseen conditions.

Challenge 2: Unstable Soil Conditions

Problem: Nairobi's geology includes collapsible soils, concealed fractures, and moisture-sensitive materials that complicate underpinning work.

Solution: Geotechnical recommendations may include soil stabilization techniques such as grouting, dewatering to control groundwater, and specialized foundation solutions like mini-piles for particularly challenging conditions.

Challenge 3: Vibration Damage

Problem: Use of heavy machinery, particularly hydraulic hammers and pile driving equipment, can transmit damaging vibrations to adjacent structures. The Parklands collapse partly resulted from vibration destabilizing soil and threatening structural integrity of nearby developments.

Solution: Conduct pre-construction condition surveys of adjacent properties, implement vibration monitoring during construction, use low-vibration construction techniques where possible, and maintain communication with neighboring property owners about construction activities.

Challenge 4: Water Ingress

Problem: Groundwater infiltration during excavation can reduce soil bearing capacity, cause flooding of the excavation, and accelerate soil erosion.

Solution: Implement dewatering systems using sump pumps or wellpoints, install sheet piling to create a water-tight barrier, and ensure proper drainage around the excavation perimeter.

Challenge 5: Inadequate Temporary Support

Problem: Insufficient or improperly installed shoring systems lead to excavation collapse, endangering workers and destabilizing adjacent structures.

Solution: Engage qualified temporary works designers, use properly engineered shoring systems appropriate to soil conditions and excavation depth, conduct daily inspections of shoring installations, and never allow workers in unprotected excavations.

Cost Considerations for Underpinning in Kenya

Factors Affecting Underpinning Costs

Depth of Underpinning: Deeper underpinning requires more excavation, materials, and complex temporary works, significantly increasing costs. Shallow underpinning (1-2 meters) costs considerably less than deep underpinning exceeding 4 meters.

Access and Site Constraints: Confined urban sites with limited access require manual excavation and smaller equipment, increasing labor costs and project duration.

Soil Conditions: Challenging soil conditions necessitating specialized techniques (mini-piling, grouting) or extensive dewatering substantially increase project costs.

Length of Boundary Wall: Underpinning costs typically calculate per linear meter, with longer boundary walls naturally requiring higher total investment.

Material Specifications: Higher-grade concrete, imported reinforcement steel, or specialized materials increase material costs compared to standard specifications.

Estimated Cost Ranges

While costs vary based on specific project conditions, typical underpinning costs in Kenyan urban areas range from:

Basic mass concrete underpinning: KES 15,000 - 25,000 per linear meter
Beam and base underpinning: KES 25,000 - 40,000 per linear meter
Piled underpinning: KES 40,000 - 80,000 per linear meter

These estimates include materials, labor, and basic temporary works but exclude professional fees, regulatory approvals, and extensive site investigation costs.

Best Practices and Recommendations

For Developers and Property Owners

Engage Qualified Professionals: Work with NCA-registered contractors and consultants with demonstrable experience in underpinning work. Verify registrations with professional bodies (EBK, BORAQS).
Invest in Proper Investigation: Comprehensive site investigation identifies potential issues before construction, allowing appropriate design solutions and accurate cost estimation.
Maintain Neighbor Relations: Proactive communication with adjacent property owners prevents conflicts and facilitates cooperation when access to neighboring land becomes necessary.
Budget Adequately: Underpinning represents a significant construction cost. Adequate budgeting prevents mid-project financial constraints that might compromise safety or quality.
Document Everything: Maintain thorough documentation including pre-construction surveys, approval certificates, material test results, and as-built drawings.

For Contractors

Follow Approved Designs: Strictly adhere to structural engineer's designs and specifications. Never improvise or take shortcuts in underpinning work.
Prioritize Safety: Implement comprehensive safety measures including proper shoring, regular inspections, worker training, and emergency response procedures.
Quality Materials: Use only KEBS-certified materials meeting specified grades and standards. Counterfeit materials contribute significantly to building failures in Kenya.
Continuous Monitoring: Establish monitoring protocols and maintain vigilance throughout construction. Early detection of problems enables corrective action before serious damage occurs.
Professional Supervision: Ensure continuous supervision by qualified site engineers throughout critical underpinning operations.

For Regulatory Bodies

Strengthen Enforcement: Rigorous enforcement of building codes and NCA regulations prevents substandard construction practices.
Address Corruption: Transparent approval processes and accountability measures reduce corruption that enables dangerous construction practices.
Capacity Building: Training programs for contractors, artisans, and inspectors improve overall construction quality and safety awareness.
Public Awareness: Educational campaigns inform property owners about risks of unregulated construction and importance of engaging qualified professionals.

Case Studies: Lessons from Recent Failures

Parklands Boundary Wall Collapse (May 2025)

This incident demonstrates consequences of multiple failures in underpinning practice:

Foundation works extended to boundary wall edges without adequate support
Insufficient root barriers destabilized remaining wall sections
Hydraulic hammer use caused soil destabilization
Improper underpinning techniques and inadequate temporary works
Lack of proper safety equipment for workers

Lessons: This case highlights the need for comprehensive temporary works design, proper equipment selection to minimize vibrations, adequate protection for existing structures, and strict adherence to safety protocols.

South C Building Collapse (January 2026)

The collapse of a multi-story building under construction in Nairobi's South C area left at least four people trapped, with rescue workers digging through rubble. While not specifically an underpinning failure, this incident reflects broader construction safety issues including poor methodologies and inadequate regulatory oversight.

Lessons: This tragedy underscores the importance of proper construction supervision, adherence to approved designs, and effective regulatory enforcement throughout construction projects.

Future Outlook and Emerging Technologies

Digital Monitoring Systems

Advanced monitoring technologies including automated total stations, inclinometers, and real-time settlement sensors enable continuous monitoring of adjacent structures during underpinning work. These systems provide early warning of potential problems, allowing immediate intervention.

Building Information Modeling (BIM)

BIM technology facilitates better planning and coordination of underpinning work, enabling clash detection, construction sequencing visualization, and improved communication among project stakeholders.

Geotechnical Innovations

Emerging soil stabilization techniques including jet grouting, soil mixing, and ground improvement methods offer alternatives to traditional underpinning in challenging ground conditions.

Regulatory Evolution

Kenya's adoption of Eurocodes and updates to the National Building Code represent ongoing modernization of construction standards. Future regulatory developments may include specific provisions for party wall works and underpinning procedures similar to international best practices.

Conclusion

Underpinning of existing boundary walls and stabilization of adjacent buildings during excavation represents critical aspects of responsible construction practice in Kenya's urban areas. Proper underpinning techniques enable developers to maximize usable building space while protecting neighboring properties from damage or collapse.

Success requires comprehensive site investigation, qualified professional involvement, adherence to regulatory requirements, proper construction techniques, and continuous monitoring. The cost of proper underpinning represents essential investment in structural safety and legal compliance, preventing far greater expenses from building failures, legal disputes, or loss of life.

Recent building collapses in Kenya emphasize urgent need for stricter enforcement of construction standards, elimination of corrupt practices, engagement of qualified professionals, and accountability throughout the construction process. Property owners, developers, contractors, and regulatory bodies all share responsibility for ensuring safe, compliant construction that protects both workers and the public.

As Kenya's construction sector continues growing to meet housing and infrastructure demands, professional underpinning practices will become increasingly important. By following established best practices, investing in quality work, and maintaining commitment to safety, the industry can minimize risks while enabling responsible urban development.

About Makaobora.com: This article provides educational information about underpinning practices in Kenya. For specific projects, always consult with qualified structural engineers, registered contractors, and relevant regulatory authorities. Construction requirements vary by location and project specifics—professional guidance ensures compliance with all applicable regulations and standards.