Conducting underground surveys is a critical aspect of land surveying in Kenya, particularly in urban areas with complex infrastructure networks such as Nairobi and Mombasa. The ability to accurately transfer horizontal and vertical controls from surface benchmarks to underground points ensures that construction, mining, and utility projects proceed according to design specifications and safety standards. This chapter focuses on the technical procedures and considerations necessary for precise control transfer, which forms the foundation for reliable underground mapping and development.

2.1 Transfer of horizontal and vertical controls from surface to underground

Transferring control points from the surface to underground environments is a specialized process that requires careful planning and precise execution. In Kenya, underground surveys support activities in tunnels, sewer systems, mining operations, and basements. The integrity of horizontal and vertical controls is paramount to avoid costly errors and ensure compliance with engineering and regulatory standards.

2.1.1 Principles of Horizontal Control Transfer

Horizontal control transfer involves establishing underground points that maintain consistent positional relationships with surface control points in the horizontal plane. This process is essential for aligning underground works with surface layouts and existing infrastructure.

Establishing Reference Surface Controls

• Accuracy of surface control points: Surface controls must be established with high precision using geodetic methods such as GPS or total station traversing to serve as reliable references.
• Selection of stable benchmarks: Preferably, surface benchmarks should be located on immovable and stable ground to minimize positional changes over time.
• Use of control networks: A well-distributed network of horizontal control points on the surface facilitates accurate point transfer underground.
• Documentation and verification: Comprehensive records of control point coordinates and monument descriptions ensure traceability and facilitate re-occupation.
• Coordination with project design: Surface control points should correspond with the engineering design to ensure alignment between survey data and construction plans.

Techniques for Horizontal Control Transfer

• Plumb line method: Using a plumb bob suspended from the surface to mark vertical alignment directly downwards to underground points.
• Optical methods: Employing total stations or theodolites positioned at surface or underground stations to sight and establish horizontal coordinates.
• Use of shafts or boreholes: Vertical openings provide access points for transferring horizontal positions through direct line-of-sight or instrumentation.
• Triangulation and trilateration underground: Establishing a local control network underground tied to surface points through precise angular and distance measurements.
• Electronic distance measurement (EDM): Utilizing EDM instruments to measure distances between surface and underground control points accurately.

2.1.2 Principles of Vertical Control Transfer

Vertical control transfer ensures that underground points have accurately determined elevations relative to surface benchmarks. This is vital for drainage design, structural integrity, and safety in underground construction.

Establishing Vertical Reference Points on Surface

• Use of permanent benchmarks: Surface benchmarks with known reduced levels (RLs) provide starting points for vertical control.
• Selection of stable elevation points: Locations less prone to subsidence or disturbance are preferred to maintain consistent vertical references.
• Integration with national height datum: Surface elevations should be referenced to Kenya’s established vertical datum to ensure consistency.
• Verification of surface benchmarks: Regular checks and re-leveling confirm the stability of vertical controls.
• Coordination with project requirements: Vertical control points must align with design elevation criteria to avoid structural conflicts.

Methods of Vertical Control Transfer Underground

• Direct plumb line drop: A plumb line suspended from a surface benchmark allows direct determination of underground point elevation.
• Differential leveling: Using precise leveling instruments to measure vertical differences through shafts or accessible openings.
• Barometric leveling: Employing barometers to estimate elevation differences based on atmospheric pressure changes, useful where direct line-of-sight is unavailable.
• Use of laser levels: Laser instruments provide continuous vertical reference lines for transferring elevations over distances.
• Trigonometric leveling: Calculating elevation differences using measured vertical angles and horizontal distances with total stations.

2.1.3 Equipment and Instrumentation for Control Transfer

Accurate transfer of controls underground necessitates specialized equipment capable of operating in confined and challenging environments while maintaining measurement precision.

• Total stations are used extensively for both horizontal and vertical control transfer due to their ability to measure angles and distances precisely.
• Digital levels facilitate accurate vertical control where direct plumb lines are impractical.
• Plumb bobs serve as simple yet effective tools for transferring points vertically in shafts or boreholes.
• Laser levels offer flexibility in establishing reference lines in tunnels or underground chambers.
• Reflector prisms improve EDM accuracy by providing a precise target for the instrument’s infrared beam.

2.1.4 Challenges in Control Transfer and Mitigation Strategies

Underground environments pose unique challenges that can compromise the accuracy of control transfer. Understanding these challenges and implementing appropriate mitigation measures is essential for successful surveys.

Environmental and Physical Challenges

• Limited line-of-sight: Underground passages and shafts may obstruct direct sighting, complicating measurements.
• Vibrations and movements: Machinery and construction activities can induce vibrations, affecting instrument stability.
• Poor lighting conditions: Insufficient illumination hampers visual observations and instrument readings.
• Restricted space and access: Tight spaces limit instrument placement and operator movement.
• Humidity and dust: Environmental factors can degrade instrument performance and measurement reliability.

Mitigation Strategies

• Use of specialized underground instruments: Devices designed for low-light and confined spaces improve data quality.
• Stabilizing instrument setups: Employing tripods with vibration dampeners and securing instruments firmly reduces measurement errors.
• Establishing control points in accessible locations: Whenever possible, setting control points in areas with better access facilitates easier measurements.
• Regular instrument calibration and maintenance: Ensures optimal performance despite harsh conditions.
• Employing multiple measurement methods: Cross-verification using different techniques enhances reliability and identifies discrepancies.

Chapter Summary

This chapter focused on the essential process of conducting underground surveys in engineering projects. It began with methods for transferring horizontal and vertical controls from the surface to underground environments, ensuring accurate spatial referencing below ground. Following this, detailed underground survey procedures were explored, highlighting the systematic steps required to collect reliable data in confined and complex subterranean settings. The chapter also examined various applications of underground surveying, demonstrating its critical role in construction, mining, and infrastructure development. Finally, the preparation of built survey maps was discussed, emphasizing the importance of translating raw survey data into clear, usable visual representations for project planning and execution. Together, these topics provide a comprehensive foundation for managing underground surveys effectively in engineering contexts.