Soil Investigation for Foundation Projects

A thorough understanding of ground conditions is essential for the success of foundation projects. Soil investigation plays a vital role in identifying site constraints, determining the geological profile, and understanding the properties of different strata. This ensures that foundation systems are designed efficiently and safely, taking into account the ground’s physical characteristics.

Objectives of Soil Investigation

The primary goal of a soil investigation is to assess the ground conditions and determine the geological sequence. This involves gathering information through methods such as:

• Borehole Drilling: Boreholes are drilled to retrieve soil and rock samples for identification and laboratory testing. This helps establish the geological profile.
• In-situ Tests: These tests provide data on the ground’s properties in its natural state. Common methods include Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), and pressuremeter tests.
• Geophysical Studies: In cases where the project is large or the geological conditions are complex, geophysical methods such as seismic refraction or resistivity surveys may be employed.

The data obtained from these investigations help construct a geological model that guides the selection of the most suitable foundation system for the project.

Importance of a Thorough Investigation

A well-planned and executed site investigation prevents costly design changes and delays during construction. It also helps in:

• Avoiding Complex Ground Conditions: If areas with complex geological formations are identified, project layouts can be adjusted to avoid them.
• Designing Efficient Foundations: Accurate soil information allows for the design of a foundation system tailored to the site conditions, ensuring both stability and cost-effectiveness.

Inadequate or oversimplified investigations can lead to unforeseen issues, potentially causing delays and cost overruns.

Phased Investigations

For large projects, soil investigations are often phased to allow for a preliminary geological assessment before detailed investigation work. This phased approach enables adjustments to be made as new subsurface conditions are encountered, optimizing the investigation process and reducing costs.

Desk Studies

Before starting physical investigations, desk studies are conducted to gather background information on the site. These studies include:

1. Site History

Understanding the historical usage of the site is critical. Historical data can provide insight into the presence of old foundations, wells, or tunnels that might interfere with the new foundation. For sites built on reclaimed land, it is essential to assess the possibility of continued ground settlement.

2. Adjacent Structures

Due to the high density of development in urban areas, it is important to consider the proximity of existing structures and foundations. This ensures that the construction of the new foundation does not disturb nearby buildings or utilities. In some cases, existing foundations can be reused, provided they are tested for integrity.

3. Geological Studies

Geological studies form the foundation for planning the ground investigation. Understanding the types of rocks and soils present at the site helps in interpreting subsequent field and laboratory data. Geological maps and memoirs offer valuable information about the site’s subsurface conditions, guiding the design and investigation process.

Methods of Site Investigation

Various methods are employed during a soil investigation. The choice of method depends on the complexity of the site and the scale of the project.

Method	Purpose	Description
Borehole Drilling	Retrieve soil and rock samples	Boreholes are drilled to various depths, and samples are tested in the laboratory for classification and strength.
Standard Penetration Test (SPT)	Measure soil strength	A split-spoon sampler is driven into the ground to determine soil resistance, providing a measure of the soil’s density or consistency.
Cone Penetration Test (CPT)	Assess soil behavior	A cone-shaped probe is pushed into the soil to measure resistance, offering continuous data on soil stratigraphy.
Pressuremeter Test	Determine soil deformation characteristics	A cylindrical probe is inserted into the borehole and expanded to measure the soil’s response under pressure.

Soil investigation is a critical component of foundation design, providing essential data on site constraints and geological profiles. Thorough planning and execution of these investigations can significantly reduce the risk of project delays and cost overruns.

In large and complex projects, phased investigations and continuous reappraisal of site conditions ensure optimal results, leading to a successful foundation system that supports the long-term stability of the structure.

Soil investigation is a critical step in determining the suitability of a site for construction. It involves studying soil properties and identifying potential challenges such as soil variability, rock formations, and groundwater.

In Hong Kong, due to its complex geology and varied weathering profiles, soil investigations are particularly challenging. This document discusses the various aspects of soil investigation, focusing on the impact of different rock types, weathering profiles, and geological features on foundation design and construction.

Weathering Profiles and Rock Types

Influence of Weathering Profiles

Weathering profiles significantly vary based on rock type, location, and geological history. In Hong Kong, corestone-bearing profiles are typically associated with medium- to coarse-grained granites and volcanic rocks like coarse ash tuffs. However, not all volcanic rocks display corestones. Fine-grained volcanic rocks, such as fine ash tuffs and fine-grained granites, often lack corestones.

The presence of corestones tends to increase with depth, but lateral variations are common. The depth and extent of weathering fluctuate depending on rock type and discontinuity spacing. These variations are crucial in foundation design, as the undulating weathering front must be considered for accurate engineering assessments.

Granitic Saprolites

Granitic saprolites, which retain the original texture and structure of the parent rock, behave like granular soils. Depending on the degree of weathering and cementation, they may also possess secondary bonding. The engineering behavior of these soils must be carefully assessed, as it influences the foundation design.

Volcanic Rocks

The lithological variability of volcanic rocks presents significant challenges in foundation design. These rocks, including tuffs and lavas, vary in grain size, structure, and weathering characteristics. Soils derived from volcanic rocks are generally silty, and many contain fragile, decomposed grains. The relict bonding found in these soils further complicates the engineering properties, making generalizations difficult for volcanic rock formations.

Colluvium and Clay Profiles

Colluvium, consisting of debris flow and rockfall deposits, often accumulates on hillsides, containing large boulders within coarse-grained material. This can impede pile driving. Additionally, while clay profiles are rare in weathered rock formations in Hong Kong, fine-grained products from meta-siltstones, such as the Lok Ma Chau Formation, can present challenges in alluvial deposits.

Marble and Karst Features

Marble formations, found in certain regions of Hong Kong, pose unique challenges due to the occurrence of karst features such as pinnacles, dissolution channels, and underground caves. These karst features significantly influence foundation stability and must be thoroughly investigated during soil exploration.

Groundwater Considerations

Groundwater plays a critical role in the design and construction of foundations. The groundwater regime must be understood, particularly when working with cast-in-place piles. Artesian water pressures can affect shaft stability, and tidal variations near the seafront may influence foundation design. In sloping terrain, groundwater flow and hydraulic gradients must be considered, as they can impact pile construction.

Ground Investigation Execution

Importance of Competent Contractors

Soil investigation requires experienced and competent contractors capable of producing high-quality work. The Buildings Department and the Environment, Transport and Works Bureau maintain registers of qualified contractors for both private and public developments. Proper design, direction, and supervision of the investigation by qualified engineers or geologists are essential to ensure accurate results.

Extent of Ground Investigation

The extent of ground investigation depends on several factors, including the complexity of the ground and the proposed development’s nature. While no strict guidelines exist, borehole spacing of 10 to 30 meters is often appropriate for general building structures. In reclamation areas, closer boreholes may be necessary to identify buried obstructions, such as remnants of old seawalls.

Borehole Sinking and Geological Modeling

Boreholes should extend through unsuitable materials into competent ground to ensure that no boulders or weak materials are encountered. For sites involving pile foundations, pre-drilling is typically conducted to confirm design assumptions. Boreholes for large-diameter bored piles should penetrate at least 5 meters into solid rock to confirm the piles’ founding level.

Table 1: Borehole Spacing Recommendations

Structure Type	Borehole Spacing	Pre-Drilling Depth
General Building	10 – 30 meters	5 meters into solid rock
Large-Diameter Piles	Borehole at each pile	5 meters into solid rock
Small-Diameter Piles	5 meters from each pile	–
Diaphragm Wall Panels	10 meters between boreholes	–

Use of Geophysical Methods

Geophysical methods, such as surface, cross-hole, and down-hole surveys, can augment traditional borehole drilling. These techniques help screen site areas and plan subsequent investigations. Geophysical data must be processed in the field to resolve any anomalies that may arise.

Investigating Rock Slopes and Discontinuities

The stability of foundations built near rock slopes depends on a comprehensive geological study and detailed mapping of joint conditions. Where direct access to the rock face is unavailable, inclined drill holes may be necessary to determine the orientation and continuity of discontinuities. Low-strength materials, such as kaolin, can affect slope stability and must be carefully assessed.

Table 2: Geophysical Techniques and Their Applications

Technique	Application
Surface Methods	Screening and preliminary assessment
Cross-Hole Surveys	Determining rock mass properties
Down-Hole Surveys	Investigating deeper discontinuities

Conclusion

Soil investigation in complex geological environments like Hong Kong requires a combination of advanced techniques and careful observation. The variability of weathering profiles, rock types, and groundwater conditions necessitates thorough analysis to ensure stable and safe foundation designs. By employing competent contractors, using both borehole drilling and geophysical methods, and continually updating geological models, engineers can navigate the complexities of soil investigation and optimize construction outcomes.