A geotechnical report (also called a soil report or geotechnical investigation report) is a comprehensive document that provides an analysis of the subsurface conditions of a construction site. It is produced by geotechnical engineers after conducting field and laboratory tests on soil, rock, and groundwater at the site. The purpose of this report is to offer valuable data that helps engineers and architects design safe, stable, and cost-effective foundations and structures based on the site’s unique geological conditions.
1. Introduction
This geotechnical report presents the findings from subsurface exploration and laboratory testing conducted for the proposed New Multi-Story Office Building at 123 Main Street, Cityville. The purpose of this investigation was to assess the soil properties at the site and provide recommendations for foundation design and other construction considerations.
2. Site Description
- Site Location: The project site is located at the southeast corner of Main Street and Oak Avenue in Cityville, USA. The site is currently vacant and is relatively flat with no significant vegetation or structures.
- Topography: The site is level with an elevation of approximately 200 feet above sea level. The surrounding area consists of commercial buildings to the north and residential properties to the south.
3. Field Exploration
- Date of Exploration: The field exploration was conducted from January 1, 2025, to January 5, 2025.
- Test Borings: Two soil borings were drilled at the proposed foundation locations at depths of 30 feet (Boring 1) and 40 feet (Boring 2).
- Equipment Used: A truck-mounted drilling rig with a continuous sampler for soil samples and a standard penetration test (SPT) sampler were used.
- Boring Logs: Detailed soil profiles for each boring are attached in Appendix A. Below are the summarized findings:
| Depth (ft) | Boring 1 (Soil Type) | Boring 2 (Soil Type) |
|---|---|---|
| 0 – 5 | Topsoil, 6 in. thickness, loose | Topsoil, 8 in. thickness, loose |
| 5 – 12 | Silty Clay, soft, moist | Silty Clay, soft, moist |
| 12 – 20 | Clayey Sand, firm | Silty Sand, medium dense |
| 20 – 30 | Silty Clay, soft to medium dense | Clay, stiff |
| 30 – 40 | Sand, medium dense | Clay, stiff |
4. Laboratory Testing
Several laboratory tests were performed on the soil samples to determine important geotechnical parameters that will guide the foundation design. The results are summarized below:
- Atterberg Limits (Liquid Limit: 42, Plastic Limit: 25) for the silty clay layers indicate high plasticity.
- Standard Penetration Test (SPT) results for Boring 1 at depths of 10 ft and 15 ft are 5 and 6 blows per foot, respectively, indicating soft soil conditions in the top layers.
- Consolidation Test showed a coefficient of consolidation of 0.3 inches per year for the silty clay layer at 15-20 ft depth, suggesting potential for settlement under load.
5. Groundwater Conditions
- Water Table Depth: During drilling, groundwater was encountered at a depth of approximately 18 feet below ground surface in Boring 1 and at 15 feet in Boring 2.
- Groundwater Fluctuations: Based on local historical data and current site observations, the groundwater table may fluctuate seasonally by approximately 2 to 3 feet.
6. Soil Properties and Bearing Capacity
The soil at the site consists primarily of soft silty clay and some medium-dense sand layers. The general soil profile indicates the following key characteristics:
- Topsoil: Loose, approximately 6-8 inches thick.
- Silty Clay: Soft to medium stiff, bearing capacity of 2,000 psf (pounds per square foot).
- Clayey Sand/Silty Sand: Medium dense to dense, with an ultimate bearing capacity of 3,500 psf.
- Sandy Layer (below 30 feet): Medium dense, bearing capacity 4,000 psf.
7. Foundation Recommendations
Based on the subsurface conditions, the following foundation recommendations are provided:
- Shallow Foundations (Spread Footings):
- Use: For areas with medium-dense sand and clayey sand layers below 12 feet depth (Boring 2).
- Size: Spread footings with a minimum width of 3 feet.
- Depth: Minimum depth of 2 feet below grade for frost protection and to avoid soil heave.
- Bearing Capacity: Use a safe bearing capacity of 3,000 psf for the medium dense soil layer.
- Deep Foundations (Piles):
- Use: For areas with soft silty clay and silty sand layers (Boring 1).
- Type: Use drilled concrete caissons or precast concrete piles to penetrate down to the medium dense soil layers.
- Design: The piles should be designed to resist axial and lateral loads with a minimum factor of safety of 3.0.
- Bearing Capacity: The ultimate bearing capacity of piles should be assumed to be 10,000 psf in deeper, more competent layers.
- Waterproofing and Drainage Considerations:
- Since the water table is encountered at shallow depths, we recommend the installation of a drainage system around the foundation perimeter to reduce hydrostatic pressure. Additionally, waterproofing on foundation walls should be considered to prevent water infiltration.
8. Settlement Considerations
- Settlement of Shallow Foundations: Estimated total settlement for shallow foundations is expected to be less than 1 inch if designed within the recommended bearing capacity.
- Settlement of Deep Foundations (Piles): Piles should experience minimal settlement (<0.5 inches) as they will bear on deeper, more stable soils.
9. Conclusion
Based on the geotechnical investigation, the following is concluded:
- Shallow foundations are suitable for areas where medium-dense sand and silty sand layers are present.
- Deep foundations (such as piles or caissons) should be used where the soil is soft and weak, as identified in the upper layers.
- Appropriate drainage and waterproofing measures must be implemented to address groundwater and prevent soil erosion.
This report provides the necessary soil analysis and foundation recommendations required for safe and cost-effective design and construction.
10. Appendices
- Appendix A: Boring Logs
- Appendix B: Laboratory Test Results
- Appendix C: Site Photographs
This sample geotechnical report serves as a basis for understanding soil conditions and making informed decisions about the type of foundation needed for a construction project. The findings and recommendations are tailored to the specific site and its geotechnical conditions.
Clext Engineering & Construction Management 