Site Investigation
Learning Objectives
- Understand the primary purpose and objectives of site investigations in civil engineering.
- Explain the progressive phases of a site investigation, from desk study to direct exploration.
- Compare different drilling and sampling techniques, including disturbed vs. undisturbed samples.
- Understand in-situ testing methods like SPT, CPT, and Vane Shear, including necessary corrections.
- Evaluate rock mass quality using core recovery metrics and the Rock Quality Designation (RQD).
Purpose of Site Investigation
The critical first step in mitigating geotechnical risk and ensuring project feasibility.
Site Investigation
The systematic process of collecting physical information about the subsurface soil, rock, and groundwater conditions at a proposed construction site.
Importance of Geotechnical Exploration
Site Investigation (or Geotechnical Exploration) is arguably the most important phase of any civil engineering project, as unknown subsurface conditions are the leading cause of construction delays, cost overruns, and structural failures.
Primary Objectives
Objectives of Site Investigation
- Assess Feasibility: Determine if the proposed project can actually be built at the chosen location safely and economically.
- Determine Design Parameters: Provide the structural engineer with accurate values for soil bearing capacity, settlement estimates, active earth pressures, and groundwater elevations.
- Hazard Identification: Detect hidden geohazards such as active faults, subsurface cavities (karst), ancient landslides, or loose liquefiable soils.
- Construction Planning: Allow contractors to select appropriate excavation equipment (e.g., excavators vs. blasting), design temporary excavation support systems (shoring), and plan dewatering methods.
Phases of Investigation
A progressive, phased approach from cheap regional data to expensive site-specific drilling.
Phase 1: Desk Study
Phase 1 Overview
The most cost-effective phase. Before stepping foot on the site, engineers review existing regional data to anticipate what they might find.
Desk Study Activities
- Reviewing existing geological and topographic maps.
- Studying historical aerial photographs to identify old stream beds, abandoned mines, or previous land uses.
- Reviewing nearby well logs or previous geotechnical reports from adjacent properties.
Phase 2: Field Reconnaissance
Phase 2 Overview
A physical "walkover" survey by a geotechnical engineer or engineering geologist to observe the site's current condition.
Field Reconnaissance Activities
- Observing rock outcrops to measure strike and dip.
- Noting vegetation types (e.g., willow trees indicate shallow groundwater).
- Identifying signs of instability like curved tree trunks, tension cracks in the soil, or seepage zones on slopes.
Phase 3: Direct Exploration
Phase 3 Overview
The core of the investigation, involving physical penetration of the ground to retrieve samples.
Drilling and Sampling Types
Sample Quality
A critical distinction during exploration is the quality of the sample retrieved for laboratory testing.
Types of Samples
- Disturbed Samples (Split-Spoon): The soil structure is completely altered or destroyed during retrieval (e.g., auger cuttings, SPT split-spoon samples). The split-spoon sampler is a thick-walled steel tube that splits in half to reveal the recovered soil. These are perfectly fine for classification tests (grain size, Atterberg limits) but useless for determining the soil's actual in-situ strength or compressibility.
- Undisturbed Samples (Shelby Tube): Specialized tools are used to retrieve a sample while minimizing structural disturbance. The most common is the Shelby Tube, a thin-walled, seamless steel tube with a sharpened cutting edge. It is pushed smoothly and steadily into soft to stiff clays (never hammered) using the hydraulic pressure of the drill rig. These expensive, high-quality samples are mandatory for accurate consolidation and triaxial shear strength laboratory testing.
Exploration Methods
Common Exploration Techniques
- Test Pits: Digging shallow trenches (usually ) with a backhoe. This is relatively cheap, allows the engineer to visually inspect the soil profile in 3D, and take large block undisturbed samples. However, it is limited by the reach of the machine and usually cannot go below the water table without trench collapse.
- Soil Borings (Auger Drilling): Using a hollow-stem auger to drill deep into the soil profile. This is the standard method for soil exploration, allowing samplers (split-spoon or Shelby tubes) to be lowered through the hollow center to sample the undisturbed soil ahead of the drill bit.
- Rock Coring (Rotary Drilling): When bedrock is encountered, a diamond-studded drill bit is used to cut and retrieve intact, continuous cylindrical samples of the rock (cores). Wireline coring is the modern standard, where the inner core barrel is retrieved rapidly via a wire cable, eliminating the need to pull the entire drill string out of the hole for every sample.
The Standard Penetration Test (SPT)
The Standard Penetration Test (SPT) is the most widely used in-situ geotechnical test worldwide. A hammer is dropped onto a drill rod, driving a split-spoon sampler into the soil. The engineer counts the number of blows required to drive the sampler the final . This blow count, known as the N-value, correlates directly to the soil's strength and relative density.
SPT Corrections ( and )
SPT Corrections
The raw, field-measured SPT N-value is notoriously variable due to differences in drill rig efficiency, rod length, and borehole diameter. Therefore, the raw N-value must be mathematically corrected to a standard energy efficiency of 60%, known as .
SPT N60 Correction
Formula to correct the raw, field-measured SPT N-value to a standard energy efficiency of 60%.
Variables
| Symbol | Description | Unit |
|---|---|---|
| The raw, field-measured SPT N-value | - | |
| Hammer efficiency (can vary from 45% for a donut hammer to 90% for an automatic hammer) | - | |
| Borehole diameter correction | - | |
| Sampler correction (whether a liner is used) | - | |
| Rod length correction | - |
Overburden Correction
Furthermore, for granular soils (sands), the N-value is highly dependent on the depth (overburden pressure). To compare the density of sand at 2 meters vs. 20 meters, is further corrected for overburden pressure to calculate .
Advanced In-Situ Testing and Monitoring
Advanced Testing and Monitoring
Beyond basic SPT, modern investigations heavily utilize specialized testing and long-term monitoring to gather precise data.
Advanced In-Situ Techniques
- Cone Penetration Test (CPT): Pushing an instrumented steel cone continuously into the ground at a constant rate. It measures tip resistance, sleeve friction, and pore water pressure in real-time, providing a continuous, high-resolution soil profile without retrieving physical samples. CPT is vastly superior to SPT for identifying thin, weak liquefiable sand layers.
- Vane Shear Test: Used exclusively in very soft, saturated clays. A four-bladed steel vane is pushed into the clay at the bottom of a borehole and slowly rotated. The torque required to shear the clay cylinder provides a highly accurate, direct measurement of the soil's undrained shear strength ().
- Groundwater Monitoring (Piezometers): Knowing the precise location of the water table is absolutely critical for calculating effective stress and slope stability. Engineers install Piezometers (slotted PVC pipes or vibrating wire sensors) into finished boreholes to measure and monitor static groundwater levels and pore pressures over months or years.
Core Logging & Rock Mass Quality
Evaluating the structural integrity of the rock mass from retrieved core samples.
Core Logging Procedure
When rock is drilled, the retrieved core is placed in a core box and "logged" by a geologist or engineer. They record the rock type, weathering grade, fracture spacing, and structural defects. Three primary quantitative metrics are calculated for every core run:
Core Recovery Metrics
Metrics for Core Recovery
- Total Core Recovery (TCR): The total length of the core recovered (including solid rock, highly crushed rock, and soil-like clay seams) divided by the total length drilled (the core run length), expressed as a percentage. Low TCR indicates highly weathered, weak, or cavernous rock that washed away during drilling.
- Solid Core Recovery (SCR): The total length of only the solid, full-diameter, cylindrical pieces of rock recovered divided by the total length drilled. This completely ignores any crushed rock or soil gouge.
Significance of RQD
The most vital and universally used metric for engineering design is the Rock Quality Designation (RQD).
RQD (Rock Quality Designation)
Interactive Simulation
Interact with the RQD Calculator below to see how core piece lengths affect the rock mass classification.
RQD Calculator
Only sound core pieces greater than 10 cm (highlighted in grey) are counted towards RQD.
Core Pieces
Rock Quality Designation (RQD)
Formula to calculate the Rock Quality Designation, which quantifies rock mass integrity.
Variables
| Symbol | Description | Unit |
|---|---|---|
| The length of solid, intact pieces of rock recovered that are longer than 10 centimeters (4 inches) | cm | |
| The total length drilled for that core run | cm |
RQD Classification System
RQD Classifications
- 0% - 25%: Very Poor (Heavily crushed, highly faulted rock)
- 25% - 50%: Poor
- 50% - 75%: Fair
- 75% - 90%: Good
- 90% - 100%: Excellent (Massive rock, extremely widely spaced joints)
- Site Investigation is a mandatory, phased process consisting of a Desk Study, Field Reconnaissance, Direct Exploration (drilling/test pits), and Geophysical testing.
- Shelby Tubes are critical for retrieving undisturbed clay samples for strength testing, while Split-Spoon samplers provide disturbed samples during SPT testing.
- The raw SPT N-value must be rigorously corrected to to account for hammer efficiency and equipment variations before being used in engineering design.
- CPT provides continuous, high-resolution profiling, while Vane Shear accurately measures soft clay strength in-situ.
- RQD (Rock Quality Designation) quantifies rock mass integrity by measuring the percentage of a core run consisting of solid pieces .