1.1 Site Data Interpretation & Boring Plans

Introduction to Subsurface Exploration

Subsurface exploration is the foundational phase of geotechnical engineering design. The primary objectives of a site investigation are to determine the stratigraphy, soil and rock engineering properties, groundwater conditions, and potential geo-hazards (such as sinkholes, faulting, or expansive clays). A poorly planned or insufficient subsurface exploration program leads to unpredicted foundation settlements, slope failures, cost overruns, and severe structural failures.

Guidance from major agencies—such as the Federal Highway Administration (FHWA), the American Association of State Highway and Transportation Officials (AASHTO), and the American Society of Civil Engineers (ASCE)—dictates systematic procedures for planning exploration extent, selecting boring locations, sampling soils and rocks, and interpreting borehole records.

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Subsurface Exploration Depth Criteria

Determining the minimum depth of exploration ($D_{max}$) is critical to ensure that compressible or weak strata beneath the foundation are identified. Exploration depths must capture all zones of significant stress increase caused by the structure.

Shallow Foundations

For spread footings and mat foundations, borings must extend below the proposed foundation level ($D_f$) to a depth where stress increments become negligible. Two primary criteria are applied:

1. The 10% Stress Increase Rule ($\Delta \sigma \le 0.10 \sigma'_{v0}$): Exploration must continue until the vertical stress increase ($\Delta \sigma$) induced by the structural foundation drops to $10%$ (or in highly sensitive soils, $5%$) of the initial effective vertical overburden stress ($\sigma'_{v0}$).
2. Geometric Footing Rule ($1.5B$ to $2.0B$): For square or isolated footings of width $B$, borings must extend $1.5B$ to $2.0B$ below $D_f$. For continuous (strip) footings of width $B$, where stress dissipation is slower due to 2D plane-strain conditions, borings must extend $3.0B$ to $4.0B$ below $D_f$.
3. Mat Foundations: For large raft/mat foundations of width $B_{mat}$, exploration depth should extend to at least $1.5 B_{mat}$ unless competent bedrock is encountered at a shallower depth.

$\Delta \sigma = \frac{q \cdot B \cdot L}{(B + z)(L + z)} \quad \text{(2:1 Stress Distribution Method)}$

Where $q$ is the net applied foundation bearing pressure, $B$ is foundation width, $L$ is foundation length, and $z$ is depth below the footing base.

Deep Foundations (Piles and Drilled Shafts)

For deep foundations, exploration must extend significantly below the anticipated pile toe elevation:

• Deep Foundation Groups: Minimum $5$ meters ($15$ feet) or $3$ times the pile group width ($3B_g$) below the estimated pile tip elevation.
• Bedrock Termination: Borings terminating on competent bedrock must penetrate a minimum of $3$ to $5$ meters ($10$ to $15$ feet) into intact rock to confirm sound rock rather than an isolated boulder.

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Boring Layout and Spatial Spacing Guidelines

Boring locations must be distributed strategically across the project footprint to capture spatial variability in soil stratigraphy. Minimum spacing requirements specified by AASHTO and FHWA are summarized below:

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Soil and Rock Sampling Techniques

Subsurface sampling is divided into disturbed sampling (suitable for index tests such as sieve analysis and Atterberg limits) and undisturbed sampling (essential for strength and consolidation testing).

Common Soil Samplers

• Standard Split-Spoon Sampler (ASTM D1586): Outer diameter $50.8\text{ mm}$ ($2.0\text{ in}$), inner diameter $38.1\text{ mm}$ ($1.5\text{ in}$). Driven by a $63.5\text{ kg}$ ($140\text{ lb}$) hammer falling $760\text{ mm}$ ($30\text{ in}$). Yields disturbed soil samples.
• Thin-Walled Shelby Tube (ASTM D1587): Seamless steel tube with outer diameter $76.2\text{ mm}$ ($3.0\text{ in}$) or $127\text{ mm}$ ($5.0\text{ in}$) pushed hydraulically into soft-to-stiff fine-grained soils. Area ratio $AR \le 10%$ minimizes disturbance: $AR(\%) = \frac{D_o^2 - D_i^2}{D_i^2} \times 100$
• Pitcher & Denison Samplers: Double-tube core barrels with inner liners designed for sampling hard, stiff, or cemented soils and fragile soft rocks.

Rock Core Evaluation Metrics

When coring rock using diamond core barrels (e.g., NQ, HQ size), core quality is quantified using two primary metrics:

1. Core Recovery (CR): Percentage ratio of total length of rock core recovered to the total length of the core run ($L_{run}$): $CR (\%) = \frac{\sum L_{recovered}}{L_{run}} \times 100$
2. Rock Quality Designation (RQD - ASTM D6032): Percentage ratio of the cumulative length of sound, intact rock core pieces $\ge 100\text{ mm}$ ($4\text{ inches}$) long to the total core run length ($L_{run}$): $RQD (\%) = \frac{\sum L_{pieces \ge 100\text{mm}}}{L_{run}} \times 100$ Note: Fractures caused by mechanical drilling operations must be fitted together and counted as one continuous piece.

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Standard Penetration Test (SPT) Corrections

The field-measured blow count ($N_{field}$) represents the number of hammer blows required to drive a standard split-spoon sampler the final $300\text{ mm}$ ($12\text{ in}$) of a $450\text{ mm}$ ($18\text{ in}$) penetration interval. Because field hammers deliver varying percentages of theoretical kinetic energy, $N_{field}$ must be standardized to an energy ratio of $60%$, denoted as $N_{60}$:

$N_{60} = N_{field} \cdot \frac{E_m}{60\%} \cdot C_B \cdot C_S \cdot C_R$

Where:

• $E_m$ = Hammer energy efficiency factor ($E_m = 60%$ for safety hammer with rope and cathead; $80%$ for automatic trip hammer; $45%$ for donut hammer).
• $C_B$ = Borehole diameter correction factor ($1.0$ for $65\text{--}115\text{ mm}$; $1.05$ for $150\text{ mm}$; $1.15$ for $200\text{ mm}$).
• $C_S$ = Sampler liner correction factor ($1.0$ for standard sampler with liner; $1.2$ for sampler without liner).
• $C_R$ = Rod length correction factor ($0.75$ for rod length $< 4\text{ m}$; $0.85$ for $4\text{--}6\text{ m}$; $0.95$ for $6\text{--}10\text{ m}$; $1.0$ for $> 10\text{ m}$).

To normalize $N_{60}$ for effective overburden stress ($\sigma'{v0}$) in cohesionless soils (sands and gravels), the fully corrected blow count $N{1,60}$ is calculated:

$N_{1,60} = C_N \cdot N_{60}$

Where $C_N$ is the overburden correction factor. According to Liao & Whitman (1986):

$C_N = \sqrt{\frac{P_a}{\sigma'_{v0}}} \le 1.7$

Where $P_a$ is atmospheric pressure ($100\text{ kPa}$ or $1.0\text{ tsf}$ or $2000\text{ psf}$) and $\sigma'_{v0}$ is the effective vertical overburden stress at the test depth.

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Detailed Worked Numerical Example

Problem Statement

A subsurface exploration program for a bridge bent requires an SPT test at a depth of $7.5\text{ m}$ below the ground surface. The subsurface profile consists of:

• $0.0\text{ to } 3.0\text{ m}$: Moist medium dense sand, bulk unit weight $\gamma = 18.0\text{ kN/m}^3$.
• $3.0\text{ to } 10.0\text{ m}$: Dense saturated sand, saturated unit weight $\gamma_{sat} = 19.5\text{ kN/m}^3$.
• Groundwater table is located at a depth of $3.0\text{ m}$ below ground surface ($\gamma_w = 9.81\text{ kN/m}^3$).

The field SPT was performed using an automatic hammer ($E_m = 80%$), a borehole diameter of $150\text{ mm}$ ($C_B = 1.05$), a standard split-spoon sampler without liner ($C_S = 1.20$), and a rod length of $8.5\text{ m}$ ($C_R = 0.95$). The recorded field blow counts for three successive $150\text{ mm}$ ($6\text{ in}$) increments were $6$, $12$, and $14$.

Calculate:

1. Field $N_{field}$.
2. Effective overburden stress $\sigma'_{v0}$ at the test midpoint ($7.5\text{ m}$).
3. Standardized blow count $N_{60}$.
4. Overburden-corrected blow count $N_{1,60}$.

Solution

Step 1: Calculate field blow count $N_{field}$ The field blow count $N_{field}$ is the sum of the last two $150\text{ mm}$ increments: $N_{field} = 12 + 14 = 26$

Step 2: Calculate effective vertical overburden stress $\sigma'_{v0}$ at $z = 7.5\text{ m}$

• Total stress above water table ($0\text{--}3.0\text{ m}$): $\sigma_{v1} = 3.0\text{ m} \times 18.0\text{ kN/m}^3 = 54.0\text{ kPa}$.
• Total stress below water table ($3.0\text{--}7.5\text{ m}$): $\sigma_{v2} = 4.5\text{ m} \times 19.5\text{ kN/m}^3 = 87.75\text{ kPa}$.
• Total stress $\sigma_{v0} = 54.0 + 87.75 = 141.75\text{ kPa}$.
• Pore water pressure $u = (7.5\text{ m} - 3.0\text{ m}) \times 9.81\text{ kN/m}^3 = 44.15\text{ kPa}$.
• Effective overburden stress $\sigma'{v0} = \sigma{v0} - u = 141.75 - 44.15 = 97.60\text{ kPa}$.

Step 3: Calculate $N_{60}$ $N_{60} = N_{field} \cdot \left(\frac{80\%}{60\%}\right) \cdot 1.05 \cdot 1.20 \cdot 0.95$ $N_{60} = 26 \cdot 1.3333 \cdot 1.05 \cdot 1.20 \cdot 0.95 = 41.51 \approx 42$

Step 4: Calculate $C_N$ and $N_{1,60}$ Using $P_a = 100\text{ kPa}$: $C_N = \sqrt{\frac{100\text{ kPa}}{97.60\text{ kPa}}} = \sqrt{1.0246} = 1.012 \le 1.7$ $N_{1,60} = C_N \cdot N_{60} = 1.012 \times 41.51 = 42.01 \approx 42$

The normalized blow count $N_{1,60} = 42$ confirms a dense sand layer suitable for high-capacity foundation support.

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Desk Study: Site Data Sources and Exploration Planning

Before the first boring is drilled, a competent site characterization program begins with a desk study — a systematic review of existing published and archival data that establishes the geologic context, flags potential hazards, and focuses the field exploration budget on the areas of highest uncertainty. NCEES treats desk-study data identification, validation, and interpretation as a distinct exploration skill because misreading (or skipping) it is a common source of scope-inadequate boring programs.

Aerial Photography and Remote Sensing

Historical and current aerial photography reveals site history that a single boring cannot: former ponds, quarries, or borrow pits later backfilled with uncontrolled material; buried stream channels and cut/fill scars from prior grading; agricultural drainage tile; and waste-disposal or fill-placement areas. Multi-decade photo sets (available from the USGS Earth Explorer and state historical imagery archives) are compared sequentially to detect land-use changes invisible in current conditions. LiDAR-derived bare-earth topography and satellite multispectral imagery extend this to vegetation-obscured karst depressions, landslide scarps, and fault scarps.

Geologic and Topographic Maps

USGS and state geological survey quadrangle maps identify bedrock formation, structural features (faults, folds), and surficial (Quaternary) deposit types at regional scale, which sets expectations for bedrock depth, rock quality, and groundwater regime. USGS 7.5-minute topographic quadrangles (or LiDAR contours) are used to estimate natural slope angles, drainage divides, and floodplain extents that affect boring accessibility and scour/erosion exposure. These maps are mapped at scales (commonly 1:24,000) that cannot resolve small, site-specific anthropogenic features — a critical limitation discussed below.

GIS and Geotechnical Databases

State DOT geotechnical GIS portals, the National Geotechnical Experimentation Sites database, and local subsurface-data repositories allow the engineer to pull nearby historical boring logs before mobilizing a rig. This desk-study step often reveals stratigraphic trends (e.g., a known perched water table, a regionally consistent clay layer, or corrosive soil pockets) that directly justify closer boring spacing, additional in-situ testing, or targeted sampling for chemical/index testing.

As-Built Plans and Planning Studies/Reports

As-built construction drawings document prior foundation types and bearing elevations, existing utility alignments and depths (critical for boring-location clearance and utility-conflict avoidance), and prior grading records showing cut/fill limits and dates. Prior geotechnical reports, environmental site assessments, and master planning studies for the same parcel often contain groundwater monitoring history and geohazard flags that should be re-verified, not assumed current.

How Desk-Study Findings Drive the Exploration Plan

Desk-study conflicts and gaps directly modify the boring program: suspected fill zones require borings extended well below the mapped fill base into competent native material; karst-prone geology (from geologic maps) triggers systematic probe borings on a tight grid to detect voids between conventional borings; and expansive-clay regions justify a higher frequency of Atterberg-limit sampling. Utility as-builts are overlaid on the proposed boring layout before mobilization to eliminate strike hazards.

Worked Example: Reconciling Conflicting Map and Boring Data

A 1962 aerial photograph shows an approximately $40\text{ m} \times 25\text{ m}$ pond/quarry at the proposed building footprint. The regional geologic map classifies the site as "residual soil over granite bedrock" with no fill noted. A new boring (B-3), drilled at the mapped pond location, encounters $6.5\text{ m}$ of loose-to-medium-dense heterogeneous fill containing brick and wood debris before reaching native residual soil — directly contradicting the geologic map's implied shallow-bedrock condition.

Using the aerial photo footprint and an assumed average fill depth of $5\text{ m}$ (consistent with the as-built grading record), the estimated fill volume is: $V_{fill} \approx 40\text{ m} \times 25\text{ m} \times 5\text{ m} = 5{,}000\text{ m}^3$

Applying a typical fill-delineation guideline of one supplemental boring per $500\text{–}1{,}000\text{ m}^2$ of suspect footprint ($1{,}000\text{ m}^2$ pond area) indicates 2 to 3 additional borings are warranted to define the fill's lateral limits and confirm depth to competent native soil across the footprint, rather than relying on the single-boring/geologic-map interpretation. The published geologic map is correct at regional scale but cannot resolve site-specific anthropogenic fill; site-specific boring and as-built data always govern over generalized mapping when they conflict.