Understanding Geotechnical Reports

Key Sections of a Geotech Report

A complete geotechnical report for construction includes these sections:

• Executive Summary — high-level findings and recommendations for foundations
• Scope and Methodology — where borings were drilled, how deep, testing performed
• Boring Logs — detailed description of soil at each depth
• Laboratory Test Results — classification, strength, and compaction data
• Recommendations — allowable bearing pressure, compaction standards, dewatering notes
• Conclusions — summary of key findings and limitations

Reading Boring Logs

Boring logs are the core of a geotech report. Each log represents a hole drilled at a specific location. Learn to read them:

Allowable Bearing Pressure

The geotech engineer recommends an allowable bearing capacity (usually in PSF — pounds per square foot) for each soil layer. This is the maximum load a footing can safely support.

Example from a report: "Glacial clay below 8 feet depth: allowable bearing capacity = 3,000 PSF." The structural engineer uses this value to design footing sizes. If the bearing capacity is low, footings must be larger (and more expensive).

The geotechnical report will also specify depth of footing (typically 3–4 feet minimum to avoid frost heave and poor surface conditions).

Lateral Earth Pressure for Retaining Walls

If the project includes retaining walls, the geotech report provides lateral earth pressure values. These are used to design wall thickness and reinforcement.

Key values provided:

• At-rest pressure (K₀): Force exerted on wall with no wall movement. Used for walls that cannot move (e.g., basement walls).
• Active pressure (Kₐ): Force when wall leans away from soil (typical for cantilever walls). Lower than at-rest.
• Passive pressure (Kₚ): Resistance provided by soil in front of wall. Used for design of wall toe depth.

The geotech report will state these as coefficients (e.g., Kₐ = 0.35 for a particular soil). The structural engineer multiplies this by soil weight and wall height to calculate the total lateral load.

Compaction Requirements

For fills (areas where soil is added and compacted), the geotech report specifies compaction standards. This is critical for preventing settlement.

Typical specification: "All fill shall be compacted to 95% of maximum dry density (ASTM D698) in 6-inch lifts."

What this means:

• 95% maximum dry density: Soil is compacted to remove voids. Higher percentage = denser, more stable soil. 95% is standard; 98% is more stringent.
• ASTM D698: Standard test method (there's also D1557 for heavier equipment; contractor must know which applies).
• 6-inch lifts: Fill is placed and compacted in 6-inch-thick layers. Helps ensure uniform compaction.
• Proof rolling: After compaction, heavy equipment (vibratory roller or loaded truck) is driven over the area. If ruts appear, additional compaction is required.

Poor compaction leads to settlement, which can crack slabs and damage structures. This is why geotech inspectors are on site during fill placement.

Groundwater: Seasonal Variation and Dewatering

The groundwater level reported in the geotech investigation represents the level on the day of boring. But groundwater fluctuates with seasons, rainfall, and nearby surface water.

A good geotech report will note:

• Historical high water table: Highest level anticipated based on seasonal data. Plan excavation and drainage for this level.
• Seasonal variation: Example: "GWL typically rises 3–4 feet in spring due to snowmelt."
• Dewatering requirements: If excavation extends below GWL, the contractor must pump water to keep the excavation dry. This adds cost and schedule time.

If dewatering is required, the geotech report may recommend a groundwater control system (sumps, wells, or French drains). This should be detailed in the specifications and reflected in site plans.

How Geotech Recommendations Translate to Structural Drawings

The structural engineer uses geotech data to design foundations:

Changed Conditions: When Field Differs from the Report

During excavation, you may encounter soil different from what the geotech report predicted. Example: report says clay at 8 feet, but you hit sand with water at 6 feet.

This is a "changed conditions" situation:

• Stop work at that location. Do not assume the design is still valid.
• Notify the engineer and owner immediately. You need written direction before proceeding.
• The engineer may: Approve continued work if conditions are still acceptable, require additional borings to understand the change, redesign foundations, or recommend dewatering.
• Cost and schedule impact. If conditions are worse than anticipated, the contractor may be entitled to a change order for additional dewatering or redesign.

The geotech report includes a disclaimer that borings are at specific locations and soil may vary. This is why contractors take changed conditions seriously — they can cost significant time and money if not managed properly.

Common Pitfalls When Reading a Geotech Report

• Ignoring limitations. Geotech reports always end with a limitations section. Read it. It explains that boring locations are limited, and soil may vary between borings.
• Not checking boring locations on the site plan. Are borings clustered in one area? If the site is large and only one boring was done, soil could be very different elsewhere.
• Assuming GWL is constant. It fluctuates. Plan for the higher level mentioned in the report.
• Not coordinating with the structural engineer. If you see something concerning in the geotech report (low bearing capacity, high water table, clay), ask the engineer how it affects the design. Don't assume it's been accounted for.
• Missing compaction specs in the narrative. Compaction requirements are sometimes buried in the text, not highlighted. Read carefully.

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