Geotechnical Reports and Construction Drawings: Reconciling the Subsurface

The Geotech Gap

A foundation crew is excavating for spread footings on a commercial building. The plans call for footings founded at 4 feet below finished grade on "dense sand and gravel." At 3.5 feet, they hit clay. At 5 feet, they hit more clay. The soil encountered is nothing like what the geotech report predicted.

Was the report wrong? Maybe. But more likely, the boring location shown in the geotech report doesn't correspond to the actual footing location shown on the structural drawings. The report showed subsurface conditions at boring location B-1, which was 50 feet away from where the footing is actually being constructed.

What follows: the foundation contractor requests a change order because actual conditions differ from the plans. The owner argues the plans were based on the geotech report. The geotech consultant argues their report is valid at the boring locations but can't guarantee conditions between boreholes. Discussions escalate into disputes about responsibility.

The root cause: nobody reconciled the geotech report with the structural drawings to verify that:

• Boring locations on the geotech plan corresponded to planned structure locations
• Subsurface conditions predicted by the report matched the footing depths and bearing capacity assumptions in the structural design
• Special site conditions noted in the report (groundwater, weak layers, settlement potential) were reflected in the specifications

What's in a Geotechnical Report

A comprehensive geotech report includes several key sections that are relevant to construction drawings:

Boring Logs and Subsurface Profile

Boring logs show the soil and rock layers encountered at each boring location, from the surface to the depth that was bored. Each log includes:

• Soil classification: Sand, silt, clay, gravel, rock, etc., with grain size and plasticity descriptions
• Depth intervals: The depth range where each soil type was encountered
• Consistency or density: Qualitative descriptions (loose, medium, dense) or SPT N-values (standard penetration test blow counts)
• Special observations: Color, cementation, groundwater, voids, fossils, etc.

A site plan shows the locations of boreholes relative to the planned structure. If boreholes are clustered in one area and the building footprint is much larger, significant areas of the site have no direct boring data. The geotech report typically notes this limitation.

Groundwater Conditions

The report documents the depth of the water table at each boring location and the time it was measured. Seasonal variation in groundwater elevation is typically discussed. If the water table is high, the report addresses implications for excavation, dewatering, foundation waterproofing, and basement construction.

Construction documents must address groundwater:

• If a basement or sub-grade level is planned, foundation waterproofing and drainage details must account for actual groundwater elevation.
• If dewatering will be required during construction, the specifications must address pump capacity, discharge location, and environmental controls.
• If a sump pump system is needed, the drawings and specifications must show its location, size, and discharge routing.

Bearing Capacity and Settlement Analysis

The report calculates the bearing capacity of the soil at various depths and recommends foundation design parameters:

• Allowable bearing capacity: The maximum pressure (in kips per square foot) that the soil can safely support. This directly feeds into footing size calculations on the structural plans.
• Recommended footing depth: The depth at which bearing capacity is adequate. The structural plans must show footings at this depth or deeper.
• Settlement calculations: For certain sites, the report estimates total and differential settlement. If settlement is significant, special detailing (settlement joints, sloped floors) may be required.

If the structural engineer uses a bearing capacity value different from the geotech report's recommendation, the structural calculations and geotech report are now in disagreement. This mismatch must be resolved before construction.

Slope Stability and Lateral Earth Pressure

For sites with significant slopes or where excavation will create slopes, the geotech report analyzes slope stability and recommends:

• Slope angle and height: The steepest slope that can be safely excavated without support
• Lateral earth pressure: For retaining walls, the pressure that the retained soil exerts on the wall, which determines wall design requirements
• Slope stabilization measures: If slopes require protection, the report may recommend berms, terracing, soil nailing, or slope drains

These recommendations must be reflected in the construction drawings and specifications. If the site plan shows a 60-degree excavation slope and the geotech report recommends a 35-degree maximum, the structural plans need to account for this constraint through either shallower cuts or supported slopes.

Foundation Recommendations

The geotech report typically recommends the type of foundation best suited to the site:

• Spread footings: For sites with adequate bearing capacity at shallow depths
• Mat foundations: For sites with variable bearing or where spread footings are uneconomical
• Pilings: For sites where bearing soil is deep, compressible, or where settlement cannot be tolerated

If the structural design uses a different foundation type than recommended, the reasons should be documented and the geotech engineer should be notified.

Special Conditions and Cautions

Geotech reports often flag special site conditions:

• Expansive soils: Soils that swell when wet and shrink when dry, which requires special foundation design
• Corrosive conditions: Soils or groundwater that attack concrete or steel, which requires protective measures
• Collapsible soils: Certain granular soils that lose strength rapidly when saturated
• Potential for liquefaction: In seismic areas, soils that may liquefy during earthquakes
• Contamination: Evidence of fill, mining, or industrial activity that may have left contaminants

These conditions must be addressed in the construction documents. If the geotech report notes expansive soils but the specifications call for standard concrete footings, the design is not coordinated with the report.

How Geotechnical Information Translates to Structural Drawings

The structural engineer uses geotech recommendations to design foundations. The translation process includes:

1. Footing Depth

If the geotech report recommends footings at 3 feet below finished grade, the structural plans must show footings at this depth (or deeper, if the structural engineer chooses). A foundation plan that shows footings at 2 feet is inconsistent with the geotech recommendation and will encounter inadequate bearing soil.

2. Footing Size and Reinforcement

The allowable bearing capacity from the geotech report determines footing size. If the soil can support 3 kips per square foot at the recommended depth, larger footings will be required than if it could support 4 kips per square foot. The structural plans must show footing sizes that are consistent with the allowable bearing capacity used in calculations.

3. Pile Type and Length (if applicable)

If the site requires piles, the geotech report recommends pile type (driven, drilled, helical) and design parameters (bearing layer depth, bearing capacity). The structural plans must specify pile details, lengths, and capacities consistent with the geotech recommendation.

4. Special Details (Waterproofing, Dewatering, Backfill)

Based on groundwater conditions, site history, and soil type, the geotech report recommends specific construction practices. The specifications and details must include:

• Basement waterproofing: If high groundwater is present
• Foundation drainage: Perimeter drains and sump pump systems if needed
• Backfill material and compaction: Specifications for fill material type and compaction density
• Dewatering method: If required during excavation

Common Geotech-Drawing Coordination Failures

1. Boring Locations Don't Match Structure

The geotech site plan shows boreholes clustered in one corner of the property, but the building will be constructed in a different area. The geotech recommendations are based on subsurface conditions in the boring locations but may not apply to where the building actually sits.

During preconstruction review, overlay the geotech boring plan with the structural plan. If significant areas of the building footprint lack boring data nearby, flag the gap. Consider additional boreholes in critical areas before finalizing foundation design.

2. Footing Depths Are Too Shallow

The geotech report recommends 4-foot footing depth, but the structural plan shows 3-foot footings. Maybe the designer was trying to minimize excavation costs or didn't fully read the recommendation. Either way, the footings are underdesigned relative to the geotech analysis.

3. Bearing Capacity Assumptions Are Inconsistent

The geotech report calculates allowable bearing capacity at 3 kips per square foot. The structural calcs assume 3.5 kips per square foot. This 17% discrepancy means footings are underdesigned, or the geotech engineer's recommendation is being ignored.

4. Special Site Conditions Are Ignored

The geotech report notes "loose fill material observed in the eastern portion of the site" but the structural plans show footings and basements in that area at depths that assume competent natural soil. The conflict is unresolved.

5. Groundwater Conditions Aren't Addressed

The geotech report documents groundwater at 6 feet below finished grade, but the structural plans show a basement at 8 feet below grade with no waterproofing details or drainage system specified. The design hasn't accounted for the reported groundwater condition.

6. Pile Recommendations Aren't Reflected

The geotech analysis recommends piles driven to a certain depth and capacity, but the structural plans show the piles to a shallower depth. Either the structural engineer has reasons for the change (which should be documented), or the geotech recommendation is being ignored.

What GCs and Project Managers Need to Know

The geotech report is not just a design consultant document—it's a critical construction reference. GCs and project managers should:

1. Read the Full Geotech Report

Don't just file it away. Read the recommendations section and the special cautions. Understand what soil conditions are expected. If the report says "weak clay layer at 8 feet," your crews need to know that when they excavate.

2. Reconcile Geotech with Structural Plans

During preconstruction planning, compare the geotech report with the structural plans. Verify:

• Footing depths match geotech recommendations
• Bearing capacity assumptions in the structural calcs are consistent with the geotech report
• Special site conditions are addressed in construction documents
• If actual conditions during construction differ from the report, flag the variance immediately

3. Be Ready for Field Condition Changes

Even thorough preconstruction planning can't predict every subsurface surprise. Be prepared for field conditions that differ from boring logs. If excavation reveals soil types not expected, document the condition, notify the geotech engineer and structural engineer, and get guidance before continuing.

4. Keep Geotech Accessible

The geotech report should be part of the project construction documents. The site superintendent and foundation contractor need access to it. If they're aware of what conditions to expect, surprises are fewer.

Best Practices for Preconstruction Review

1. Create a Reconciliation Matrix

Develop a simple matrix comparing geotech recommendations to structural plan details. For each boring location, verify that the nearby structure (footings, piles, basement) is designed consistent with the geotech data at that location.

2. Overlay Boring Locations on Structural Plans

Plot boreholes on the structural plan to visually assess coverage. Areas with no nearby boreholes are areas where geotech extrapolation is larger and risks are higher.

3. Verify Bearing Capacity Assumptions

Request the structural design calculations and confirm that the allowable bearing capacity used matches the geotech recommendations. If they differ, get written explanation from the structural engineer.

4. Check Specifications for Geotech Requirements

The specifications section should include details on dewatering, backfill, excavation support, and waterproofing that are responsive to geotech recommendations. If critical items are missing, add them before construction begins.

5. Use Comprehensive Plan Review

During preconstruction document review, specifically compare geotech recommendations with structural and specification details. Use this as an opportunity to surface any gaps or inconsistencies before construction begins.

The Prevention Value

A geotechnical report that goes unread and unreconciled with structural plans is a wasted investment. The report is developed to guide design and construction decisions. When it's ignored, the benefit is lost, and field surprises multiply.

Spending a day or two reconciling geotech recommendations with structural plans before construction begins can prevent weeks of delays and tens of thousands of dollars in change orders when subsurface conditions are encountered.

The geotech report is project knowledge. Use it.