Geotechnical Reports: Boring Logs, Foundation Recommendations, and Program Implications

Interpreting geotechnical investigation reports including boring logs, soil classifications, Standard Penetration Test data, and foundation recommendations. Evaluating how subsurface conditions affect building program feasibility, foundation system selection, and site development decisions during the programming and analysis phase.

ARE · Programming & Analysis

Overview

Before you can design a foundation, you need to know what's underneath the site. That's where geotechnical reports come in. These documents translate raw subsurface data into actionable recommendations that directly shape your building program, foundation system, and construction budget.

A geotechnical report typically includes boring logs showing soil strata at specific locations, laboratory test results on recovered samples, groundwater observations, and engineering recommendations for foundation design. The boring log is the backbone of the report. It records depth, soil type, blow counts from the Standard Penetration Test (SPT), sample recovery data, and groundwater levels encountered during drilling.

For the ARE, you won't need to design foundations yourself. But you absolutely need to read a geotechnical report and understand what it means for the project. Can the site support shallow footings, or does the soil demand deep foundations? Is there a high water table that will complicate below-grade construction? Are there fill layers or organic soils that can't support loads? These answers drive program decisions about basement levels, structural systems, and construction cost.

The architect's role is to synthesize this technical data with the building program, evaluate feasibility, and coordinate with the geotechnical engineer and structural engineer to align site realities with project goals.

Essential

A geotechnical report lands on your desk. It's 40 pages of soil descriptions, test data, and engineering jargon. Here's how to extract what matters for your building program. ## What a Geotechnical Investigation Includes The investigation starts in the field. A drilling crew advances borings at planned locations across the site, recovering soil and rock samples at regular intervals. According to USACE guidance, only qualified geotechnical professionals should plan the investigation, and only competent personnel should perform the drilling and data collection. The number, depth, and spacing of borings depend on the project size, the anticipated foundation system, and known geological conditions. The investigation program typically includes several components: soil borings with continuous or interval sampling, in-situ testing (Standard Penetration Tests at minimum), groundwater monitoring, and laboratory analysis of recovered samples. For sites with suspected sinkholes or karst geology, the program may also include ground-penetrating radar or electrical resistivity surveys.

Professional

An architect receives a geotechnical report for a proposed four-story mixed-use building on a downtown infill site. The boring logs reveal 8 feet of miscellaneous fill over soft clay extending to 25 feet, underlain by dense sand. The SPT N-values in the fill range from 3 to 8, and the clay shows N-values of 2 to 5. The water table sits at 10 feet below grade. The fill is unsuitable as a bearing stratum. It's heterogeneous, poorly compacted, and potentially contains debris. The soft clay below won't support spread footings without excessive settlement. The geotechnical engineer recommends deep foundations: auger-cast piles bearing in the dense sand at 25+ feet. This changes the project economics. The original budget assumed conventional shallow footings. Deep foundations add significant cost per pile, plus a pile cap system and grade beams.

TLDR

Boring logs record soil layers, SPT blow counts, sample recovery, groundwater depth, and rock quality at each borehole location
The SPT N-value equals the blow count for the second and third 6-inch increments only; the first 6 inches is the seating drive and is excluded
Higher N-values mean denser/stiffer soil: N < 4 in sand is very loose, N > 30 in clay is hard
RQD (Rock Quality Designation) measures intact core pieces longer than 4 inches as a percentage of the core run; 90-100% is excellent, below 25% is very poor
Foundation recommendations hinge on two factors: bearing capacity (can the soil support the load?) and settlement (will it deform too much over time?)
Clay soils are the classic trap: adequate bearing capacity but excessive long-term consolidation settlement
Deep foundations (piles, drilled shafts) transfer loads through weak soils to competent bearing strata; they cost significantly more than shallow footings
The architect synthesizes geotechnical data with the building program, not designs the foundation

ELI5

Think of building a sandcastle near the water. You wouldn't just start building without checking the sand first, right? Is it wet? Dry? Packed tight or loose and crumbly? That's what a geotechnical report does for real buildings. Engineers drill holes into the ground at different spots across the site. They pull out tubes of dirt and rock, like taking a core sample out of a layer cake. Each hole gets a "boring log," which is a detailed record of every layer they found on the way down. Sandy layer at 3 feet. Clay at 10 feet. Rock at 25 feet. They also do a test at each level called the Standard Penetration Test. Imagine dropping a heavy weight onto a tube and counting how many times you have to drop it before the tube pushes 6 inches into the ground.

AREProgramming & Analysis

Geotechnical Reports: Boring Logs, Foundation Recommendations, and Program Implications

2 min read204 words

Geotechnical Reports: Reading the Ground Beneath Your Building

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