Chapter 18 geotechnical investigations, soil classification, foundation types, driven piles, drilled shafts.
3
hours
0.3
CEUs
Codes and Standards
1.7.3
This course covers material relevant to the following ICC certification exams:
Chapter 18 geotechnical investigations, soil classification, foundation types, driven piles, drilled shafts.
Format
On-Demand Online
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Self-Paced
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Certificate of Completion
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Contact our support teamUnderstand geotechnical investigation requirements and soil classification systems
Every load a building carries eventually reaches the ground, and the ground is the one structural "material" nobody manufactured to a specification. Concrete, steel, and wood arrive with known properties; the soil beneath a foundation is whatever nature left behind, and it can vary within the footprint of a single building. That is why soils and foundations work sits at the front of any serious plan review: if the ground assumption is wrong, everything built on top of it — walls, floors, roof, life-safety systems — inherits that error, and foundation failures are notoriously expensive and difficult to correct after the fact compared with catching the problem on paper.
The starting question on every project is simple to ask and consequential to answer: is the soil good enough, in a known and documented way, to rely on ordinary assumptions, or does this site need an engineer to go look? Most low-risk, conventional construction on obviously competent ground can lean on the presumptive load-bearing values the code allows for recognized soil and rock classifications — a simplified path meant for ordinary conditions where the classification is not in real dispute. But it is a default for the ordinary case, not a substitute for judgment. The code requires a full soils investigation and written geotechnical report whenever conditions take the site out of the ordinary case: unknown or unverified soil classification, expansive or otherwise problematic soil, questionable bearing capacity, groundwater complications, larger or heavier structures, or any site where the reviewer has reason to doubt that presumptive values fairly represent what is actually there.
A geotechnical report earns its place in the submittal package because it replaces assumption with investigation. A competent report classifies the soil, states the bearing capacity the design may rely on, describes groundwater conditions, and gives specific recommendations for foundation type, depth, and any special ground preparation needed before the site can be trusted to carry the building. Those recommendations become the design basis, and the construction documents should reflect them directly, not generic prescriptive assumptions borrowed from an unrelated site.
Consider a site where a preliminary geotechnical report flags variable fill in one portion of the lot and expansive clay in another, so a single presumptive bearing value cannot honestly describe the whole footprint. A reviewer working this file methodically starts by confirming whether the investigation was triggered correctly and whether its scope matches the conditions actually found — not just the conditions the applicant assumed going in. From there, the review traces every design decision on the foundation drawings back to a specific recommendation in the report: bearing depth relative to competent material, any required overexcavation or engineered fill, groundwater or drainage measures, and the foundation type selected for each portion of the building. Anywhere the drawings diverge from the report's recommendations without an engineer's sign-off, that is a flag, not a technicality — it means the structure may be designed for ground conditions never actually verified where the design assumes them.
In the field, an inspector on this same project is verifying that the soil actually exposed in the excavation matches what both the report and the drawings describe. If a footing excavation reveals loose, disturbed fill where the geotechnical report and the approved drawings assumed undisturbed, competent native soil, that is not a condition an inspector should judge informally by eye and wave through. The correct response is to stop the pour, document what was actually observed against what was assumed, and route the discrepancy back to the geotechnical engineer and engineer of record for a documented resolution — whether that means overexcavation and engineered fill, a redesign of the footing, or confirmation that the material is acceptable after all. Approving concrete on unverified bearing material because the crew is on-site and the schedule is tight is exactly the shortcut that later produces settlement cracks and a much more expensive repair than the delay would have cost.
The most common failure is treating the presumptive-values path as the default for every project rather than as the fast lane reserved for genuinely ordinary ground — reviewers wave a project through on assumed bearing capacity when site history, visible fill, or regional soil behavior should have triggered a real investigation. A related mistake is accepting a geotechnical report as a compliance checkbox without reconciling its recommendations against the foundation drawings line by line; a report that recommends a specific foundation type or drainage measure does nothing if the structural drawings quietly specify something else. In the field, the parallel error is inspecting a footing excavation against a mental image of "looks fine" rather than the documented soil classification the design relied on.
The correction is the same discipline every time: treat the geotechnical report, when one exists, as the governing design basis, verify that every foundation-related decision on the drawings traces back to it, and require a documented, engineer-reviewed resolution — not a field judgment call — whenever installed conditions do not match what the report and drawings assumed.
Code Reference: IBC Section 1803 - Establishes geotechnical investigation scope and reporting requirements.
Apply foundation design provisions for different soil conditions
Once the soil is understood, the next decision is which foundation system fits it. Shallow foundations — spread footings under isolated columns, continuous strip footings under walls, and mat or raft foundations that spread the whole building load over a broad slab — work when reasonably competent bearing material is available at a practical depth near grade. They are simpler to build and inspect, and they are the default choice whenever the soil investigation supports them. Deep foundations become necessary when competent bearing material is too far below grade for a shallow footing to reach economically, when the near-surface soil cannot be trusted to carry the load without excessive settlement, or when the structure needs resistance to uplift or lateral forces that a shallow footing cannot reliably provide. Choosing between the two families is not a matter of preference; it follows directly from what the soils investigation actually found.
Whichever foundation type is selected, several protective concepts apply regardless of soil classification. Footings are required to bear at a depth sufficient to stay below the zone where seasonal freezing and thawing of the ground can occur, because a footing sitting inside that zone can be pushed upward by frost heave as the surrounding soil freezes and expands — a movement that cracks foundations and telegraphs damage straight up through the structure above. Good drainage design and appropriate damp-proofing or waterproofing at foundation walls exist for the same underlying reason: water allowed to pool against or beneath a foundation undermines bearing capacity over time, promotes frost action, and drives hydrostatic pressure against walls never designed to resist it. Where expansive soils are present, the code layers on additional protective provisions specifically because ordinary shallow-foundation assumptions do not hold — the same clay that carries a building safely when dry can lift and crack that foundation as it absorbs moisture and swells.
Retaining walls deserve the same engineering rigor as building foundations, not less, because they carry both a foundation problem and a lateral-load problem at once. A wall holding back soil on a sloped site must be designed for the pressure that retained soil exerts against it, must manage the water that collects behind it so hydrostatic pressure does not compound the soil pressure, and must itself bear on soil capable of carrying its weight without excessive settlement or rotation. Where a structure sits near the top of a descending slope, the code also addresses the setback relationship between the building and the slope face, recognizing that a foundation placed too close to an unsupported slope edge can lose support as the slope erodes or fails over time.
Picture a sloped residential lot where the design calls for a retaining wall near the property line to create a level building pad, with site drainage designed to route stormwater around and below the wall rather than letting it pond behind it. A thorough plan review does not stop at confirming the wall has a stamped structural design; it checks that the drainage plan and the wall design were actually coordinated with each other — that weep holes, drainage fill, or a subsurface drain system behind the wall match what the civil or grading plans show, and that the building foundation's relationship to the slope and to the wall has been evaluated for the setback protections the code intends. A wall and a drainage system engineered independently, without cross-checking, is a common source of failures that show up years later as bulging walls or waterlogged, unstable soil.
In the field, the inspector's job on this same project is to verify that what gets built matches both the structural drawings for the wall and the drainage details that make the wall's design assumptions valid — backfill material, drainage layers, and outlet points installed as shown, not substituted informally. A retaining wall that looks sound on the day of backfill can still fail years later if the drainage behind it was never actually built as designed.
A frequent mistake is selecting a foundation type based on what is fastest or cheapest rather than what the actual soil investigation supports — defaulting to a shallow footing where the report calls for deeper bearing material, or skipping frost protection on an addition because the existing building next to it does not appear to need it. Retaining walls are commonly under-scrutinized as "just a wall" rather than evaluated as the combined foundation-and-lateral-load structure they actually are, with drainage treated as an afterthought.
The correction is to hold foundation and retaining wall reviews to the same standard as any other structural element: confirm the selected system is supported by the soils information for that specific location, confirm frost protection and drainage measures are addressed everywhere they apply, and require any retaining wall submittal to include coordinated structural and drainage documentation.
Code Reference: IBC Sections 1805 through 1809 - Governs foundation wall, footing, slab, and retaining wall design/application.
Understand pile and drilled shaft design, installation, and testing requirements
Deep foundations exist to solve a problem shallow footings cannot: reaching bearing material that is too deep, too weak, or too variable near the surface to support the building directly. Driven piles and drilled shafts (also called caissons or piers, depending on installation method) transfer the building's load down through weak or unreliable upper soil layers to a stronger stratum below, either by bearing directly on that stronger material or by developing friction resistance along the surrounding soil. Because the load path runs through material the design team cannot see once construction begins, deep foundation work depends far more heavily on verification during installation than shallow foundation work does — there is no equivalent to simply looking into an open excavation and confirming the bearing soil matches expectations.
That is why testing and quality control are treated as integral parts of deep foundation work rather than optional add-ons. Installation records — how a pile behaves as it is driven, or what material a drilled shaft actually encounters as it is excavated — are compared against the capacity the geotechnical report and structural design assumed. Where field conditions match those assumptions, the foundation is performing as intended; where they diverge, the divergence has to be resolved by the geotechnical engineer and engineer of record before construction proceeds, not worked around informally. Special inspection has a particularly important role here, because deep foundation work is exactly the kind of high-consequence, hard-to-verify-after-the-fact work that the code's structural plan review framework is designed to catch in real time, before it is buried and irreversible.
Consider a deep foundation project where the piles being installed are behaving noticeably differently than the geotechnical baseline predicted — reaching apparent refusal earlier than expected in some locations, or requiring far more effort to reach anticipated depth in others. A disciplined response does not treat this as a contractor problem to be pushed through on schedule; it treats it as a design-assumption problem that needs to be resolved before more piles go in. That means pausing affected work, documenting exactly what was observed at each location, and routing the discrepancy to the geotechnical engineer and engineer of record so they can determine whether the variation reflects normal soil variability, or whether the subsurface conditions differ meaningfully from what the original investigation found — in which case the foundation design itself may need to be reevaluated for those locations.
The plan-review and inspection roles work together on this kind of finding: the reviewer's job earlier in the process was to confirm the geotechnical report's recommendations were reflected in the foundation design and that a special inspection and testing program was specified for the deep foundation work; the inspector's job in the field is to make sure that program is actually executed and that anomalies get documented and escalated rather than quietly absorbed into "the way it went."
A recurring failure is accepting deep foundation installation records as paperwork to file rather than data to evaluate — logging that piles were installed without actually checking the installation behavior against the criteria the design relied on for capacity. Another is allowing installation to continue at full pace after an anomaly appears, on the theory that it will "probably be fine," rather than pausing for the engineer of record and geotechnical engineer to weigh in. A third is treating special inspection for deep foundations as a formality rather than the primary means of verifying work that cannot be checked any other way once complete.
The correction is to build verification into the installation sequence itself: require that installation records be compared against design criteria as work proceeds, require documented engineering sign-off on any anomaly before continuing in that area, and treat special inspection reports for deep foundation work with the same weight as the structural drawings, since they may be the only direct evidence of what is actually holding the building up.
Code Reference: IBC Section 1810 - Regulates deep foundation design, detailing, installation, and testing procedures.
Soils and foundations work rewards a consistent habit more than any single piece of technical knowledge: always ask what the ground actually is before trusting what the drawings assume it to be. That means recognizing when ordinary assumptions are appropriate and when a genuine investigation is required, matching the foundation system to soil conditions actually verified, protecting every foundation against frost, water, and expansive soil behavior, and holding deep foundation work to the standard of continuous verification it demands because so little of it can be checked after the fact.
The common thread through plan review and field inspection alike is documentation that connects design assumptions to what is actually found and built. A geotechnical report never reconciled against the drawings, a retaining wall designed without its drainage system, or an installation record nobody compared against design criteria all represent the same failure: a gap between what was assumed and what was verified. Closing that gap is what keeps foundation failures rare instead of routine.