2021 IBC Chapter 18
Every load in a building, from the roof snow or rain to the people on the top floor, eventually ends up in the soil. Chapter 18 of the 2021 IBC is about that last handoff: how much the ground can carry, how it behaves when it gets wet or dry, and how deep and wide the footings have to be. In the Houston area the soil question is not academic. Much of the region sits on clay that swells in wet weather and shrinks in dry weather, and the cracked slabs, sticking doors and sloping floors in the older buildings GCHS inspects usually start there.
Section 1803.2 says a geotechnical investigation is conducted under Sections 1803.3 through 1803.5 unless the building official waives it. The waiver is allowed only where satisfactory data from adjacent areas show that an investigation is not necessary for the conditions the section lists. That is a judgment call for the official, not a right of the builder, and a waiver is easy to defend on a small addition next to a building with a known soil history and hard to defend on a vacant lot. Under 1803.3 the soil is classified from borings, test pits or other exploration, and under 1803.4 a fully qualified representative of the registered design professional must be on site during all boring or sampling. Section 1803.5.2 adds a useful trigger: where soil classification, strength or compressibility is in doubt, or where someone claims a bearing value better than the code's own presumptive values, the official can require an investigation.
The report itself covers more than bearing capacity. Section 1803.5.4 asks whether the ground-water table is above or within 5 feet below the lowest floor where that floor is below the finished ground next to the foundation, unless waterproofing under Section 1805 is provided. Section 1803.5.5 lists what a deep foundation report must contain, such as recommended types and capacities, installation procedures and load tests.
Section 1803.5.3 gives a strict definition. A soil is considered expansive only if it meets all four of these:
The first three tests can be skipped if the expansion index test is performed, so in practice the geotechnical lab often runs the expansion index alone. The word "all" matters. A soil with a high plasticity index but a low expansion index is not expansive under this definition, and an inspector should read the report's classification rather than guess from the color or stickiness of the dirt.
Once a soil is expansive, Section 1808.6 gives the designer two ways to deal with it. Under 1808.6.1 the foundation is designed to resist the differential volume change and prevent uplift. Under 1808.6.2 a slab-on-ground, mat or raft foundation is designed using the Wire Reinforcement Institute and Concrete Reinforcing Steel Institute design method for slab-on-ground foundations or the Post-Tensioning Institute design standard PTI DC 10.5. Two exceptions let the designer skip that work: the expansive soil is removed to a depth that gives a constant moisture content and the replacement fill is not expansive (1808.6.3), or the building official approves stabilization of the soil (1808.6.4).
Section 1806.2 caps the load-bearing values used in design for soils near the surface at the values in Table 1806.2, unless data to substantiate higher values are submitted and approved. The table sorts soil and rock into five classes. The layout below is rebuilt with only what this lesson teaches.
| Class | Material | Vertical bearing (psf) | Lateral bearing (psf per ft of depth) |
|---|---|---|---|
| 1 | Crystalline bedrock | 12,000 | 1,200 |
| 2 | Sedimentary and foliated rock | 4,000 | 400 |
| 3 | Sandy gravel and gravel (GW, GP) | 3,000 | 200 |
| 4 | Sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC) | 2,000 | 150 |
| 5 | Clay, sandy clay, silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH) | 1,500 | 100 |
Two footnote-style rules matter in the field. Lateral bearing may be increased by the tabular value for each additional foot of depth, up to 15 times the tabular value (1806.3.3), and for class 5 soils the lateral sliding resistance cannot exceed one-half of the dead load (1806.3.2). Notice where the everyday Gulf Coast soils land: the clays are class 5, the lowest bearing value in the table, and they are also the soils most likely to be expansive. A soil can be weak and moving at the same time.
A worked example. A column on a small community building carries 60,000 pounds and bears on class 5 clay at the presumptive 1,500 psf. The footing area needed is 60,000 divided by 1,500, which is 40 square feet. A square footing needs a side of the square root of 40, about 6.32 feet, so a 6 foot 4 inch square footing (about 40.1 square feet) is the smallest whole-inch size that works. The same column on class 4 sand at 2,000 psf needs only 30 square feet, roughly a 5 foot 6 inch square.
Section 1809.4 sets the floor for every footing: the bottom is at least 12 inches below the undisturbed ground surface, and the width is at least 12 inches. "Undisturbed" is the key word. Fill placed last week does not count as ground, so the depth is measured from the natural surface (or the cut surface), not from a graded pad. Section 1809.5 adds frost protection where it applies. Footings extend below the frost line of the locality, or are built to ASCE 32, or bear on solid rock. There is an exception for free-standing Risk Category I buildings of 600 square feet or less if light-frame (400 square feet if not light-frame) with an eave height of 10 feet or less. Section 1809.5.1 also protects exterior landings of required exits that have outward-swinging doors, to the extent needed to keep the doors operable. Frost depth is a local number and it is shallow on the Gulf Coast, so in Houston the 12-inch minimum and the soil report usually govern; check the adopted code and local amendments for the value your jurisdiction uses.
Where a specific design is not provided, Table 1809.7 gives footing sizes under light-frame walls by the number of floors supported. One floor calls for a 12 inch wide, 6 inch thick footing. Two floors call for 15 inches wide and 6 inches thick, and three floors call for 18 inches wide and 8 inches thick. A footing may carry a roof in addition to the floors listed, and interior stud-bearing walls may use isolated footings twice the table width spaced not more than 6 feet on center. Under walls other than light-frame, Section 1809.8 requires plain concrete footings to be at least 8 inches thick at the edge (with a 6 inch allowance for Group R-3 within limits). A prescriptive table does not override an engineered design or an expansive-soil report; if the plans include a design, build the design.
Water is what moves clay, so the code controls it at the surface. Section 1804.4 requires the ground next to the foundation to slope away at a rate of at least 5 percent (1:20) for at least 10 feet measured perpendicular to the wall. If a lot line or obstruction prevents 10 feet, the 5 percent slope runs to an approved alternative means of diverting water. Swales within 10 feet slope at least 2 percent, and impervious surfaces within 10 feet slope at least 2 percent away from the building. Where climate or soil conditions warrant, the slope may be reduced to 1:48 (2 percent).
In the field
On a rehab of a one-story community hall built on a slab, GCHS found a long diagonal crack at the corner, doors that stuck in August, and a downspout that discharged at the foundation. The bare dirt against the wall sloped toward the building, and a hose bib leaked into the planting bed. Moisture at the edge of a slab on clay makes that edge heave while the dry center stays put. Before quoting any slab repair, look first at grading, downspouts, leaks and trees, ask for the original geotechnical report, and check that the fix drains water 10 feet away from the wall. Otherwise the crack comes back.
When shallow footings cannot carry the load, or the soil near the surface is too weak or too moving, the designer uses deep foundations such as driven piles or drilled piers, covered in Section 1810. Section 1810.1.1 requires a geotechnical investigation under Section 1803 for them, and the inspection of deep foundations is a special inspection item you will meet in Lesson 16. For the rest of the chapter, use this map: 1805 dampproofing and waterproofing, 1806 presumptive values and lateral resistance, 1807 foundation walls, retaining walls and embedded posts, 1808 foundations in general, and 1809 shallow foundations.
1. Under 2021 IBC Section 1803.5.3, a soil is considered expansive only when it meets:
2. A geotechnical lab tells a designer that the first three tests under 1803.5.3 need not be run when the expansion index test is performed. Which statement is correct?
3. A slab-on-ground for a Group B office is on expansive soil and is designed under Section 1808.6.2. The design method that section names is:
4. An inspector reads the plans for a Group S-1 warehouse on expansive clay. The designer avoids designing for soil movement by taking the exception in Section 1808.6.3. What must the plans show?
5. Under Table 1806.2, the presumptive vertical foundation pressure for class 5 soil (clay, sandy clay, silty clay, clayey silt, silt, sandy silt) is:
6. A column carries 90,000 lb and bears on class 4 sand (SW, SP, SM, SC, GM, GC) at the presumptive vertical value. The minimum footing bearing area is:
7. An inspector checking a Group R-2 plan sees a continuous footing that carries 4,500 lb per linear foot on class 5 clay at the presumptive value. The minimum footing width needed for bearing is:
8. Under Section 1806.3.3, lateral bearing may be increased for each additional foot of depth by the tabular value, up to a maximum of:
9. For class 5 soils, Section 1806.3.2 limits lateral sliding resistance to no more than:
10. An inspector measures a footing bottom at 10 in. below the undisturbed ground surface on a Houston-area site where frost is not a factor. Under Section 1809.4, this footing:
11. Which building may be exempt from frost protection of footings under Section 1809.5?
12. A light-frame wall on a small multifamily building supports two floors, and no specific footing design is provided. Table 1809.7 calls for a footing of at least:
13. An inspector measures 2 in. of fall in 10 ft away from the wall of a Group A-3 community building. Under Section 1804.4, this grading is:
14. Section 1803.5.4 requires a geotechnical report to address the ground-water table when: