Foundation & Soil Terms —
Plain Language.

A fan-shaped deposit of sediment laid down where a stream or wash exits a mountain canyon and spreads across a flatter surface. In New Mexico, alluvial fans extend from every mountain range — the Sandias, the Organs, the Guadalupes, the Peloncillos — onto the adjacent basin floors. The material is sorted by energy: coarser gravel and cobble near the canyon mouth, finer silt and sand farther out. Older fan surfaces are often cemented by caliche. Foundation implications vary by position on the fan: upper fans are typically granular and potentially collapsible; lower fans are finer-grained and more likely to contain clay. A foundation sitting near the transition between two fan deposits can span two different bearing conditions within a single footprint.

A layer of calcium carbonate that accumulates in desert soils when calcium-rich groundwater evaporates near the surface, leaving mineral deposits behind. In New Mexico it appears as a white to tan hardpan ranging from a soft, friable crust to a layer hard enough to turn a drill bit. Caliche is present at variable depths throughout most of the state and is one of the most commonly encountered features in foundation work here. It is frequently mistaken for competent bearing material — a boring that hits caliche refusal may appear to have found good ground, but soft or collapsible material can lie directly beneath it. Any investigation in caliche country requires penetrating below the hardpan to characterize what actually underlies it.

Soil that carries load adequately under dry conditions but consolidates — loses volume — when moisture infiltrates it. The open particle structure that gives collapsible soil its dry-state strength was built up under arid conditions where the soil was never consistently wet. When water arrives and dissolves or softens the bonds between particles, the structure collapses into a denser arrangement. Collapsible soil is the most common foundation hazard in New Mexico and appears across most developed basin floors and alluvial fan surfaces in the state. A foundation that has been stable for decades can begin moving within a single season if the moisture conditions underneath it change for the first time.

A mineral or rock deposited when a body of water evaporated and left its dissolved minerals behind as solids. Common evaporite minerals include gypsum, anhydrite, and halite (rock salt). In New Mexico, Permian evaporite formations are extensive in the southeastern part of the state — the Pecos Valley and surrounding area — where an ancient sea laid down thick deposits that are still present in the subsurface today. The foundation significance is that evaporites are soluble. Groundwater that contacts these formations dissolves them over time, creating subsurface voids. This process — karst dissolution — is the primary foundation hazard in the Pecos Valley.

Soil containing clay minerals that swell when they absorb water and shrink when they dry out — repeating that cycle indefinitely. The volume change can be significant: sodium montmorillonite, the most aggressive clay mineral found in New Mexico, can change volume by ten to fifteen percent between its dry and saturated states. A foundation bearing on expansive soil is pushed upward in wet conditions and drops back in dry conditions, every year. Unlike collapsible soil, expansive soil does not have a clear geography in New Mexico — it turns up statewide, often where the surface gives no indication. The only reliable way to know whether clay is present is to get into the ground at the specific site.

Fine-grained sediment deposited at the bottom of an ancient lake. New Mexico's closed basins — the Estancia, the Mimbres, the Tularosa — held lakes during wetter periods of the Pleistocene. As those lakes evaporated, they left behind thick sequences of clay, silt, and evaporite minerals on the basin floor. Lacustrine clays tend to be highly expansive because the fine particle size and high surface area of clay minerals is concentrated in still-water deposits. They also tend to be buried beneath younger, drier surface soils, meaning a foundation bearing on what looks like ordinary desert ground may have lacustrine clay at depth — a layer that can activate under moisture without announcing itself at the surface.

A dark gray to olive Cretaceous marine shale that outcrops and underlies much of the San Juan Basin in the Four Corners region. Deposited as the floor of a shallow inland sea roughly 85 million years ago, it left behind organic-rich shale loaded with sodium montmorillonite — one of the most expansive clay minerals that exists. Volume changes of ten to fifteen percent between dry and saturated states are documented. The Mancos Shale is the most severe expansive soil environment in New Mexico and the most consistently misdiagnosed: the damage pattern from Mancos heave is routinely mistaken for settlement, with costly consequences when the wrong repair is applied as a result.

A clay mineral with a layered atomic structure that absorbs water between its molecular layers, causing the mineral — and the soil containing it — to swell. Sodium montmorillonite is the most expansive form, with a net negative charge that attracts water molecules strongly into the interlayer space. It is present in the Mancos Shale of the San Juan Basin, in basalt-derived soils on the Taos Plateau, and as a component of clay assemblages throughout New Mexico. Smectite is the broader mineral group; montmorillonite is the specific smectite most associated with severe foundation heave in New Mexico. When a soil report references smectite content or plasticity index, it is measuring the presence and activity of this clay family.

A shallow, flat-floored depression in arid terrain that collects water after rain events and drains by evaporation rather than by outflow to a stream. Playas are common across eastern New Mexico and the High Plains. They are significant in foundation work because the finest, most clay-rich sediment in the surrounding landscape settles out in still water — meaning playas concentrate the most expansive material in the formation. A structure on or near a playa margin consistently sees more moisture-driven foundation movement than structures on the surrounding interplaya surface. Playas are not always obvious on the surface, particularly in developed areas where grading has obscured the subtle topographic difference that defines them.

A zone of active crustal extension running the length of New Mexico, where the earth's crust is pulling apart along a series of faults. The pulling-apart process dropped elongated basins while adjacent mountain ranges rose — producing the distinctive pattern of north-south trending basins and ranges that defines New Mexico's topography. Those basins have been filling with sediment for millions of years, creating the thick sequences of alluvial fan deposits, river sediments, lake beds, and wind-blown material that now underlie most of New Mexico's developed areas. The rift is still active, which contributes to the seismicity relevant to foundation evaluation in some parts of the state — particularly the Socorro corridor, which has documented elevated earthquake frequency.

The depth of soil that experiences meaningful moisture variation — and therefore meaningful volume change — in response to seasonal and long-term surface moisture conditions. Above the active zone, the soil wets and dries with the seasons. Below it, moisture content is relatively stable year-round. The depth of the active zone is a critical design parameter for any deep foundation repair in expansive clay country. A pier that bears within the active zone is still subject to the uplift forces generated by the surrounding clay when it swells. A pier that bears below the active zone transfers load to stable material unaffected by seasonal clay movement. Active zone depth varies by climate, soil type, and local conditions — it is established by investigation, not assumed from general tables.

Ground settlement caused by the removal of groundwater from an aquifer. When water is pumped from fine-grained sediments — silts and clays — the water pressure that was partly supporting those sediments is reduced and they consolidate under the weight above them. The ground surface settles, slowly and relatively uniformly over large areas. In the Deming area of the Mimbres Basin, decades of agricultural pumping have produced measurable subsidence that is beginning to affect foundations and infrastructure. Unlike most foundation movement, aquifer compaction subsidence is a regional process — it cannot be reversed by foundation repair. Stabilization means bearing below the compressible zone so the structure remains level as the surrounding ground continues to settle.

A condition where different parts of a foundation bear on soils with significantly different load-carrying capacity or compressibility. One footing is on dense, cemented material; another is on loose, compressible fill. One end of the structure is on collapsible alluvium; the other is on clay. When soil conditions vary across a foundation footprint, different parts respond differently to the same load and the same moisture changes — one corner settles while another heaves, or one side is stable while the other moves. Differential bearing is a common cause of the diagonal cracking and racking that homeowners typically notice first. It is particularly common on alluvial fan margins, at zone boundaries in mixed-soil environments, and in laterally variable formations.

The consolidation of a collapsible soil when it is subjected to sustained moisture for the first time in its history. The soil's open particle structure — stable under dry conditions — collapses when water infiltrates and softens or dissolves the bonds holding it together. The defining feature of first-wetting collapse is that it can happen to a foundation that has been perfectly stable for decades. Forty years of no movement does not mean the soil is competent — it may mean the soil has simply never been wet. When landscape irrigation is established, a utility line fails, or an unusually wet year saturates previously dry ground, the collapse can begin. Movement can progress rapidly once the moisture front reaches the collapsible layer.

Upward movement of a foundation driven by expanding soil beneath it. The most common cause in New Mexico is expansive clay swelling as it absorbs moisture. Frost heave — water in the soil expanding as it freezes — is also significant at higher elevations. Heave is the most consequential concept to understand about New Mexico foundations because it is so consistently misdiagnosed as settlement. Heaving soil eventually drops back when it dries, and the residual damage pattern — a foundation that has gone up and partially back down — often looks like the structure settled. That misreading leads to the wrong repair: installing settlement piers on a heaving foundation pins the structure while the surrounding soil continues to push up, creating a worse problem than the original.

A landscape and subsurface condition produced by the dissolution of soluble rock — typically limestone, gypsum, or other evaporites — by groundwater. As the rock dissolves, voids form in the subsurface. Those voids grow over time. The surface above them may remain intact until the void is large enough that its roof collapses. Karst is the primary foundation hazard in the Pecos Valley of southeastern New Mexico, where Permian evaporite formations are present in the subsurface throughout the Roswell, Artesia, and Carlsbad areas. Carlsbad Caverns is the most dramatic expression of what this geology does given water and time — the same process operates at smaller scales, more quietly, under structures throughout the valley.

Significant changes in soil type, density, or bearing capacity over short horizontal distances — sometimes within a single foundation footprint. Lateral variability is a defining characteristic of fluvial environments like river floodplains, where overbank clays are deposited adjacent to sandy channel fills, and of alluvial fan margins, where dense cemented zones sit next to loose unconsolidated material. In practice, lateral variability means a single boring does not characterize the site — you find one soil type, but the footing five feet away may be bearing on something completely different. The Hatch–Rincon Corridor and Corrales are both examples where lateral variability within a single lot is the rule rather than the exception.

A process by which saturated, loose granular soil temporarily loses its strength and behaves like a liquid when subjected to rapid cyclic loading — most commonly from an earthquake. The shaking causes water pressure in the soil pores to increase until the soil particles are effectively floating, no longer in contact with each other, and unable to support load. In New Mexico, liquefaction risk is most relevant along the Rio Grande corridor in the Albuquerque Basin, where loose sandy alluvial deposits near the river can be saturated by a shallow water table. The Socorro area — which has documented elevated seismicity — also warrants liquefaction assessment for susceptible soil profiles.

Ground settlement caused by the collapse or deterioration of underground mine workings. As wooden supports rot or unmaintained tunnels deteriorate, the ground above can settle gradually or — in more acute cases — subside suddenly in a bowl-shaped depression. Mine subsidence is a relevant foundation hazard in three New Mexico locations: the Raton Basin, where Colfax County coal mines operated through much of the 20th century; Gallup, where Mentmore and Navajo mine areas underlie portions of the city; and Silver City, where historic copper and silver mining has left subsidence risk in older neighborhoods. New Mexico Mining and Minerals Division records are the starting point for site assessment in these areas.

Downward movement of a foundation caused by compression or consolidation of the soil beneath it. Settlement can be uniform — the entire foundation moves down together, which rarely causes structural damage — or differential, where different parts move different amounts, producing the cracking, racking, and distortion that homeowners typically observe. The critical diagnostic distinction in New Mexico is between settlement and heave — both produce visible distress, and the surface patterns overlap enough that one is routinely mistaken for the other. The correct repair for each is different; the wrong repair applied to the wrong diagnosis produces a worse outcome than the original condition.

The repeated expansion and contraction of clay-bearing soil in response to changes in moisture content. The soil swells when it absorbs water and shrinks when it dries — and this cycle repeats with every wet season and drought, indefinitely. A foundation bearing on shrink-swell soil is never fully at rest. The question is not whether it will move but whether the movement is within a range the structure can tolerate. Over years and decades, cumulative displacement from repeated cycles can produce significant structural distress even when no single cycle caused obvious damage. Annual movement of a quarter inch can accumulate to two inches of net displacement over time. In New Mexico, the shrink-swell cycle runs with the summer monsoon and the dry winter-spring period.

Chemical degradation of concrete caused by sulfate ions in soil or groundwater reacting with Portland cement. The reaction produces expansive compounds within the concrete matrix that cause cracking, spalling, and progressive loss of strength over time. The concrete itself deteriorates — not just the soil around it. Sulfate attack is relevant wherever gypsum or other sulfate-bearing minerals are present in the soil, which includes the Tularosa Basin, the Pecos Valley, and the Estancia Basin. Any foundation repair work in these environments requires sulfate-resistant mix design (ASTM Type V or equivalent) to ensure the repair does not degrade over the life of the structure.

An open space in the subsurface created by the dissolution of soluble rock or soil, the decomposition of organic material, or the deterioration of mine workings. Voids are significant in foundation work because they represent the complete absence of bearing material at a given depth. The dangerous characteristic of dissolution voids is that they have no surface expression while they are forming. A void growing beneath a foundation produces no visible indication at the surface until it is large enough to cause movement — or until its roof collapses suddenly. In the Pecos Valley, gypsum dissolution voids are the primary concern. In mine subsidence areas, the voids are the remnant openings of tunnel and room-and-pillar workings.

The ability of soil or rock to support the load placed on it without failing or compressing excessively. Bearing capacity depends on soil type, density, moisture content, and the geometry of the load being applied. "Competent bearing" is the practical term used in foundation work to refer to material dense, stable, and strong enough to support a pier or footing without long-term compression. In New Mexico, finding competent bearing often means getting through caliche, through loose alluvial material, through active clay zones, and through compressible sediments to reach a layer that meets the load requirements of the repair. The depth to competent bearing varies enormously by location and is established by investigation, not assumed.

A ground improvement technique in which low-slump grout is injected under pressure into the soil, compacting the surrounding material and filling voids. The grout does not flow into the soil pores — it forms a bulb that displaces and densifies the adjacent material. Compaction grouting is used when the problem is loose, poorly consolidated soil rather than a condition that requires piers to transfer load to a deeper bearing layer. In the Gallup area, where loose sediments extend to significant depth, compaction grouting has been used where the soil profile does not offer the kind of discrete bearing layer that makes helical piers or micropiles the right approach. It is a site-specific method — the soil profile determines whether it is applicable.

The permanent, static weight of a structure — the weight of the framing, roofing, concrete, and all other fixed components. Dead load does not include the variable weight of occupants, furniture, or snow. Dead load is the load that push piers use to drive themselves to bearing. The pier is installed by jacking against the structure's dead load — pressing down on the building to push the pier into the ground. This is why push piers are problematic for most New Mexico residential construction: wood-frame stucco homes on slab-on-grade foundations do not have enough dead load to drive piers to adequate depth in our soil profiles. Helical piers are installed by torque rather than by jacking against dead load, which is why they are the appropriate residential pier method for most New Mexico conditions.

Settlement that occurs unevenly across a foundation — one part of the structure moves down more than another. The structural damage associated with foundation problems is almost always the result of differential settlement rather than uniform settlement: when the foundation moves as a unit, the building rides along without distress; when different parts move different amounts, the structure is forced out of plane and begins to crack, rack, and distort. Diagonal cracking at door and window corners is the classic indicator of differential settlement. The crack geometry indicates which part of the foundation dropped and which direction the movement traveled — information that is part of reading the evaluation before a boring goes in.

A steel shaft with one or more helical plates welded to it, installed by rotating it into the ground using hydraulic torque equipment. The helical plates advance the pier into the soil like a screw, allowing it to reach bearing depth without requiring the structure's dead load as a reaction force. Helical piers are the correct residential pier solution for most New Mexico foundation work. They can be installed in light structures — wood frame, stucco, slab-on-grade — that do not have the dead load required to drive push piers. Installation torque is monitored in real time and correlates directly to bearing capacity, allowing the installer to confirm adequate bearing as the pier advances. In heave situations, helical piers can be designed to resist uplift forces from clay expansion — but that design requires swell pressure data from laboratory testing, not standard specifications.

A small-diameter drilled and grouted deep foundation element, typically 3 to 12 inches in diameter. Micropiles are drilled rather than driven or rotated, which allows them to penetrate materials that resist other installation methods — hard rock, cobbles, dense caliche, and certain configurations of compressible soil. They are used when the soil profile requires reaching bearing at significant depth through materials that would prevent helical pier installation, or when torque capacity cannot be reliably correlated to bearing in very loose or variable soils. In the Gallup area, micropiles drilled to rock have been the appropriate solution on specific sites where helical piers could not confirm bearing.

The horizontal wood member that sits directly on top of the foundation wall and anchors the structural framing above it to the foundation below. The mudsill is the transition point between concrete or masonry and wood framing. It is also the most vulnerable point in the foundation system in elevated-moisture environments: if water consistently reaches the mudsill from inadequate clearance above grade, from direct soil contact, or from drainage problems, it will rot. A rotting mudsill produces the same visible symptoms as foundation settlement — the structure above drops — but the repair is replacement of the wood, not installation of piers. In mountain communities like Angel Fire and Cloudcroft, mudsill deterioration from freeze-thaw cycling and seasonal moisture is a common evaluation finding.

A structural element installed beneath a foundation to transfer load from a shallow footing to deeper, more competent bearing material. Piers are the primary structural repair tool in foundation work — they are installed when the soil immediately below the foundation is inadequate and the goal is to reach material at depth that can reliably support the load. Piers solve a load-transfer problem — they do not address the soil conditions causing movement unless those are specifically accounted for in the design. A pier that reaches bearing below a collapsible soil profile works. A pier installed in an active heave environment without accounting for uplift forces from clay expansion does not. Method, depth, and design specification all depend on what soil investigation reveals about the specific site.

A steel pier installed by hydraulically jacking it into the ground using the weight of the structure above as a reaction force. The pier is driven downward until the resistance of the soil at depth equals the capacity of the hydraulic ram, at which point the pier is locked off and load is transferred from the foundation to the pier. Push piers are not appropriate for most New Mexico residential foundation work. The installation method requires the structure to provide sufficient dead load to drive the pier to bearing depth. Wood-frame stucco homes on slab foundations common throughout New Mexico do not generate enough dead load to drive piers to the depths required in our soil profiles. Push piers are a commercial and heavy-structure solution in this market.

A foundation system consisting of a concrete slab poured directly on the prepared ground surface, without a basement or crawlspace. The slab bears directly on the soil below it and is the dominant foundation type for residential construction in New Mexico. Slab-on-grade foundations respond directly to whatever the soil beneath them does — there is no crawlspace buffer, no basement wall to redistribute load. When the soil swells, the slab heaves. When the soil consolidates, the slab settles. Differential movement across the slab produces the cracking, floor tilt, and door problems that most homeowners first notice.

The practice of continuously measuring the rotational resistance encountered during helical pier installation, and using that measurement to infer bearing capacity in real time. As a helical pier advances through soil of increasing density, the torque required to rotate it increases. The relationship between installation torque and bearing capacity is established through empirical correlations. Torque monitoring is the primary quality control method for helical pier installation and is essential in variable soil profiles where bearing conditions change significantly with depth. It is not a substitute for soil investigation — you still need to know where the target bearing layer is and why — but it confirms that the pier reached adequate bearing and was not stopped by an intermediate obstruction that could fail under load.

The general term for any method used to extend a foundation to deeper bearing material — either by installing piers beneath the existing footing or by excavating and extending the footing itself. In common usage, underpinning has become nearly synonymous with pier installation. Underpinning addresses load-transfer; it does not address moisture conditions, expansive soil forces, or any other driving mechanism unless those are specifically accounted for in the design. A correctly designed underpinning system transfers the structural load to material that is not subject to the conditions causing movement at the shallow level. An incorrectly designed one moves the load to a depth that is still within the active zone of the problem soil.

A hole drilled into the ground to collect soil samples and measure soil properties at depth. Borings are the primary tool of geotechnical investigation — they reveal what is actually in the ground below the surface horizon visible to visual inspection. Standard practice involves collecting samples at regular depth intervals, logging the soil type and color at each depth, and performing field tests that measure density and resistance. Laboratory tests on the samples establish moisture content, plasticity, swell potential, and other properties that cannot be determined in the field. A boring program — multiple borings at different locations on a site — is necessary in laterally variable environments where a single boring characterizes only one location in what may be a complex soil profile.

The point during a boring or driven investigation where the drilling equipment can no longer advance because it has hit a caliche hardpan. Refusal is a common occurrence in New Mexico investigation work and is frequently misinterpreted. Caliche refusal does not mean competent bearing has been reached. It means a hard layer has been encountered — but that layer may be thin, may be underlain by soft compressible material, and may not extend laterally across the site. An investigation that terminates at caliche refusal without penetrating below it has not characterized the bearing conditions that a pier will ultimately bear on. Rotary drilling methods that can penetrate caliche are required to determine what lies below it.

Non-invasive surface investigation methods that use electromagnetic or electrical signals to image the subsurface without drilling. Ground-penetrating radar (GPR) transmits radar pulses into the ground and records reflections from buried features and boundaries; electrical resistivity measures how well the ground conducts electricity, which varies with soil type, moisture content, and the presence of voids. These methods are used in karst terrain — particularly the Pecos Valley — to detect subsurface voids that drilling alone might miss: a boring can pass between voids without finding them. Geophysical surveys provide a continuous image of the subsurface along a transect, identifying anomalies that warrant follow-up drilling. They do not replace borings; they improve the probability that borings are placed where the critical subsurface conditions are located.

A systematic program of field and laboratory work to characterize the soil and groundwater conditions at a specific site — including what materials are present, how deep they extend, how they behave under load and under moisture, and what engineering properties they have. A geotechnical investigation is distinct from a contractor inspection: it produces data, not a sales proposal. The investigation tells you what the ground is doing and why. The repair decision follows from that understanding. In New Mexico, geotechnical investigation before repair is particularly important in areas with complex or ambiguous soil conditions — Mancos Shale heave, karst terrain, mine subsidence zones — where the wrong diagnosis leads directly to the wrong repair.

The specific source of moisture that initiated or is sustaining foundation movement. Identifying the moisture trigger is the starting point for repair design in moisture-driven foundation problems — which describes most foundation movement in New Mexico. Common triggers include landscape irrigation established for the first time on previously dry soil, broken or leaking supply lines and slab plumbing, roof drainage directed toward the foundation, and changes in surface grade that concentrate runoff at the perimeter. A repair that does not address the moisture trigger will not hold. Helical piers installed in a collapsible soil profile will continue to lose bearing if moisture continues to infiltrate and consolidate the surrounding soil. The trigger is part of the diagnosis, not background information.

A laboratory measurement that quantifies how much the water content of a soil can change while the soil remains in a plastic — workable, cohesive — state. A high plasticity index indicates a soil that can absorb a large amount of water while remaining plastic, which correlates with high swell potential and significant shrink-swell behavior. Plasticity index is used to characterize clay activity and is a key parameter in swell potential assessment. In Mancos Shale evaluation, plasticity index testing is part of the standard investigation sequence — visual identification of shale routinely underestimates the swell potential, and the lab test provides the quantitative basis for pier design specification. PI values above 35 indicate high expansivity; the Mancos Shale routinely tests above that threshold.

The tendency of a soil to increase in volume when moisture is added, expressed either as a percentage volume change or as the pressure the expanding soil can generate against a confining structure. Swell potential is measured in the laboratory on undisturbed soil samples under controlled conditions. It is the parameter that drives helical pier design in expansive clay environments. A pier specified to resist settlement without accounting for the uplift forces the surrounding clay can generate will be inadequate in high-swell-potential soil. In Mancos Shale country, swell pressure testing on undisturbed samples is a required step before pier design — the documented volume change potential translates to significant uplift force that a standard pier specification does not address.

A soil sample collected in a way that preserves the natural structure, density, and moisture content of the in-place material — as opposed to a disturbed sample, which is simply soil recovered from an auger or split-spoon that has been remolded during collection. Undisturbed samples are required for laboratory tests that depend on the natural soil structure: consolidation testing, swell potential testing, and shear strength testing. You cannot measure the swell potential of a remolded sample and apply it to in-place conditions — the structure that controls swell behavior is exactly what gets destroyed in sample collection if an undisturbed sampling method is not used. In Mancos Shale evaluation, undisturbed sampling is required for the swell pressure testing that informs pier design.