Glossary of Terms

Sinkhole Testing & Repair Glossary

Dealing with sinkholes can be overwhelming especially when the terminology is unclear. We’ve included a list of common geotechnical terms that can assist you in understanding your engineering report.

Commonly used as an indicator of the expansive potential of a clayey soil. The activity of a soil is estimated by dividing the plasticity index by the clay fraction of the soil.

Active Zone
The area of seasonal soil moisture variation. Sometimes referred to as the zone of seasonal fluctuations.

A deviation from uniformity in physical properties; a perturbation from a normal, uniform, or predictable field. An anomaly represents an area or volume of geologic materials that have unusual physical properties that can be detected by ground penetrating radar, electrical resistivity, seismic, or other investigation methods.

The aquifer is a geologic formation, group of formations, or part of a formation that is capable of receiving, storing and transmitting water. The formation is capable of yielding enough water to support a spring or well.

Atterberg Limit
Tests performed on cohesive (clayey) soils to determine the range of moisture contents over which the soil changes from on physical condition to another. These limits are used as parameters to classify and estimate behavior of the clayey soils.

A corkscrew-like drill tool designed to remove the soil from a drill hole without using air or water. This method does not require vibration or impact to create the drill hole.

Bearing Capacity
A measure of allowable bearing (load) placed on a given soil profile before a predetermined failure (yield) criteria is met.

A clay material that is mined in the mid-west and typically ground into a powder. Bentonite is normally used in test drilling to prevent the drill hole from caving. It is also used in cosmetics and even some candy bars.

Blow Count
In a Standard Penetrating Test, a blow count represents the number of times the 140-pound hammer must fall 30 inches onto the anvil in order to drive the split-spoon 6 inches. The blow counts for the second and third, 6-inch intervals are added up to obtain N-value.

Chemical Grouting
High density polyurethane expandable grout is installed under low pressure to fill voids immediately beneath foundations and/or concrete slabs. This can gently raise foundations upward in a controlled and precise method.

Rock type composed of amorphous quartz (SIO2). The material is highly insoluble and usually found in the upper zones of the limestone bedrock. It is very hard and difficult to drill.

A soil that has the finest possible particles, usually smaller than 1/10,00 in (2.5X10-4 cm) in diameter, and often possesses the capacity for extreme volume changes with differential access to water.

Soils which exhibit plastic behavior when moist which is generally comprised of clay minerals.

Compaction Grouting
Grout is pumped through steel casings into the ground under high pressure. This seals cavities in the limestone bedrock, compacts loose soils, and fills voids in the overlying soils.

Concrete Shrinkage
The volumetric change (shrinkage) of concrete due to the evaporation of water from the hardened concrete. Tensile stresses are generated and cracks are common.

The loss of volume of a cohesive soil mass due to loading (pressure) where water is “squeezed” out over time. Often misused interchangeably with settlement or deformation in sandy soils.

Cover Collapse Sinkhole
Develop abruptly (over a period of hours), and may cause catastrophic damage to houses, commercial buildings, and roadways.

Cover Subsidence Sinkhole
The most common type of sinkhole condition. Unlike the cover-collapse sinkhole, this type of sinkhole typically moves slowly over a period of time. A passageway in the limestone allows soil particles to flow into a larger void or opening, similar to the way sand from an hourglass passes from the top to the bottom. Like in the hourglass, a depression or subsidence occurs at the surface when enough sand or soil moves into the deeper void.

Cone Penetration Test (CPT)
Similar to the SPT in that it is used to determine the density of subsurface soils. This test uses a metal rod or tube fitted with a special point that is equipped with sensors that measure the resistance that is required to advance the point through the soil as it is hydraulically forced into the ground. This technique is very common in Europe.

Dissolution Sinkholes
Formed by rainfall and surface water percolating through joints in the limestone.

The zone of weathering found at the upper surface of the limestone bedrock. The weathering of the limestone surface results in an irregular surface and limestone pinnacles and boulders are common within the zone.

Measurements taken by instruments (usually optical) to establish grades.

The transportation of earth materials, usually accomplished by water, wind, or gravity.

Expansive Soil
A fine-grained clay that is subject to swelling and shrinkage of the soil, varying in proportion to the amount of moisture present in the soil.

Soils (generally sands) used to backfill excavations and to elevate site grades during construction.

Fly Ash
A fine powered ash material that is a by-product of the coal burned in an electric power plant. This material is added to the grout mixture to fill the small voids between the sand particles to produce a well-graded mixture. This is important when pumping the grout material underground.

Footing is the portion of the foundation that transfers the structural load to underlying soils.

That upon which anything is founded; that on which anything stands, and by which it is supported; the lowest and supporting layer of a structure; groundwork; basis.

A geologist is a person who specializes in geologic research and study.

Pertaining to practical applications of geological science in civil engineering, mining, etc.

Geotechnical Report
A report produced by geotechnical engineering firms used to communicate the soils conditions below a structure. This report typically will contain ground penetrating radar analysis, soil boring (STP borings), and other specific soil related test.

Geotechnical Engineer
The branch of engineering dealing with the soil and bedrock, especially aspects of foundations and earthworks.

Ground Penetrating Radar
A geophysical method for mapping interfaces underground by reflecting frequency radar waves off the interfaces and recording their reflections at the ground surface. This method is often used to detect shallow soil conditions that might represent sinkhole activity.

A cement-based material similar to the mortar that is used to lay bricks. A specially designed mixture that utilizes Portland cement, fine sand, fly ash and water. Small-diameter stone may also be added to the mixture, depending on the particular project. This material is typically delivered to the site by local concrete suppliers on residential stabilization projects. Grout material used in residential stabilization projects usually reaches about 1000 psi of compressive strength in 28 days. It will typically reach an initial set in about two hours.

Grout Casing
A high-strength steel pipe, usually about three inches in diameter. It is very important that a specially designed, flush-joint (internally threaded) grout casing be used for this purpose. Standard threaded and coupled pipe should not be used for this purpose. This casing will initially be installed to the top of the limestone and will then be withdrawn as the grout-injection process proceeds.

Grouting is a method of sinkhole repair or remediation where concrete is pumped under the ground. This is the most inexpensive method of subsurface sinkhole repair. There is no way to determine where the concrete is going or how much will be needed making this form of repair unreliable. Grouting coupled with underpinning is more widely accepted as a proper method to stabilize the foundation of a structure.

Upward movement of soils below a structure. One natural cause of heave is shrink and swell clays below the property that expand and contract with the amount of moisture it absorbs. The second cause of heave can be the result of compaction grouting operations that inject material into the soils below to relevel structures. Monitoring devices should be in use if trying to relevel structures.

Grout Injection Point
This is a location where grout casing is drilled into the ground down to limestone. The purpose for this is to transport the grout material down to the top of the limestone. Once the top of the limestone is sealed, the grout is then pumped in lifts, the entire length of the injection point, to just below ground surface.

Karst is a generic term which refers to the characteristic terrain produced by erosional processes associated with the chemical weathering and dissolution of limestone or dolomite, the two most common carbonate rocks in Florida. Karst is an area of limestone terrain characterized by ravines, sinks and underground streams.

A sedimentary rock consisting primarily of calcium carbonate, often in the form of the minerals calcite or aragonite, and sometimes with magnesium carbonate in the form of dolomite.

Liquid Limit (LL)
The moisture content at which a cohesive soil (clay) ceases to behave plastically and flows (as a viscous fluid).

Loss of Drilling Fluid Circulation (LOC)
Occurs when drilling fluid is lost to voids or openings in underlying geologic units. In addition, drilling fluid may be lost due to drilling conditions (clogging or drill bit, etc.)

Stone, brick, concrete, or similar building materials bonded together with mortar to form a wall.

Moisture Content
Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, or wood on a volumetric or gravimetric basis.

Monolithic Slab
A monolithic slab is one of four different types of foundations. One of the four types of foundations. Foundations were footings and slabs are poured together.

The sum of the blows required to drive a split spoon sampler 12 inches of soil.

Organic Soils
Layers of decaying organic matter such as wood, plants, trees and other vegetable matter. When these layers are exposed to air, decomposition continues and the soil volume continues to decrease. If the water table rises above this zone then decomposition is arrested until it is exposed to the air again.

Overburden Soils
The sediments located directly over the bedrock. These soils are generally composed of sand and clay in Florida.

An organic-rich soil or sediment with greater that 50% by dry weight organic component.

A column used to support a structure and transfer its load to the soil through a footing as opposed to direct embedment in the soil.

Plastic Limit (PL)
The moisture content at which a cohesive soil (clay) ceases to be brittle and becomes plastic (able to be molded).

Plasticity Index (PI)
The range of moisture over which a cohesive soil behaves plastically. The PL is calculated as the liquid limit minus the plastic limit and is expressed as a whole number.

An erosional process where groundwater transports soil particles by seepage forces (drag) downward into fractures and openings in underlying geologic units.

The condition reached when a pile being driven by a hammer has zero penetration per blow (as when the point of the pile reaches an impenetrable bottom such as rock) or when the effective energy of the hammer blow is no longer sufficient to cause penetration. When so stipulated, the term refusal or substantial refusal may be used to indicate the specified minimum penetration per blow. Overdriving of piles after essential refusal can damage them seriously.

Deformation of soil in the (downward) vertical direction. Settlements observed in soils may be the results of several conditions, including loading, drag forces from moving water, and erosion.

Shallow Grouting
Grout is pumped at low pressure in low volumes at 6-8 feet below a structure. This will compacted loose soils, stabilizing a structure.

Formed when soluble limestone and dolomite are dissolved by acidic rainwater. This creates solution channels, fissures, and interconnected void veins within the limestone formation. As water percolates through the overburden soils into the limestone formation, part of the soil is transported into the limestone formation.

A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote, is a natural depression or hole in the surface topography caused by the removal of soil or bedrock, often both, by water. Sinkholes may vary in size from less than a meter to several hundred meters both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. They may be formed gradually or suddenly, and are found worldwide. These terms are often used interchangeably though many will distinguish between those features into which a surface stream flows and those which have no such input. Only the former would be described as sinks, swallow holes or swallets. ¹

Solution Sinkhole
A solution sinkhole is a small diameter opening in the limestone bedrock. The small opening may be in-filled with sediment from above or (if the limestone is sufficiently shallow) may produce a small “hole” in the ground surface. The latter is often referred to as a “chimney sinkhole”.

Standard Penetration Test (SPT)
A geotechnical investigation technique that is widely used in which a split-spoon sampler is driven into a soil or sediment by a 140-pound hammer dropping 30 inches. The number of blows required to drive the sampler one footed is called the blow count (N).

Subsidence Sinkhole
A subsidence sinkhole forms when the upper surface of the limestone is dissolved away, and the thin cover overburden slowly subsides to occupy the space once occupied by limestone. Because of the continued downward movement of cover materials, voids may not be well developed.

Surficial Aquifer
The ground water contained in the upper sequences of sands or other overburden sediments in general, the upper surface is free to move with changes in ground water volume change (due to increases/decreases in rainfall, evaporation).

Underpinning is a method of subsurface repair in which metal piers are driven into the ground and imbedded in the underlying limestone. Once the piers are secured into the limestone, they are then attached to your business or home with brackets. Underpins are usually installed around the perimeter of the structure at set intervals. If needed, pins can also be placed under the interior of the structure, such as under interior load bearing walls. When interior pins are needed, the homeowner often has to move out and flooring must be removed for installation. Exterior perimeter underpinning, coupled with interior pinning, is widely considered the best way to stabilize a structure where sinkhole activity is present. Some grout may also be added to provide lateral support to the pins. Underpinning is often a very expensive process, the cost of which may exceed available property insurance limits. For this reason, insurance companies rarely recommend this method of repair because payment of full policy limits is often warranted.

Water Table
The planar, underground surface beneath which earth materials, as soil or rock, is saturated with water. The upper surface of the surficial aquifer.

The various mechanical and chemical processes that cause exposed rock to decompose.

Weight of Hammer (WOH)
An event where the drilling hammer, rod, and bit are allowed to rest on the bottom of the borehole and advance under their own weight.

Weight of Rod (WOR)
This is an event that occurs during a standard penetration testing when the drill string (drilling rod and split spoon sampler) are allowed to rest on the bottom of the borehole and they advance under their own weight. This event may reflect a void, naturally weak soils, or excessive weight of drill string.