Infrastructure and Development:
Sinkhole Evaluation and Investigation
The entire Florida peninsula is underlain by solution-weathered limestone that contains cavities and voids that range in size from a few inches to more than 100 feet wide and high. The lengths of these natural conduits are often measured in miles. The sinkhole-dotted surface of the limestone is typically buried beneath thick deposits of overlying sediments, thus the foundation hazards associated with building above the limestone are generally not visible from ground level.
Further, the level of the shallow groundwater table is frequently higher than the pressure head within the limestone, creating a downward gradient. Under this gradient, water from the upper sediments flows downward through cracks or breaches in the confining clay layer, carrying soil into cavities within the limestone, (this process is called “piping” or “raveling”). Significant raveling can create a void in the overburden soils. When this void enlarges to a size at which the beam action of the overlying soil can no longer support itself, the “roof” of the void collapses resulting in a sinkhole of the ground surface (a cover collapse sink).
Often, when the overburden soil is cohesionless, and the available void space in the limestone is small, the settlement is more gradual (a subsidence sink). In either case, the effects on structures that are positioned above or very near the sinkhole are potentially damaging. Proper exploration and engineering is, therefore, critical to evaluating sites for development and construction.
The focus of a geotechnical exploration in sinkhole-prone areas is to locate potential problematic subsurface conditions and define their extent.
Since the cavities in the rock and the voids in the overburden are buried deep underground, and since subsurface conditions vary throughout the state, various exploration and data-gathering techniques are used:
Existing Data: We first study the geology and hydrogeology through existing maps and aerial photographs, publications, contacting local agencies and landowners, and reviewing Ardaman’s extensive year database of prior investigations.
Soil Borings: Rotary wash borings with Standard Penetration Tests provide specific and accurate information on soil and rock stratigraphy as well as soil strength and consistency, and location of voids and loose raveling zones within the overburden.
Cone Penetrometer Soundings: These provide similar information as the soil borings, but they can be conducted faster and more economically, allowing more data to be acquired within a stated schedule and budget. The electric piezocone can be effective for detecting slight downward hydraulic gradients often present near breaches in the confining layer.
Groundwater Monitoring Devices: The level of the upper groundwater table will frequently depress above and near confining layer breaches. This level can be monitored and contoured through a series of selectively placed piezometers.
Geophysical Methods: Geophysical techniques such as ground penetrating radar, electro-magnetic conductivity, and seismic surveys can detect anomalies in the overburden profile. These methods are somewhat interpretive, but are useful for providing a significant amount of data, relatively quickly, that can be used to design the final drilling and testing program.
One or more of these exploration methods are used to evaluate the sinkhole potential of a site. The most important tool, however, is the knowledge and experience of the engineers conducting the investigation. They will know the most efficient and effective technique to apply in any given situation. At Ardaman, our engineers do not use technology for its own sake. Rather, we endeavor to apply the proper level of study that will obtain the appropriate information within the schedule and budget requirements.
Engineering on Existing Structures
Investigations are made for existing structures generally when distress from a sinkhole is imminent or has already occurred. After the geotechnical study has evaluated the nature and extent of the sinkhole and/or the underground features, we prepare recommendations for alleviating the distress or otherwise dealing with the subsidence.
Some of the options include:
- Doing nothing after the study reveals a low probability of additional subsidence.
- Pressure grouting with sand/cement grout to solidify loose soil conditions and “seal” the breach.
- Underpinning the structure with piles to carry the loads to deeper competent strata.
- Monitoring the structure to evaluate whether additional subsidence is occurring (this can occur in conjunction with other options).
- Removing the structure, remediating the sinkhole and rebuilding (if a value analysis reveals this as the most economical option).
New Construction Engineering
If a detailed investigation of a site does not disclose breaches or suspicious conditions above the confining layer, foundation design can proceed normally.
If, however, detrimental conditions are suspected, alternate approaches are recommended, and may include:
- Locating the facilities to avoid the problem condition and its potential zone of influence.
- Modifying the foundations (e.g. mats, grade beams, waffle slabs) to span a suspected sinkhole if it does occur.
- Designing deep foundation options to carry the structure loads below the problem zones.
- Modifying the soil conditions by artificial improvement methods such as subsurface grouting, deep dynamic compaction, vibro-densification, etc.
The risk associated with any option will vary, and we are cognizant of the need to adequately communicate the relative risk to our clients. An informed client is mandatory when sinkholes are involved, and our engineers work hard to make sure that our clients fully understand the benefits and risks of any foundation system or improvement method.
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