Recently several articles have appeared in the editorial section of local news papers regarding the new Citizens Property Insurance proposal to fix rather than litigate confirmed sinkhole claims. The articles have provided both the pros and cons of the Citizens proposal but also have questioned the effectiveness of subsurface grouting versus underpinning for sinkhole remediation purposes. The pros and cons are a separate subject; however, this engineer has been in the business of evaluating sinkhole claims as well as damaged homes and other types of buildings and proposing appropriate remediation plans consistent with the specific findings for almost 40 years and believes that comments regarding grouting versus underpinning are needed.
The debate over which is better regarding grouting versus underpinning is synonymous with consideration of which is better in treating the symptom or the cause of the damage. Settlement damage is the symptom that everyone sees while the underlying sinkhole activity progressively weakening the soil supporting the structure is the cause that very few will see.
The two terms "grouting" and "underpinning" are used by a lot of homeowners, contractors, attorneys, and legislators to describe what they believe is the appropriate method of addressing weakened soils associated with sinkhole activity. While State Law (F.S.§ 627.7073.1.b.4.c) says that only the independent engineer retained by the insurance company can provide a plan as to land and building stabilization and foundation repair when required, many proponents of underpinning attempt to convince the public or their customers that underpinning alone should be substituted for the engineer-of-record's remediation plan which may specify subsurface grouting.
The first general method of remediation is "subsurface grouting". It is used in sinkhole remediation by injecting a cement based grout material into the ground under controlled pressure beginning at a depth at which competent limestone is intercepted and progressing under additional controlled conditions from the beginning depth of competent limestone to a depth of approximately 5 to 10 feet below the surface.
The cement based grout material does not contain coarse aggregate as does normal concrete. The grout will flow under pressure and take the path of least resistance in sealing off the open rock crevices and solution channels before setting to form a hard concrete like material. Once set, the grout material is more resistant to deterioration from groundwater than is the limestone it covers. A well designed grouting plan will direct the grout beneath the damaged subject structure and minimize its flow away from the structure or subject property by limiting criteria designed to prevent it from extending in unwanted directions.
This process of "subsurface grouting" is intended to (1) seal off open solution channels leading to subterranean cavities within the limestone which are accepting soils from the identified sinkhole activity, (2) densify/compact the weakened intermediate soil profile caused by a loss of soil into the rock, and (3) restore bearing capacity to the overall soil profile supporting the structure. This process treats thecause of the sinkhole development which results in the building's settlement.
Since the limiting criteria used to control the deep compaction grouting process may also prevent it from extending all the way upward to the bottom of the foundation, many times it is complimented by a secondary program of shallow grouting typically using a chemical grout material that either permeates and binds the upper sands together thereby increasing their bearing capacity or this secondary process may consist of injecting a two part chemical grout in which the two parts set upon mixing and form a water/insect/weather resistant material that also binds loose sands together adding to their strength in addition to forming its own strong material that again increases and supplements the strength of the remaining near surface soils.
This shallow grouting process begins at the depth at which the influence of the deep grouting terminated and continues to the bottom of the structure's foundation. The need for this secondary process is determined by the strength or weakness of the soils within the upper ten feet of the surface and depending on the condition of the near surface soils may or may not be needed to fully restore bearing capacity to the soils.
A well planned and well implemented grouting program will direct the grout beneath the damaged structure not away from it, restore bearing capacity to the supporting soils, eliminate further soil raveling through open solution channels leading into the limestone, thereby taking away the potential for imminent collapse due to soil instability or soil movement and allow the structure's foundation to dissipate structure loads as it was originally designed by way of uniform load distribution into the underlying soils.
The process of "underpinning" or "pinning" generally refers to installing steel pipes or "mini-piles" into the ground beneath the structure's foundation for use in supporting the structure in weak soils and/or for use in lifting and releveling severely damaged buildings. The steel pins when properly designed by a licensed engineer familiar with their use are spaced close enough together to safely support the subject structure and its elements of construction. The detail of spacing is another matter all together.
The pins can treat the symptom of building settlement but not the cause. Steel pins advanced into the ground will not stop or slow down the loss of soils into the underlying limestone formation. If soil loss into the rock is not halted then continued soil weakening will occur and the support initially developed by the pins can also be lost resulting in more settlement and worse damage since the integrity of the foundation may have been comprised by notching to attach the pin brackets to the foundation line of the home or building. This compromise results from the underpin having some type of manufactured, engineered, steel bracket that attaches the steel pipe pin to the foundation of the structure. Because the pin uses a small diameter pipe, typically three inches or less, to aid in installation around an existing structure eccentricity developed by having the pin offset from the center of the wall and its transmitted downward load will cause bending and early failure to the pin pipe. Therefore, with a strip footing the foundation is cut or notched to move the bracket and pipe pin closer to the center of the wall to minimize eccentricity.
The notching of the strip footing is a process that requires a lot of care and specialized equipment; however, no matter how careful and skillful the contractor is with the notching process, the foundation is fundamentally changed from its design intent of uniform load distribution to one of point load distribution. The difference in changing the load dissipation method is one of how the foundation bends and the reinforcing steel necessary to resists the different types of bending.
For a standard shallow spread footer foundation the bending is downward only (positive) as the walls push on the foundation and the foundation pushes on the soil. For a point load standpoint when pins are attached to the foundation the load distribution fundamentally changes from one that experiences only positive bending to one that has positive bending between pins and negative or reverse bending over the top of each pin.
Reinforcing steel to resist this alternating positive and negative bending configuration was not included in the original foundation when it was constructed and cannot be added into the existing foundation at the time of underpinning. Additionally, the notching process can cause a stress rise at the notches, diminish the width of the footing at the notch, and even cut the existing reinforcing steel when it is located at different spacing than the contractor assumed it to be.
When the damage sustained to the building is minor and can be fixed by above ground repairs, the original foundation concept of dissipating loads into soils capable of providing sufficient support is always best. When the damage is more significant and additional support with pins is warrantied before cosmetic repairs begin, the original foundation becomes compromised.
While the above discussion of underpins and their use appears to be entirely negative, it is not intended to be so. Pins have their place in a remediation process and should be considered by the professional engineer for inclusion into the remediation plan when certain conditions exist. These conditions and the benefits derived by adding underpinning are:
First and foremost, controlled lifting to close large cracks or controlled lifting to relevel a portion of the structure or even the entire structure can be accomplished with underpins after soil stabilization has occurred. Controlled lifting cannot be accomplished by grouting alone and the need for releveling may trump the detrimental effects of changing the foundation load distribution methodology. Remember, reasonable and effective cosmetic repairs cannot be made when the structure is so far out of level that the house becomes a "funhouse" with sloped floors or slanted ceilings.
Secondly, underpins can be used to provide support in buried debris or when shallow active clay soils exist beneath the building that may be prohibitive to stabilization by cementitious or chemical grout alone. This is also the case with highly organic soils that will inhibit the setting process of cementitious grout and may partially defeat its stabilization effects. These other deleterious conditions may have been exacerbated by the effects of the ongoing sinkhole activity and may not be totally extraneous to the sinkhole claim.
Thirdly, underpins can be used to provide support for the structure on a contingency basis that will resist significant movement of the structure during the initial stages of the grouting process. Upon completion of the grouting process, lateral support for the pins is established and the potential for further deterioration of the bearing stratum upon which the pins develop their support is abated. In this case, the decrease in foundation integrity has to be weighed as opposed to the structure incurring severe damage before soil stabilization has been affected.
It can be seen by the above discussion, the dynamic process of soil weakening associated with sinkhole activity cannot be arrested or slowed down by underpinning alone. The pins are only as sound as the soil/rock they are founded in at the time the brackets are secured to the house. Underpinning will address the symptom but not the cause of the damage. If the soil supporting the pins continues to weaken due to continuing and progressive sinkhole activity the underpins will loose support and damage will reoccur.
The prudent engineer evaluates the conditions causing the damage and the condition of the building at the time of the investigation and then proposes a remediation plan that will address both the cause and the symptom considering what is best to restore the building to a pre-loss condition. This plan may include grouting alone, a combination of deep and shallow grouting or a combination of grouting and underpinning. In the case of a sinkhole claim it should never consist of underpinning alone.
George C. Sinn, Jr., P.E.
FAS3 - President
Recent events have put sinkholes and the dangers they pose to the public back in the forefront of most people's minds. National media as well as local media have all been covering "sinkholes" and the psychological as well as the physical damage they cause the public. When talking about sinkholes, sinkhole activity and sinkhole claims, specific terms and references are made by engineers, geologists, contractors and adjusters.
This message is an attempt to enlighten the General Public about those terms and regulations and how they may affect your claim. It is titled "Sinkholes – 101" and is intended to be the starting guide for an understanding of what all the technical references mean and how they may relate to you and your claim.
Terms that are frequently used in talking and/or reporting about sinkholes and the insurance coverages that address them are as follows:
Sinkhole – a landform consisting of a depression or hole at the ground surface formed by the end result of sinkhole activity. Typically, sinkholes form from the movement (raveling) of soil downward into cavities or caverns located in the underlying limestone that is the bedrock formation for Florida and several other states. Limestone is the most soluble of the common rock types that form the bedrock layer underlying the United States. The three major types of rock are igneous, sedimentary and metamorphic, with limestone being a sedimentary rock and the most soluble by mildly acidic groundwater.
Sinkholes can be as large as several hundred feet in diameter or as small as a few feet wide when they first form. Their depths can also vary by the same amounts. The initial size and depth of a sinkhole is a function of the composition of the soils above the limestone, how far below the surface the limestone begins, how much soil has raveled into the rock and how large is the cavity, void or crevice that accepting the raveling soils. After initial formation sinkholes can "grow" due to continued soil migration into the cavities or from caving due to a function of slope stability along the exposed sides of the hole.
Statute language defines a sinkhole as ……. "Sinkhole" means a landform created by subsidence of soils, sediment, or rock as underlying strata are dissolved by groundwater. A sinkhole forms by movement or collapse of soils into subterranean voids created by the dissolution of limestone or dolostone or by subsidence as these strata are dissolved.
Sinkhole Activity – the geologic process that causes sinkholes to form. This process can be slow or rapid in its progression of moving soil into cavities already existing in the limestone through solution channels or crevices located in the surface of the limestone leading to the cavities. The process works in a bottom to top direction starting at the surface of the limestone and working its way upward through the overburden soils. Sinkhole activity is driven by either groundwater saturating the soils above the rock changing from being in a static condition to being in a dynamic condition. This is similar to a bath tub full of the water having the drain plug pulled. When the plug is pulled groundwater moves downward into the rock and transports soil along with it through a breach in the confining layer of low permeability clayey soils separating the sands from the porous limestone. As this soil movement continues a soil void or reduction in soil density develops along the rock surface and with time, may progress upward getting larger and/or weaker. When the overburden soils can no longer bridge or span over the developing soil void, the soils sag forming a depression or even a collapse resulting in a sinkhole at the land surface. The sagging is called a "Cover Subsidence Sinkhole" while the collapse is called a "Cover Collapse Sinkhole". The latter is the type of sinkhole that affected the Seffner property in March of this year and the Orlando property most recently in August. The warning time prior to the collapse can be very short as in the Seffner event or slightly more prolonged as in the Orlando event.
Florida Statutes define sinkhole activity as ……. "Sinkhole activity" means settlement or systematic weakening of the earth supporting the covered building only if the settlement or systematic weakening results from contemporaneous movement or raveling of soils, sediments, or rock materials into subterranean voids created by the effect of water on a limestone or similar rock formation.
Sinkhole Loss – typically insurance policies have language that reflects coverage when a "sinkhole loss" occurs. However, a "sinkhole loss" can be as complex as having to meet a specific definition given to "catastrophic collapse" which may require unrepairable damage to have already occurred in conjunction with a landform "sinkhole" to be present before coverage takes effect; or, a "sinkhole loss" may be as simple as physical damage to the covered structure or building caused by "sinkhole activity" regardless as to level of damage which has occurred at the time of the investigation. The latter or simple policy definition for a sinkhole loss is one of dwindling numbers that generally is found in policies dating prior to May of 2011.
By new statute language (post May 2011) the definition of a "Sinkhole loss" is "structural damage" to the covered building, including the foundation, caused by sinkhole activity. Contents coverage and additional living expenses apply only if there is "structural damage" to the covered building caused by sinkhole activity."
It is also important to understand what is meant by the "covered building". The term "covered building" is generally synonymous with "principal building" and has been left to the individual insurance carrier to define. It generally includes the main living area of the structure only with any and all appurtenances such as pool decks, in-ground pools, patios, and/or detached structures excluded from the definition.
Knowing which coverage you have is very important in determining if and how your home will be covered by your homeowner's insurance in the event that damage begins to occur. Knowing which type of coverage you have is also important in determining if a Professional Engineer or Professional Geologist can investigate your concerns and determine if sinkhole activity is present beneath your property and it can be determined to be a cause of damage within a reasonable professional probability before the damage becomes too significant to repair or before the potential of imminent collapse occurs.
The post May 2011 Florida Statute §627.706 defines “structural damage” to mean a covered building, regardless of the date of its construction that has experienced the following:
1. Interior floor displacement or deflection in excess of acceptable variances as defined in ACI 117-90 or the Florida Building Code, which results in settlement related damage to the interior such that the interior building structure or members become unfit for service or represents a safety hazard as defined within the Florida Building Code;
2. Foundation displacement or deflection in excess of acceptable variances as defined in ACI 318-95 or the Florida Building Code, which results in settlement related damage to the primary structural members or primary structural systems that prevents those members or systems from supporting the loads and forces they were designed to support to the extent that stresses in those primary structural members or primary structural systems exceeds one and one-third the nominal strength allowed under the Florida Building Code for new buildings of similar structure, purpose, or location;
3. Damage that results in listing, leaning, or buckling of the exterior load bearing walls or other vertical primary structural members to such an extent that a plumb line passing through the center of gravity does not fall inside the middle one-third of the base as defined within the Florida Building Code;
4. Damage that results in the building, or any portion of the building containing primary structural members or primary structural systems, being significantly likely to imminently collapse because of the movement or instability of the ground within the influence zone of the supporting ground within the sheer plane necessary for the purpose of supporting such building as defined within the Florida Building Code; or
5. Damage occurring on or after October 15th, 2005, that qualifies as “Substantial Structural Damage” as defined in the Florida Building Code.
When one or more of the above five items has been met then a comprehensive sinkhole investigation is required to be conducted. Generally, it will require a comprehensive sinkhole investigation to determine if sinkhole activity exists within the subsurface soils unless it already has progressed near to the surface. In rare instances, the finding of no "structural damage" per the statute definition can also be accompanied by the finding of "sinkhole activity" which cannot be eliminated as being at least a contributing cause of damage. In these instances, the claim does not result in a "sinkhole loss" but "sinkhole activity" has been acknowledged to exist and can be expected to continue to cause damage to the home. At some future point in time the ongoing damage most likely will increase to the level of "structural damage" and a new claim may be warrantied. Keep this in mind that the results of the inspection and testing for a sinkhole claim are only specific to the time of the testing. Results in almost all cases cannot be backdated as to when the sinkhole activity began nor can they be forward dated as to never worsening.
As confusing as the above may be, interpretation of the above criteria and the determination as to whether one or more of the above criteria has been met is a point of contention among engineers, geologists and insurance carriers.
The criteria with the most differing opinions as to what it means is Item #4. One interpretation put forth by some engineers is that Item #4 of the "structural damage" definition requires that damage to the structure must be visible at the time of the initial investigation and that this damage must be so severe that the structure is likely to imminently collapse.
Another interpretation is that damage must be present in the structure but does not have to be so significant that visible imminent collapse is possible, but that movement or instability in the subsurface soils is present and this instability is significant enough to likely cause imminent collapse to the structure. These differences in the interpretation of Item #4 are significant to say the least.
The first interpretation or theory requires a visible inspection of the structure and its damage, however, minimal if any soil testing is actually needed as visible damage to the point of imminent collapse is all that is required. The second interpretation or theory requires a visible inspection of the structure to document damage and sufficient soil testing to eliminate the potential for significant soil movement or instability which could then translate to imminent collapse of the structure at some reasonably short term future timeframe because of the thickness and/or weakness of the underlying soils. The difference between these two theories is huge when one considers which one could have possibly prevented the Seffner tragedy or forewarned of the Orlando building collapse. If either the Seffner or the Orlando structures were inspected for a sinkhole claim a week before the event occurred, most likely neither claim would have a positive as to Item #4 if the first interpretation were used in the investigation.
Using the second interpretation, however, it is most likely that the soil arch or bridge beneath the structure and in "the ground within the influence zone of the supporting ground within the sheer plane necessary for the purpose of supporting such building"(statute language) was explored with sufficient soil testing it would have found the soil arch to be too thin or too weak for long term support and a reasonable conclusion of imminent collapse would have been determined before the damage became severe. A positive finding for Item #4 would have triggered the more comprehensive sinkhole investigation that would have determined or at least estimated the size of the underlying soil cavity and the depth to limestone, parameters that could have allowed engineering estimates as to the scope and cost of repairs to be made before the collapse actually happened.
The above differences of looking for instability in the structure and evacuating it after the damage has manifested itself to a severe level as opposed to looking for instability in the supporting soils and stabilizing them before the damage becomes unrepairable is synonymous in some ways with the old saying of "putting the cart before the horse".
Additionally, logic could be used to state that if Item #4 requires damage to the point of imminent collapse then Item #5, at least, would have been met or Catastrophic Collapse coverage (see below) would be triggered and there would be no need for an Item #4 in the first place. Conversely, if the intent of the ambiguous Item #4 language was to address concerns of imminent collapse resulting from an unseen threat in the supporting soils, then the second interpretation makes more sense and Item #5 should be included in the statute language to address the more visible damage concerns.
Since sinkhole activity begins at the surface of the underlying limestone and progresses upward through the overburden soils toward the land surface and is dependent upon time for this progression, the depth of the initial soils investigation is directly proportional to the early diagnosis of detection as well as the extent of remediation.
Too shallow of a soils investigation will reveal no meaningful problem with the underlying soils as the soil arch may not have been sufficiently measured, while too deep of a soils investigation is an oxymoron to some extent as the more data that is gathered by deeper borings and/or testing the more accurate the interpretation of the data can be. Again, the old adage, "it is better to be safe than sorry" appears to have been written with sinkhole investigations in mind.
Lastly, when the claim about suspected sinkhole damage has reached the point of no return where a hole has developed or the house or building has sustained significant damage and one would believe that it is a no-brainer in finding a "sinkhole loss" has occurred, there is a statute definition that still has to be met. This is the one for Catastrophic Collapse coverage.
Catastrophic Collapse – this type coverage is afforded by all insurance companies coming under the auspices of the Office of Insurance Regulation (OIR). This language per F.S. § 627.706 states…..
Sinkhole insurance; catastrophic ground cover collapse; definitions. –
(2)(a) Catastrophic ground cover collapse means geological activity that results in all of the following:
Contents coverage applies if there is a loss resulting from a catastrophic ground cover collapse. Damage consisting merely of the settling or cracking of a foundation, structure, or building does not constitute a loss resulting from a catastrophic ground cover collapse.
The key word in the above definition is "all". If one, two or three of the events listed have taken place, but not all four, then the definition has not been met and insurance coverage is not available under the Catastrophic Ground Cover Collapse definition. That is not to say that all four events may occur at a later point in time after the claim has been submitted and/or investigated; however, at the time of the claim investigation all four events must be in place for it to be considered a covered loss.
This leads to the question that if you have damage caused by sinkhole activity, but it has not reached the level of catastrophic ground cover collapse, how long will your home remain safe to live in before the situation may change for the worse. With respect to safety, if you believe your home is unsafe to live in, contact your local Building Official to make a determination as to whether it should be condemned or deemed unsafe for Human Occupancy. Remember, the catastrophic collapse coverage cannot be met without Item #4 of the four criteria being met.
In summary, the above synopsis of terms, definitions and interpretation of statute language goes to show that insurance coverage for sinkhole claims are being limited by legislative action that was directed to decrease the number of frivolous claims, the expense of claim investigations, and the coverages being afforded by the policy. This legislation has done that to some extent, but it has also shackled the engineer and/or geologist from actually investigating for valid sinkhole activity, limiting the data used to develop conclusions as to whether sinkhole activity may be a cause of damage, and finally, limiting the general public as to coverage for legitimate damage being caused by actual sinkhole conditions.
In order to correct some of the problems associated with the legislation, the members of FAS3 recommend that you the General Public contact your State Legislators and voice your concerns.
George C. Sinn, Jr., P.E.
(See Part I of this message below in April's message.)
Good or sound engineering always involves knowing the conditions that you are dealing with or need to overcome in the design of a structure or the engineering solution to a problem. Good engineering does not provide temporary solutions to problems unless that was the intent of the design in the first place with a more permanent solution to follow.
In nature or real life, I cannot name one disruption of normality that a "one size fits all" fix applies. Examples are floods, some are caused by rising water and raising the structure above the flood plain may be the solution, but for moving water problems raising the structure only make it more susceptible to toppling over. Earthquakes, hurricanes, tornadoes, floods, sinkholes come in various sizes or intensities. To design a fix for a structure to resist the forces of a Category 5 hurricane, more is needed than a structure designed to resist the forces of a Category 1 hurricane. Yes, either can be done because we know the force of each and what to expect from compiling years of data about them. Similarly, if you told an engineer to design a bridge to resist the current in a stream versus the current in a large river the design would be different. In either, assuming the traffic loads to be the same, the design the velocity of the current and the depth of the moving water for both the stream and the river would be needed in order to start the design.
With the above analogies, the maximum forces to be resisted are known or have been quantified in the engineering design. Bridges are designed and built to resist static conditions or a maximum anticipated condition. When the forces are dynamic and change constantly or progress/increase with time, the competent engineer looks for a way to stop the progression or increase in stress that he is designing to address and thereby works with static force parameters that have been quantified. Sinkholes are different! There is no maximum and there is no constant.
With a sinkhole or sinkhole activity, first only limited testing has been conducted to determine that it exists. Secondly, with sinkhole activity the worst of the condition may not be known before the remediation program is designed. Thirdly, with sinkhole activity what you find today may not be the same condition that you find tomorrow or next year because it will increase or grow in its width and depth of affected soils at an unknown rate.
Engineers do not get the opportunity to investigate sinkhole activity for three to five years in a row to determine its rate of progression before designing the remediation plan. To address this changing condition the best approach is to abate its progression or make the problem a static condition and then design a fix to address it. One cannot change a dynamic condition to a static condition by simply sticking pins in it (no pun intended). A doctor does not treat a growing cancer with a bandage, he tries to cut out or remove the cancer first and then fix the damaged tissue.
Turning the dynamic condition of an ever weakening soil profile into a static condition changes the design parameters from unknown toknown and the design becomes one that can fix the problem with a high probability of success.
The same applies to a remediation program for sinkhole activity. An engineer that says just stick pins into the ground and be done with it will have done nothing to address the cause of the problem nor has he done anything to stop the problem from getting worse. Therefore his fix is destined to failure. This same engineer typically also does not know if the foundation that the pins are secured to is sufficient to support the point loads being developed and if he doesn’t want to reconstruct the structure over a new foundation then his design may be flawed. As described in the FAS3 President's message for April of 2013, typical soil supported foundations are not designed to resist positive and negative bending. The best fix is to address and restore the bearing capability of the soils that the foundation rests upon and distributes its loads into, as opposed to guessing at a number of unknowns and recommending a fix that has a low probability of success.
As an example, underpins that are 50 feet long at the time of installation may appear to be founded on sound material by a good resistance to further penetration; however, with the sinkhole process (sinkhole activity) still present and still being able to grow or expand its affect on the soils, next year or the year after the 50 feet long pins may be 5 or 10 feet short of resting on sound bearing material which would exhibit the same resistance to penetration as they did when the pins were installed. These weakened soils also have no lateral support and allow the underpins to bend, buckle or yield and the house to resettle. To go back and lengthen the pins or increase their size after they are in the ground is a waste of time and money and is never done. So there goes any warranty work short of redoing the entire process with little hope of not having to return again in cases where underpins are recommended as the sole solution to remediating sinkhole activity.
This engineer asks, why not just stop the weakening process in the first place and then see what is needed to restore the house or structure to a pre-loss condition. With early detection the fix may be easy and less expensive, while with late detection of the problem, the fix may be more complicated and more expensive.
Sinkhole activity can be remediated, but it is never a "one size fits all" approach to having a successful plan.
George C. Sinn, Jr., P.E.
FAS3 - President
Grouting versus Underpinning – which is better or appropriate in the remediation of sinkhole activity and what is the purpose of each.
The two terms "grouting" and "underpinning" are used by a lot of contractors and their sales personnel to describe what they believe is the appropriate method of repair for sinkhole activity. While State Law (F.S.§ 627.7073.1.b.4.c) says that only the insurer's professional engineer can provide a plan as to land and building stabilization and foundation repair when required, many contractors attempt to convince the homeowner that they know better than the engineer-of-record and that underpinning should be substituted or added to a remediation plan specifying subsurface grouting only.
First, the term "subsurface grouting" or "compaction grouting" is used to define the process of sinkhole remediation by injecting a cement based grout material into the ground under controlled pressure beginning at a depth at which competent limestone is intercepted and progressing under additional controlled conditions from the beginning depth of competent limestone to a depth of approximately 5 to 10 feet below the surface.
The cement based grout material does not contain limestone aggregate as does normal concrete. Grout will flow under pressure and take the path of least resistance in sealing off the open rock crevices and solution channels before setting to form a hard concrete like material. Once set, the grout material is more resistant to deterioration from groundwater than is the limestone it covers.
This process of subsurface grouting is intended to (1) seal off open solution channels leading to subterranean cavities within the limestone which are accepting soils from the identified sinkhole activity, (2) densify/compact the weakened intermediate soil profile caused by a loss of soil into the rock and (3) restore bearing capacity to the overall soil profile supporting the structure.
Since the compaction grout does not extend upward to the bottom of the foundation, many times it is complimented by a secondary program of shallow grouting typically using a chemical material that either permeates and binds the upper sands together or consists of two part chemical that sets upon mixing of the two parts and forms a water/insect/weather resistant polyurethane material that also binds loose sands or forms its own strong rigid bonds. This shallow grouting begins at the depth the influence of the deep grout terminated and continues to the bottom of the structure's foundation. The need for this secondary process is determined by the strength or weakness of the soils within the upper ten feet of the surface and may or not be needed to fully restore bearing capacity.
A well planned and well implemented grouting program will restore bearing capacity to the supporting soils, eliminate further soil raveling through open solution channels leading into the limestone thereby taking away the potential for imminent collapse due to soil instability or movement and allow the structure's foundation to dissipate structure loads as it was originally designed, by way of uniform load distribution to the supporting soils.
Secondly, the term "underpinning" or "pinning" generally refers to the installing steel pipes or "mini-piles" into the ground beneath the structure's foundation. The pins are sometimes installed prior to the grouting (pre-grout pins), after the grouting (post grout pins) or in lieu of the grouting. The steel pins may are generally engineered to provide safe support capacities typical of residential structures when placed five to eight feet on center and when the sides of the pins have lateral support from the surrounding soils. This last condition of soil support is not only critical but difficult to obtain when continued soil weakening from sinkhole activity is ongoing and has not been arrested by a well implemented grouting program.
The steel pipe that composes the pins may or may not have a helix(s) attached to its end to aid in installation and/or load development, but always has some type of manufactured, engineered, steel bracket that attaches the steel pipe pin to the foundation of the structure. The combination of a steel pipe pin and an engineered bracket comprises the underpin assembly.
Because the pin uses a small diameter pipe, typically three inches or less, to aid in installation around an existing structure eccentricity developed by having the pin offset from the center of the wall and its transmitted downward load will cause bending and early failure to the pipe. Therefore, the foundation is cut or notched to move the bracket and pipe pin closer to the center of the wall to minimize eccentricity. The notching of the foundation is a process that requires a lot of care and specialized equipment; however, no matter how careful and skillful the contractor is with the notching process, the foundation is fundamentally changed from its design intent of uniform load distribution to one of point load distribution.
The difference of accomplishing the two different load dissipation methods is one of foundation bending and the reinforcing steel that resists the different bending. For a standard foundation the bending is downward only as the walls push on the foundation and the foundation pushes on the soil. Reinforcing steel for this configuration is placed in the bottom one-third of the foundation's thickness to resist this "positive" bending action. For a point load distribution the foundation is fundamentally changes from one that experiences only positive bending to one that has positive bending between pins and "negative" bending over the top of each pin. This concept changes the foundation from a long span downward bending configuration supported at its ends to one of alternating up and down (positive/negative) bending configuration. Reinforcing steel to resist this alternating bending configuration should consist of bottom reinforcing to resist the positive bending along with top reinforcing to resist the negative bending. Unfortunately, the foundation steel is already in place and top reinforcing steel is absent. It cannot be added. Additionally, the notching process can cause stress rise at the notches, diminish the width of the footing at the notch, and even cut the existing reinforcing steel when it is located at different spacing than the contractor assumed it to be in.
While the above discussion of underpins and their use appears to be entirely negative, it is not intended to be so. Pins have their place in a remediation process and should be considered by the professional engineer for inclusion into the remediation plan when certain conditions exist. These conditions and the benefits of pins are:
First and foremost, controlled lifting to close large cracks or controlled lifting to relevel a portion of the structure or even the entire structure can be accomplished with underpins. Controlled lifting cannot be accomplished by grouting alone and the need for releveling may trump the detrimental effects of changing the foundation load distribution methodology. Remember, reasonable and effective cosmetic repairs cannot be made when the structure is so far out of level that the house becomes a "funny" house with sloped floors or slanted ceilings.
Secondly, underpins can be used to provide support in buried debris that may be prohibitive to stabilization by cementitious grout alone, or highly organic soils that will inhibit the setting process of cementitious grout.
Thirdly, underpins can be used to provide support for the structure on a contingency basis that will resist significant movement of the structure during the initial stages of the grouting process. Upon completion of the grouting process, lateral support for the pins is established and the potential for deterioration of the bearing stratum that the pins develop their support from is abated.
It can be seen by the above discussion, the dynamic process of soil weakening associated with sinkhole activity cannot be arrested or slowed down by underpinning alone. The pins are only as sound as the soil they are founded in at the time the brackets are secured to the house. If the soil supporting the pins continues to weaken due to continuing sinkhole activity the underpins will loose support and damage will reoccur. A simple analogy to the Grout v. Pin discussion is if one thinks of water running down a river and carrying eroded soil with it as being similar to groundwater causing soil to ravel downward into a cavity in the limestone.
One can build a bridge over the river using pile but not stop the erosion of soils from its bank and the widening of the river channel without stopping the water flow. To stop the water flow and the continuing erosion of soils one has to build a dam and not a bridge. In the sinkhole remediation world, underpinning a structure is like building a bridge. It will do nothing to stop the process that the pins are supposed to "bridge" over. Only the injection of grout will stop the soil weakening process of sinkhole activity like building a dam.
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