Sinkholes

GENERAL SINKHOLE DISCUSSION: The Sinkhole Type, Development and Distribution in Florida Map, prepared by the U.S. Geological Survey, states: "Sinkholes are a natural and common geologic feature in areas underlain by limestone and other rock types that are soluble in natural water. The term sinkhole is used for closed depressions in the land surface that are formed by surficial solution or by subsidence or collapse of surficial materials owing to the solution of near-surface limestone or other soluble rocks..." The formation of sinkholes is the result of the long-term dissolution of underground rock formations (usually limestone or dolomite) by acidic water.  As groundwater percolates through the soil, it reacts with decaying organics and carbon dioxide and forms a weak carbonic acid. Over time the acidic water dissolves the susceptible stone.  Sinkholes are a product of the underlying geology, and are generally limited to areas where the water table is below the limestone surface. The map linked above breaks the State of Florida into four areas. As indicated on the map, Area I includes almost all of South Florida along with the central and western areas of the state, and has few sinkholes. Area II includes the east coast and areas around Lake Okeechobee and also has few sinkholes. Sinkholes, according to the Bureau of Geology are “most numerous” in Area III, which includes the greater Tampa area, middle portions of the state, and portions of the panhandle. Area IV includes areas throughout central and north Florida. Sinkholes may cause structural damage because they can contribute to or cause a loss of soil support beneath a structure.  The manifestation of sinkhole damage in a structure is often very similar to other causes of significant settlement.  These include the presence of organics or loose soil, both of which may indicate the improper preparation of the subgrade prior to the original construction of the home. Significant long term settlement damage to CMU walls is discussed here; settlement damage to concrete slabs is discussed here. Aside from the most dramatic of cases, the potential for sinkhole activity at a subject property usually cannot be determined without a geotechnical investigation. Such an investigation involves procedures such as SPT borings, GPR, floor elevation study, etc. and may cost as much as $10,000 or more. Even if potential sinkhole activity is determined, this does not necessarily mean that the sinkhole activity has or will contribute to home damage or loss. INSURANCE CLAIMS: Sinkhole activity, where existing, often occurs at depth and may or may not cause or contribute to home damage. The issue of whether or not a sinkhole loss exists (with regard to an insurance claim) was addressed by the State of Florida in 2011.  Florida Statute 627.706 took effect on May 17, 2011 and contains the following provisions:

(1)(a)  Every insurer authorized to transact property insurance in this state must provide coverage for a catastrophic ground cover collapse.

(2)(a)  “Catastrophic ground cover collapse” means geological activity that results in all of the following:

(1) the abrupt collapse of the ground cover; (2) a depression in the ground cover clearly visible to the naked eye; (3) structural damage to the covered building, including the foundation; and (4) the insured structure being condemned and ordered to be vacated by the governmental agency authorized by law to issue such an order for that structure

As indicated above, catastrophic ground cover collapse coverage is required to be provided.  However, a 'sinkhole loss' may occur without meeting the "catastrophic" criteria listed above.  The Statute defines "sinkhole loss" as follows:

(2)(j)  “'Sinkhole loss' means structural damage to the covered building, including the foundation, caused by sinkhole activity..."

This means that without 'structural damage' there is no 'sinkhole loss' with respect to an insurance claim.  'Structural damage' is defined in the Statute as follows:

(2)(k)  "'Structural damage’ means a covered building, regardless of the date of its construction, has experienced the following:

(1) interior floor displacement or deflection in excess of the 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 the 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 movement or the instability of the ground within the influence zone of the supporting ground within the sheer plan necessary for the purpose of supporting such building as defined within the Florida Building Code; or (5) damage occurring on or after October 15, 2005, that qualifies as ‘substantial structural damage’ as defined in the Florida Building Code.”

With regard to (2)(k)(1) above, acceptable variances of an interior floor displacement or deflection are found in ACI 117-90 and the Florida Building Code (FBC).  ACI 117-90 contains a level requirement of 3/4" over the entire surface of a concrete slab on grade.  In other words, it requires that the elevation of the lowest point on a slab surface be not more than 3/4" less than the highest point on the slab.  Of course, this does not apply to intentional slab elevation changes such as at a shower depression or garage floor slopes.  Table 1604.3 of the 2010 FBC, Building limits elevated floor system deflection to L/240.  This is equivalent to 1" over 20', 1/2" over 10', etc. With regard to (2)(k)(2) above, acceptable variances of a foundation displacement or deflection are found in ACI 318-95 and the FBC.  Table 9.5b of ACI 318-95 limits deflections to L/240 for "...floor construction supporting or attached to nonstructural elements not likely to be damaged by large deflections."  This table limits deflections to L/480 for "...floor construction supporting or attached to nonstructural elements likely to be damaged by large deflections."  The 2010 FBC, Building Paragraph 1604.3.1 states:  "The deflections of structural members shall not exceed the more restrictive of the limitations of Sections 1604.3.2 through 1604.3.5 or that permitted by Table 1604.3."  Paragraph 1604.3.2 applies to reinforced concrete, and references the deflection requirements of ACI 318.  The current referenced standard, listed in Chapter 35 of the 2010 FBC, Building is ACI 318-08.  Table 9.5b of ACI 318-08 remains unchanged from 318-95.  Table 1604.3 of the 2010 FBC, Building limits floor member deflections to L/240. With regard to (2)(k)(5) above, please refer to Structural Repairs to Existing Buildings which includes a discussion of Substantial Structural Damage. The Statute also provides definitions of “primary structural member” and “primary structural system” as follows:

(2)(d) “Primary structural member” means a structural element designed to provide support and stability for the vertical or lateral loads of the overall structure.

(2)(e) “Primary structural system” means an assemblage of primary structural members.