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Subsidence Investigation Mathew/Aley Rental Property Claim No. 223968 BCI Project No. 03-15564 November 2007 Page 9 4.0 CONCLUSIONS It is BCl's professional opinion that the distress to the Mathew/Aley rental property can be attributed to a combination of factors, including possible sinkhole activity. Based on the results of our investigation, the minor cracking distress to the house is due to minor differential settlement of the foundation, which can be attributed, in part, to the to the long-term densification of the very loose to loose sandy soils that locally underlie the structure. However, the occurrence and morphology of the ground collapse feature below the front porch exhibited the typical characteristics of a cover-collapse sinkhole. When this condition is considered with the localized zone of very soft and unstable limestone encountered in boring SPT-3, it appears that sinkhole activity may be affecting the house to some extent. The analysis described in this report is of sufficient scope to identify sinkhole activity, as defmed by S627. 706-707 Florida Statutes, as a possible contributing cause of the damage with a reasonable, professional probability. Primary factors in our conclusions include the following: . Based on our test pit excavations, the house foundation is embedded at a depth of 16 inches below grade. The addition foundation is embedded at a depth of approximately 4 inches below grade. The hand cone penetrometer probings indicate the surficial sandy soils around the perimeter of the house are very loose to loose to depths ranging from 64 to greater than 96 inches below grade. In the SPT borings, very loose to loose sands were encountered to depths up to 10 feet below grade. These data indicate the house and addition foundations are locally bearing on very loose to loose sandy soils, which are susceptible to the effects of long-term densification. Given these soil conditions, some minor differential settlement of the foundation and slab structures and associated minor cracking would be expected to occur. In addition, a lack of rain gutters along the roof drip edges can result in excessive amounts of moisture being introduced into the near surface sandy soils. This would act to increase the densification effects. . The occurrence and morphology of the collapse feature under the front porch was typical of a recently formed cover-collapse sinkhole. The feature widened with depth, was steeped sided and lacked any association with buried debris that would suggest a shallow origin for the collapse. 8 Evidence of unstable limestone was encountered in boring SPT-3, located on the south side of the house. In the boring, a significant interval of very soft limestone (weight-of-hammer) was encountered from 38Y2 to 47 feet below grade. In addition, a loss of drilling fluid circulation was associated with the very soft rock zone. When these subsurface conditions are considered in conjunction with the cracking distress to the house and the nearby ground collapse, it appears that minor ground settlement associated with soil raveling and a developing sinkhole condition may be impacting the structure to some extent. Overall, the current level of cracking distress to the house is minor. However, remedial measures to stabilize the house against further sinkhole-related settlement should be implemented. The ground collapse feature should be filled with flowable fill (sand-cement mixture) to stabilize and prevent further enlargement of the collapse. In addition, a program of compaction grouting is recommended to address the deeper subsurface zones of very soft and apparently raveled soils. Since the grouting will likely not be fully effective in stabilizing the shallow very loose sands directly below the foundation, a program of shallow chemical grouting is also recommended. While marginally anomalous elevation differentials were found in the floor elevation survey, the minor cracking distress to the structure indicates the elevation anomalies largely are not related to significant movement of the foundation or floor slab. Given these conditions, foundation underpinning to re-level the structure is not necessary. We also recommend that rain gutters with downspout extensions that discharge stormwater a minimum of 4 feet from the foundation be installed on all roof drip edges. A remediation plan is included with this report.