On March 5, 2019 our staff was notified of a depression forming and a large crack in the pavement along southbound lanes of North Pine Hills Road just north of Pipes O The Glen Way. Upon receiving the complaint, our staff immediately visited the area and according to their initial field assessment, this area requires urgent attention, as shown on photo 1.
North Pine Hills Road is a major travel way consisting of two north bound turn lanes, two southbound lanes, a center turn lane, and sidewalk on both sides. This particular section of North Pine Hills Road is built up and retained along the western side by a Mechanically Stabilized Earth (MSE) retaining wall. There is a large depressional area adjacent to this wall that is said to have been formed by a historical sinkhole. The depressional area has been used as a stormwater pond for decades. Due to the prevalence of the sinkhole formations in this part of Orange County, the County requested continuing engineering services to perform an emergency geotechnical investigation, analysis, and remediation of the subsidence.
Moreover, since this main travel way is the route for school buses and commercial traffic a detour and traffic plan was immediately put in place and arrangements were made to have traffic police officers at the main intersection of Pine Hills Road and Indian Hill Road at peak hours.
The geotechnical consultant performed Ground Penetration Radar (GPR), shallow and deep boring, in this area within 24 hours. To assist in this operation, our in-house crew removed the pavement in this area and closed the southbound lanes of Pine Hills Road. While performing these tests, the retaining wall between the road and the pond showed significant movement and strengthening the foundation of the wall is now immediately needed to prevent a total wall collapse. A video of the existing stormwater drainage pipes in the vicinity of the work area was conducted to check for structural integrity and potential pipe joint failures. The results displayed the pipes were not structurally impacted by the road embankment movement and were functional.
The roadway embankment widening (early 1980’s) in the zone of interest is a high fill placed within the slope/body of a very deep natural, land-locked sinkhole lake which is subject to significant water level fluctuations during periods of excess/deficient cumulative rainfall, see photo 2.
The water level in the lake can reach as high as +91 ft. NAVD but declines to below +69 ft. NAVD during drought periods. In the early 1980’s, Pine Hills Road was widened with roadway embankment created by filling in the eastern slope of the pronounced circular depression (ancient sinkhole, also known as a paleosink) and constructing a 15 ft. high retaining wall for grade change. The reinforced earth company designed a segmental concrete panel 15 ft. high retaining wall which is supported by reinforcing strips extending 14 ft. into the roadway soil back fill.
Further geotechnical assessment test were performed:
- A series of piezocone probes were set within the roadway in the vicinity of the observed cracking. A total of eight (8) test points were performed with a total footage of 584 ft. Four (4) Standard Penetration Tests (SPT) at the base of the Retaining Wall (Lake Side) along the observed cracking to assess the deep subsurface conditions in this area, see photo 3. A total footage of 246.5 ft. was drilled in this area.
- Ground Penetrating Radar (GPR) imaging of the affected area of the roadway and its immediate vicinity to detect the presence of any anomalous subsurface features within 20 ft. of ground surface.
- An Electrical Resistivity Imaging (ERI) survey (deeper imaging than GPR) of the affected area of the roadway and at the base of the retaining wall along the observed cracking to detect the presence of any anomalous subsurface features.
- An aerial drone survey (photos, video, topo, etc.) of the vicinity including the area of the observed cracking on the roadway and adjacent retaining wall using drones.
Based on the geotechnical test results and consultant literature review of information, it was revealed that Karst sinkhole activity were present. The roadway embankment in the fracture area was filled over a part of the sinkhole lake which is now a remnant of the original sinkhole depression. Moreover significant settlement of the wall footing was determined. The retaining wall and sidewalk showed signs of splitting away.
From the soil information, as shown in figure 1 that the soils are very loose to 10 ft. below the retaining wall footing for a significant length (50 ft.). There are access difficulties to work at the base of the footing otherwise a sheet pile wall could have been installed as toe cutoff. A structural engineer has also been engaged to review the movement of the concrete panels in the reinforced earth retaining wall.
The consultant recommended along with our concurrence to have the contractor install seventeen (17) pin piles along the base of the retaining wall to underpin the wall and stabilize any further movement, caused by the loose subsurface foundation soils at the base of the wall, see photo 4.
The pin pile embedment depths range from 15 ft. to 23 ft. based on the subsurface conditions revealed by the SPT boring in the area, see figure 1.
The contractor dewatered the area under the footing so they can install additional beam support under the wall foundation, see conceptual sketch figure 2. The piles and beams installation process was completed within three days and the wall movement appears to be stabilized with underpinning and grating. Overall the situation of the soil movement is very dynamic which requires daily adjustments to the plan according to our consultant and contractor.
To permanently address this subsidence, the consultant has completed the Standard Penetration Test (SPT) for this area and they have designed a plan to grout the loose area located 48 to 52 ft. below the road surface. Upon completion of grouting, with the area fully stabilized, the roadway reconstruction commenced and the expectation was to get the roadway open to traffic by Tuesday, March 26, 2019. Staff had to work all weekend and late Monday, March 25, 2019, to accomplish this feat.
The cross section of the roadway reconstruction is 165 ft. long and 23.4 ft. wide. Staff had 14 ft. offset from the retaining wall to avoid disturbance of the reinforced earth straps which encroached about 3 ft. into the roadway area. Crew then excluded disturbing the area and commenced re-construction with a composite of roadway section using the geogrid as shown in figure 3.
The total cost of the evaluation and re-construction retrofit was a little more than $280,000.00. The work was performed expeditiously to shorten the motoring public inconvenience as well as to lessen the public school and emergency vehicle impact. All went well and the public at large was very happy with the outcome.
Deodat Budhu, M.S.; P.E., is the manager of the Roads and Drainage Division for Orange County Public Works, Florida.