The illustration shown below depicts the system analyzed. As-built drawings and survey information were not available; aerial photos and survey shots of inverts were used to determine the system’s geometries.
3
Findings
A site visit was conducted on December 13th to determine damages caused by the flood event which occurred on April 29-30th, 2014. From discussions with the County, sink holes were evident in two areas; however, these sinkholes have been filled in and the only evidence is the stabilization measures taken. Figure 2 illustrates these areas:
Figure 2
The length of 18” pipe (HDPE) north of Almax Court was video inspected. After reviewing this video, cracks in the pipe and improper fitting joints were evident at several locations (Figure 3 & 4). In addition, the pipe contains a large degree of sediment near the north sinkhole (Figure 5). The downstream ditch also contains a large degree of loose sediment (Figure 6).
Given the large amount of rain which occurred over the night of April 29th, the ground surrounding the pipes was totally saturated. The hydrostatic pressure from the saturated ground coupled with the unsealed pipes allowed for soil to be drawn into the pipe once they stopped flowing full. Surcharged pipes, in this instance, would prevent soil intrusion. Sinkholes formed as the ground became unstable from the sediment being transported away underneath.
Figure 3 – Pipe Cracking
5
Figure 5
Erosion from surface runoff can be ruled out given the confined nature of the displaced earth. Surface runoff would continue downstream until a mild grade or a more stabilized surface was reached. This is not evident in here.
The pipes under Almax and to the south are 12” in diameter. These pipes are too small for video inspection. The southernmost inlet is filled with sediment. A similar sinkhole appeared south of Almax; therefore, even without visual evidence in the small pipes, it can be deduced that the same mechanism caused this sinkhole.
In conclusion, it can be said with confidence that the sinkholes which appeared after April 29th were caused by the extreme rain. Given the volume of water experienced, the prolonged period of saturated earth exacerbated the damage. Given the age of this system and its unproblematic history, these damages would likely only occur during a flood event of this magnitude. See “Flood Remediation” below for corrective measures.
Other Considerations
Other damages to the stormwater system are evident. These damages include cracked pipes and improper fitting joints. In addition, from the capacity analysis of the stormwater system, it appears the inlets and 12” pipes are inadequately sized. All replaced pipe should meet County Specification 2600; Sections 3.3.G and 3.3.I require concrete pipe under roadways and 18” minimum pipe size, respectively.
It is believed that the cracked pipes and improper fitting joints are due to the construction methods used. Overburden load which can add excessive stress to the pipes does not appear to be a factor; the pipes appear to have adequate cover (min 12”) and consolidation is not evident at the surface meaning proper backfill compaction was likely achieved. Most notable is the damage appears frequently along the total length of pipe only at joints, with no other indication of damage between joints.
Sinkholes will not likely form except for extreme rain events; however, pipe clogging and soil transport downstream will likely occur over time. Therefore, it is recommended that all underground pipes in the location considered be replaced. All 12” pipes should be replaced with 18” pipes. HDPE pipe, similar to the damaged pipe, should be adequate for this application given it is installed per the manufacturers and County specifications.
7 The stormwater inlets at the intersection of Almax Court should be replaced to properly accept new storm pipe. Inlet efficiency (i.e. inlet capture & bypass flow) is an important design factor. The 18” slots in the inlets do not provide enough length along the gutter to effectively capture the flow. These inlets should be replaced with a properly designed curb inlet, such as a County Type “A”.
The County should consider restoring the downstream drainage ditch. Soil from the sinkholes has been transported into the ditch. Also, there is no end treatment on the pipe. It is suggested that the pipe receive a concrete mitered end section, the loose sediment removed from the ditch, and the ditch reshaped immediately down stream of the new mitered end.
Flood Remediation
The following photos, figures 7 & 8, illustrate the areas which have been previously repaired:
Figure 7 – Almax, looking south
9 From visual inspection, proper backfill methods and surface stabilization have been utilized. The following calculation is provided to estimate fill material needed:
Average sinkhole depth = 18”
Volume of Sinkhole 1 = (100 SF) * (1.5’) = 150 CF Volume of Sinkhole 2 = (110 SF) * (1.5’) = 165 CF Total Volume = 315 CF = 12 CY
The following estimate provides a value to the repairs needed and is based on the current Escambia County Pricing Agreement:
Item
No. Description Aprrox
QTY Unit Unit Price
Approx Total
1 Mobilization 1 LS $2,650.00 $2,650.00
2 MOT Plan 1 EA $750.00 $750.00
3 Clearing & Grubbing 390 SY $2.00 $780.00 4
Earthwork Fill (In Place &
Compacted) 12 CY $9.80 $117.60
5 Earthwork Establishing Grade 390 SY $1.60 $624.00 6 Stabilization of Sinkhole Areas 1 LB $13.00 $13.00
7 Pipe Removal 389 LF $12.50 $4,862.50
8 Install 18" HDPE Pipe 341 LF $31.50 $10,741.50 9 Install 18" Concrete Pipe 48 LF $35.50 $1,704.00
10 Sawcut Existing Asphalt 48 LF $1.75 $84.00
11 Sawcut Existing Concrete 6 LF $2.00 $12.00
12 Remove Existing Asphalt, 2" Thick 60 CF $2.00 $120.00
13 Remove Inlets 2 EA $525.00 $1,050.00
14 Lateral Pavement Patch w/ 6" GAB 40 SY $43.00 $1,720.00 15 County Type A Inlets 2 EA $3,133.00 $6,266.00
16 County Roll Type Curb 6 EA $14.85 $89.10
17 Misc Concrete 2 CY $368.00 $736.00
18 18" Concrete Mitered End Section 1 EA $855.00 $855.00 19 Sod Downstream Drainage Ditch 440 SY $2.35 $1,034.00
20 Silt Fence, Type III 400 LF $2.25 $900.00
21 Hay Bales 40 EA $9.35 $374.00
22 Performance bond 1 LS $32,168.43 $3,246.25
Sub-Total $38,728.95
23 MOT $1,774.76
24 Design Fees @ 20% Sub Total (Engineering & Surveying) $7,745.79
Total $48,249.50
