Overview

The culvert to be repaired consists of a 36-in cast-in-place concrete circular culvert with approximately thirty-six inch wing walls. From visual rain event observations on September 1, 2007, the culvert appears to be installed properly with the size, length, and slope providing adequate capacity prior to neglect and post installation modifications. Since installation, the culvert appears to have received no maintenance and consequently has severely eroded inlet approach and exit apron. The haphazard addition of a misaligned 18-foot section of a 24-inch reinforced concrete pipe has severely restricted the capacity of the drainage components. The 24- inch culvert section is assumed to have been added for the protection of nearby business and residential structures. Unfortunately, the additional culvert only alleviated or diverted the erosion problem long enough for additional structures to be built near the outfall of the high flow culvert. The adjacent market and a newly constructed public gathering building were in eminent danger of collapse if erosion is allowed to continue at the present rate.

Existing Culvert Removal and New Channel Excavation

The first step in the project was the removal of the existing 24-inch inch culvert to make room for the new trapezoidal channel. The culvert section was approximately fifteen feet long and buried halfway in clay and large rock with an estimated weight of 15,000 lbs.  The removal of the material obstructing the culvert was conducted on Monday, August 18th. For safety considerations, removal of the existing 24” culvert was deemed necessary by heavy construction equipment. Figure 15 depicts the trapezoidal channel design.

Through the local Assemblyman, arrangements were made to remove the lower 24-inch culvert with heavy equipment from Mansco Quarry on Monday, August 18th. The area was then shaped to accommodate the forms for the new trapezoidal channel. Due to the enthusiastic work attitude of the residents of Nsumia, more native material was removed  than necessary, however, the EWB viewed the event as a demonstration of the  commitment by the villagers to work with the EWB Team. Sediment and debris were also removed from within the 36-inch culvert and the existing channel downstream to provide adequate flow. The 36-inch culvert’s cross-sectional area was reduced by approximately 75-percent due to sedimentation prior to cleaning.

Concrete Placement

The base of the concrete channel was poured on Wednesday, August 20th.  All concrete was mixed by hand in an open area adjacent to the project site. It was placed with a thickness of six inches and reinforced with rebar to ensure adequate strength if the channel were to reach full flow capacity. The most difficult part of pouring was convincing the aiding community members that a 1-2-3 mix was needed, (a mixture that is one part cement, two parts sand, three parts aggregate).  The traditional mix utilized by the local population is composed of seventy-five percent sand and twenty-five percent concrete, which explains the short design life for many locally engineered structures.  The traditional mixture is cheaper, but also weaker than the proper way of mixing concrete and often lasts only 1-2 years. The base of the concrete channel, over two cubic yards, was entirely poured on day five by four team members and a dozen community volunteers.

Existing Culvert Removal and Channel Excavation

The day after pouring concrete Friday, August 22nd the outflow of the trapezoidal channel was lined with rip rap. The rip-rap used to line the channel extended 115 feet and was composed of six-inch to 24-inch diameter stone. This was significantly longer and consisted of larger rip rap than estimated. With the help of the villagers 30 cubic yards of stone was placed in a single day.

The additional rip rap rock was placed around the 36-inch culvert inlet to act as a filter for sand preventing clogging of the culvert. The rock would also protect against the erosion of the culvert’s inlet by dissipating energy to ensure the roadway does not continue to deteriorate over time. The Community Youth Leader, Nicholas, was appointed site manager and agreed to maintain the channel by keeping it free of debris such as sand and garbage.

On Saturday (day eight), two days after the pouring the sides of the channel, the Nsumia village removed the forms for the top of the channel prior to the EWB group’s arrival.    The third day after pouring the concrete, day nine, the bottom of the forms were removed voluntarily by the community prior to the team’s arrival at the site The village was in the process of backfilling the void behind the channel’s wings using the soil left from excavating the area for the new channel. Also, several masons from the community were smoothing out the face of the channel with mortar. Rip-rap was used to cover the edges by the channel.