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Oakbend Addn-CS 900806 GINN, August 6, CONSULTING ENGINEERS Mr. Steven G. Goram Director of Public Works city of Coppell P.O. BOX~~ , ~ __ Re: ~akbend Addition ~r. Goram: '' As per o~'-~t~h~ne conversation of Friday afternoon, August 3, 1990, we have prepared this letter to address the issues discussed regarding the storm sewer system proposed in Thweatt Rd. and escrow amounts relating to the future improvements of the roads adjacent to the subject addition. With the short notice we have been given for the comparisons of probable cost for the roadway escrow amounts included herein, we have prepared the comparisons on the basis of cost figures which are readily available to us, which may or may not be representative of either current or future costs of construction. In our review of the plans for this subdivision we became acquainted with the storm sewer system proposed in the south side of the R.O.W. of Thweatt Road. To the best of our knowledge neither Ginn, Inc. nor the city of Coppell has specifically required that the proposed enclosed system in Thweatt Rd. be installed as a part of this subdivision. Typically, the storm sewer systems for a residential subdivision adjacent to an unimproved thoroughfare such as Thweatt Road are allowed to outfall through headwalls into the roadside ditches as long as adequate runoff capacity for the more frequent rainfall events can be accommodated. With regard to the future use of the proposed storm sewer in Thweatt Road when the roadway is improved, there is no guarantee that the proposed system can be effectively utilized at that time. Without even preliminary engineering to develop the proposed grades and hence, the drainage areas served by the future improvements it is difficult to predict the necessary pipe capacity which must be provided or the depth at which the line must be installed to allow them to properly serve the adjacent properties. In our review of the proposed system we noted that the storm sewer in Thweatt Road is probably too shallow to serve the future westbound lanes on the north side of the road. We have also commented on the fact that the proposed outfall for this system presents some serious problems to the safety of the traffic on the existing roadway during storms. 17103 Preston Road · Suite 100 · LB 118 · Dallas, Texas 75248 · Phone 214/248-4900 In consideration of the above it appears to us that it is unlikely that the storm sewer system proposed to be installed in Thweatt Road as a part of the Oakbend Addition will be used in the future improvement of the road. The estimated escrow amount that has been recommended to be levied on the developer by the City of Coppell does not include any consideration for the credit the developer is proposing, nor does it include the possible additional costs of removal of either all or part of the proposed system upon improvement of the road. During our conversation you also requested that we clarify estimates for escrow amounts applicable to two lane and four lane undivided streets, 37' and 45' widths, respectively. Without R.O.W. considerations the basic difference in construction costs are for the additional width of roadway including pavement, subgrade preparation and earthwork. Our recent experience indicates that the construction cost of a 37' undivided road is approximately $200/LF, or $100/FF. Our quick analysis of the probable cost of a four lane undivided street, 45' in width, indicates that it would cost approximately $19.99 more per lineal foot. This would then result of an approximate probable cost of $219.99/LF, or $110.00/FF for the 45' undivided road. The third item you requested on Friday was a quick analysis of the difference in cost of a four lane divided road built in the R.O.W. for a six lane divided road (110'), vs. a normal six lane divided road. Our recent experience indicates that a six lane divided road has an approximate construction cost of $400/LF, or $200/FF. Our quick analysis of the approximate cost of a four lane divided road in the same R.O.W. indicates that this would cost $59.19/LF, or $29.60/FF less than the six lane divided road, which translates to an estimated construction cost of $340.81/LF, or $170.40/FF. The fourth item you requested was a comparison between the construction costs of a four lane undivided roadway in a normal R.O.W. and a four lane divided roadway in a six lane divided road R.O.W. (110'). As noted above the normal four lane undivided road costs approximately $200/LF and the four lane divided road in a six lane R.O.W. costs approximately $340.81/LF. It was unclear during our conversation whether you wanted a comparison between a four lane divided road in its normal 85' R.O.W. and a four lane divided road in a six lane divided road R.O.W. (110'). In any case we've done a quick analysis of this also. The results of our analysis indicate that the four lane divided road in a six lane R.O.W. should cost approximately $6.45/LF, or $3.23/FF more than a normal four lane divided road in the usual 85' R.O.W. The main difference being in the amount of R.O.W. preparation and median preparation required in the wider R.O.W. and median of the six lane divided road R.O.W. If necessary we will be happy to transmit copies of the worksheets we used in determination of the differences in cost figures included herein. Please call me if you have any questions. Sincer. ely,/ John C. Karlsruher, P.E. Senior Engineer attachments cc: H. Wayne Ginn, P.E.