By Jon A. Brake
The Sub-surface Exploration Geotectonical Report is done, complete and it is not good. What the report tells the City of Manhattan is that it will be expensive to build on about half of the new Manhattan Corporate Technology Park land.
Of course the City knew that when they sold a lot in the northeast corner to Western Wireless a few years ago. When construction was started equipment got stuck in mud and the company move to lot 28 which was high and dry.
The City purchased the 190 acres paying almost $1 Million for the land and then put in streets, sewer and water which added to the cost. In last weeks Free Press it was reported that the City is paying $250,000 per year for the improvements and will continue to pay until the lots are sold.
Here is part of the report:
ANALYSIS AND RECOMMENDATIONS
Based on the information obtained from the borings and other sources, GeoSystems developed Figure 101 in Appendix A, which indicates the constructability rating for the remaining lots in the MCTP development. As can be seen from the figure, the MCTP development has been divided into areas that have been shaded in green. red and blue. ~ green shaded areas have been assigned a high constructability rating, blue areas have been assigned an intermediate constructability rating and red shaded areas have been assigned a low constructability rating. The following paragraphs provide an explanation of the constructability ratings.
The areas that are shaded in green on Figure 101 were assigned a high constructability rating. In these areas, there was no evidence of or shallow groundwater or subsurface conditions that would require atypical construction techniques or time frames to develop buildings, parking lots and roads. For the areas with a high constructability rating, the data indicates that office/warehouse type structures can generally be supported on conventional shallow spread footings. Multi-story structures with column loads in excess of 250 kips and/or buildings with very low settlement tolerances may have to be supported on other types of foundations, such as piling or drilled piers.
The subsurface data also indicates that standard subgrade preparation methods can be used to develop subgrades that would be suitable for support of building floor slabs and pavements in the high constructability areas. Standard subgrade preparation would include stripping of topsoil, moisture conditioning, recompaction and proofrolling of the subgrade prior to placement of structural fill and/or construction of floor slabs and pavements. In areas where fat clays are present near finished floor levels, standard subgrade preparation would also include undercutting of building areas to allow placement of 12 to 18 inches of select, low volume change material or stabilized soil below grade supported floor slabs. Underdrains or other special measures to control groundwater levels would generally not be required in areas assigned a high constructability rating. The soils encountered below any organic topsoil layers would be suitable for use as structural fill.
Areas shaded in red on the Figure 101 were assigned a low constructability rating. In these areas, there was evidence of subsurface conditions and/or shallow groundwater that may require atypical construction techniques and/or time frames to develop buildings, parking lots and roads. Most of the lots that were assigned a low constructability rating had shallow groundwater, which could hamper site grading and development of a suitable building pad. For the areas with a low constructability rating, the data indicates that dewatering in conjunction with soil stabilization, undercutting and/or preloading of the onsite soils may be required to develop stable subgrades for support of building foundations, floor slabs and pavement sections. Sites located in the red shaded areas will require site-specific geotechnical reports that address temporary dewatering requirements during construction as well as underdrains and/or other measures to permanently lower groundwater levels in areas that will be impacted by the planned construction.
Buildings constructed in the red shaded areas could be supported on conventional spread footings, post-tensioned slabs, piling and/or other foundation systems. The type of foundation best suited for a specific site will depend on the magnitude of the foundation loads, type and configuration of the proposed structure, finished grades and other aspects that are unique to the project being developed. Because of this, the selection of foundation systems should be made by a geotechnical engineer that has specifically been retained for the subject site.
Subgrade preparation in the red shaded areas may require undercutting to remove existing fill and/or soft, naturally deposited soils in order to develop stable subgrades for support of building floor slabs and pavements. Soil stabilization, preloading and other options can be used in lieu of undercutting. The selection of appropriate methods for site preparation, stabilization and/or subgrade preparation should be the prerogative of the geotechnical consultant retained for the project lot in question.
The intermediate blue rating on Figure 101 is indicative of those areas that fall between high and low rated areas. The areas that have been shaded in I)Iue are not expected to have significant groundwater problems or problems with soft, natural soils. Rather, the blue shaded areas within the MCTP site have been identified as having been previously filled or disturbed by previous site grading or construction. Site development in the blue shaded areas may require more extensive site preparation work to develop grades that are suitable for placement of structural fill or for support of building foundations, floor slabs and pavements.
It is anticipated that most of the sites in this area can be developed by undercutting and removing the uncontrolled fill that was previously placed, and replacing this material with controlled structural fill. Based on the information obtained from the borings, it is anticipated that much of the existing fill can probably be reused to develop structural fill sections for support of building foundations, floor slabs and pavement sections. Dewatering in localized areas may be required during construction of foundations and deeper utility lines.
With proper site preparation methods, it is anticipated that most single-story office/warehouse type structures can be supported on conventional shallow spread footings that bear in the naturally deposited soils or controlled structural fill. The final selection of appropriate foundations will depend on the specific requirements of the individual project, and should be the prerogative of the geotechnical consultant retained for specific projects.
The subsurface data obtained from the borings and other sources indicated that the MCTP site has areas that are underlain by shallow groundwater and subsurface conditions that may require atypical construction techniques and/or time frames to develop buildings, parking lots and roads. These areas have been delineated in Figure 101 in Appendix A to the extent possible, given the limitation of the exploration budget for this project. As can be seen from the figure, the vast majority of the MCTP site is underlain by typical river valley and upland soil deposits that are not expected to require any special construction techniques or extended time frames for development. Even the sites that have assigned a low constructability rating can be successfully developed with the aid of an experienced and knowledgeable geotechnical engineer.