A CASE STUDY
GEOSYNTHETICS IN PORT CONSTRUCTION
WILMINGTON HARBOR SOUTH PROJECT
- Design Characteristics
- 1 General
- 2 Foundation Conditions
- 3 Design Parameters
- 4 Recommended Plan
- Construction Methods
- Problem with Geotextiles
- 1 Seam Strength
- 2 Seam Alignment
Geosynthetics are used as reinforcement for port constructions. Wilmington Harbor South Project includes a 2440 m long dredged soil adjusted region that has 3.65 m height containment embankment closed to Daleware. Woven geotextile was used in construction as tensile reinforcement over foundation. Foundation incorporates soft, weak silts and clays forming the tidal flats and shallows of the Delaware River. Planning of construction period was seen as 12~18 month. Design characteristics and construction methods along with conundrums confronted to date are addressed.
Geosynthetics are man-made materials, and they have a large usage in civil engineering. A specific example of geosynthetics’ usage is port constructions. The main aim of these kind of projects is stabilization of weak, soft soil in construction area. According to the special design of Wilmington Harbor South Project, geosynthetics usage could be investigated.
The Wilmington Harbor South Disposal Area Project is situated close to the connection of the Christina River and Delawere River about 47 km south of Philadelphia. The general plan of project is indicated in Figure 1. The project was allow content for dredged material from harbor dredging of about 765 000 m3 per year. The prime contractor is American Dredging Company, and original bid price of this project was $19.315.638. Besides comprehensive price is approximately $23.500.000 with modifications. The contract period was from 1987-1990. Geotechnical Project belongs to Geotechnical Section of the Design Branch, Planning/Engineering Division of the Philadelphia District of the US Army Corp. of Engineers, which was incorporated uniquely for this project.
Fig. 1. Wilmington Harbor South general plan. (Bruce L. Uibel-Overview of Wilmington Harbor South Project)
The disposal area containment structure will be an embankment that has a length as 2440m assembled of granular material. This material was taken from a borrow area that located in the Delaware River of Cherry Island. Also this area was just upstream of the site that decreased cost of embankment.
2.2. Foundation Conditions
Subsurface investigations were accomplished at the site to define the conditions below the dike embankment built up over weak highly compressible silts and clays at depths averaging 27.4 m. The borrow area accommodates sands and gravel with varying amounts of cobbles and boulders dropped in a Pleistocene channel situated on the west side of Delaware River about one mile upstream of the new disposal area. These deposits are concealed with a thin silt and clay deposits differing in depth from 0 to 7.6 m. These layers were excavated. Besides, Figure 2 represents the stratigraphy of the disposal area development.
Fig. 2. Wilmington Harbor South Disposal area development. Typical cross-section. (Bruce L. Uibel-Overview of Wilmington Harbor South Project)
2.3. Design Parameters
Soil demonstration appointed for testing were typical of the materials confronted within the main representative of the foundation, and within the borrow area. Tests were done to classification of demonstration, and had a conjecture about behavior of soil, consolidation, permeability, triaxial shear, unconfined compression.
Design parameters were chosen according to slope stability, bearing capacity and settlement analysis. In addition this formation was cogitated the “firm base” in the design analysis. Furthermore the consequences of these tests are represented in Figure 3.
Fig. 3. Adopted Design Parameters (Bruce L. Uibel-Overview of Wilmington Harbor South Project)
2.4. Recommended Plan
Although the poor condition of foundation, construction had to be built more economic, suitable, with minimal displacement and maximal stability. The appropriate clarification was using the high strength geotextile for tensile reinforcement. Woven of high-tenacity polyester yarn geotextile was chosen which was specifically designed for this project. Placement of this reinforcement material could be examined in two stages.
First stage can be explained as, averages about 3 m in thickness by 183 m in width and forms the wide berm of the dike section and has 5 separate hydraulic fills. Vertical drains through the granular fill to a depth 12.2 m below the geotextile. Wilmington Harbor South disposal area development are seen in Figure 4 with Typical Stage I embankment detail.
This speed the consolidation and strength gain of this critical portion of the foundation. In order to balance the loads on the geotextile, the total weights of segment 1 and 2 were kept approximately equal.
Fig. 4. Wilmington Harbor South disposal area development. Typical Stage I embankment detail (Bruce L. Uibel-Overview of Wilmington Harbor South Project)
Second stage can be explained as, embankment consisting of a 3.3 m high dike with a slope 3:1 and a 3.7 m top width as Figure 5 represents. A layer of lighter weight geotextile is used as filter between the granular embankment material and riprap.
Fig. 5. Wilmington Harbor South disposal area development. Typical Stage II embankment detail (Bruce L. Uibel-Overview of Wilmington Harbor South Project)
The high strength geotextile tensile reinforcement from panels was measuring 3.6 m and 5.2 m in width. These panels are continuous for their full length in the warp direction.
The seamed sections are then rolled onto axles for shipment to the site. When field seaming is completed, the tensile reinforcement fabric is placed on the prepared foundation surface. The vertical drains are installed following completion of the stage I fill as previously explained. Generally areal view of construction site on 1 September 1987 is shown in Figure 6.
Fig. 6. Areal view of construction site on 1 September 1987(Bruce L. Uibel-Overview of Wilmington Harbor South Project)
Problems With Geotextiles
4.1. Seam Strength
The nominal seam strength was established as 140 kN/m. This strength was considered destined based upon the use of low ground pressure equipment. Moreover this value was considered necessary based upon the fill slopes which were specified in the contract.
During the early period of construction extending can be neglected but at 1988 the value of extending increases and it was seen that seams have to be controlled. On the other hand the development of seam strength was not contented as seen. A completely new project had been done to find new clarification to the conundrum. The under strength seams were arranged but these were placed constraints on the placement of construction equipment on the fill. As a successful result of this project, this method provided the same factor of safety. used in the main design.
2. Seam Alignment
Irregular alignment was accepted as the proper alignment of the fabric seams. But there were constant or persistent problems. Conundrum was that material, acted unforeseen under combined effects. Then the geometry of bottom configuration and barge orientation culminated in a slightly different variation on the some theme.
Solution was found as improve fabric anchoring systems or barge maneuver to a degree sufficient. These were all done to allow the construction to continue without major costly correction.
Wilmington Harbor South Project was completed acceptable well contemplating its capacity and claims. Project design was accomplished as it was planned for. The American Dredging Company and their subcontractors, suppliers are expected to complete the project successfully and on schedule. This project also represents how serviceable is the usage of geotextile in port study.
- Koerner R.M. – Soft Soil Stabilization Using Geosynthetics
- Yong R.N. – Geoenvironmental Engineering