As the character of the soil at the project site was critical to the construction of the ponds, a soils investigation programme was initiated as soon as possible. The programme was to include about 12 bore holes, dug by power auger of 4 in Æ to a maximum depth that could be reached without casing. The field and laboratory work was accomplished by the Materials Testing and Research Section of the Ministry of Works, Power and Water; Public Works Affairs. Due to equipment failure and inaccessibility, fewer than the planned number of holes were sampled but as there was a consistency of the soil type over the area, the work accomplished was deemed to be satisfactory.
The final soils investigation report had not been issued at the time of writing the present report. Some sample analyses were studied, however, and indicated a soil made up of silt in the upper 0.5 m, a sandy organic layer with some small amount of clay about 0.5 m thick, and then another 0.5 m of fine coarse sand with silt. There was no indication of an appreciable clay layer. Silt in the samples ranged from 8 to 45 percent. The water table appeared at about 0.5 m depth but varied.
The draft report recommendations by Dr J. Sceal, Soils Engineer are reproduced here below:
Over most of the site, the ground conditions can be summarized as comprising a thin sandy layer at the surface overlying organic silty clays of variable thickness, which in turn overly silty sands. The water table is high and the soils are saturated at depths of less than 1 m over the areas investigated.
Without some form of ground treatment or soil stabilization, presumed bearing values for these soils will be exceptionally low. With reference to British Standards Code of Practice for Foundations (GP 2004), where guides to presumed bearing values and estimates of undrained shear strengths are offered, a presumed bearing value of less than 75 kN/m2 for the sandy silts would be appropriate and an undrained shear strength of less than 20 kN/m2 might be anticipated for the organic clays. The soils are exceptionally weak.
Some form of stabilization of the site is essential before any development can take place. As has been witnessed, even four wheel drive vehicles have difficulty in crossing the site.
Bearing in mind the low loadings required of the site and its ultimate after use, it is suggested that overlaying the site with a suitable fill, together with some form of dynamic compaction (e.g., a vibrating roller), should provide a reasonably stable surface for the lagoon and embankment construction proposed. The recent construction of the access road bears witness to the effectiveness of this form of ground treatment over the site. However stabilizing a large surface area in this fashion will be less successful, since a far larger area has to be dewatered before stabilization will be effective. Some plastic deformation of the clay layer is likely to occur - there is clear evidence of “heave” adjacent to the access road recently constructed. Also, settlement over the site is likely to be significant and some allowance should be made for additional volumes of fill that may be required to bring the site up to final construction levels.
Provision should also be made in the design to account for possible long term settlement, which is likely to occur in these soils. A further point worthy of consideration is that the access road as constructed is likely to be suitable for light traffic only. The importation of large volumes of fill by heavy vehicles using this road is likely to lead to subgrade failure beneath the road.
It has been suggested that the existing ground could be treated to hold lagoon water without the need for any form of impervious lining. Noncohesive sandy materials dominate the soils on site; these could not provide an impervious barrier adequate to retain lagoon waters. Nothing could be gained by pursuing this particular proposal; some form of impervious lining to the lagoons will be essential.
Based on field observations and the above report of the soils engineer, action will be taken as follows:
A fill averaging 0.6 m will be placed and compacted over the entire pond site and extending 5 m beyond the base of the pond slopes on all sides. This will allow construction vehicles to operate and turn during pond construction. It is recommended that the fill be placed as soon as possible to allow maximum compressive effect on the water-filled strata of the subgrade before pond construction starts. Calculated maximum bearing values under the dikes are below the maximum bearing values suggested.
As the access road constructed as part of the laboratories contract has been constructed only to the top of the base course, the contractor will defer putting on the final lifts until the laboratories are finished and the fill brought in for the ponds. This will allow rebuilding of any failed portions of the road after the heavy hauling has been accomplished.
Although the soils report summary reports that settlement over the site is likely to be significant, the soils engineer felt that a good bit of the settlement would have taken place by the time the ponds were completed due to material loads and equipment movement and compaction. Long-term settlement would perhaps result in some elongation of the pond linings but otherwise would not affect them structurally.
Due to the noncohesive and permeable nature of the soil on the site and the fill that can be imported, no attempt to build compacted soil ponds with no lining treatment is considered feasible. Both ponds will be lined (see Section 5, Lining for Ponds). Maximum compaction of the soil will be specified during dike construction and, when liners are placed, a 30 cm compacted soil cover will be put over the liner for protection from the weather and mechanical injury.
