The following are summaries of papers published in the recent themed issue of Proceedings of ICE, Geotechnical Engineering on the subject of coastal stabilisation and near-shore geotechnical engineering that readers of Maritime Engineering may find of interest. Summaries of all papers in ICE journals are freely available and fully searchable at the ‘journals on-line’ section of the ICE website. See www.ice.org.uk/journals for details.
Building a breakwater with prefabricated caissons on soft clay
S. W. Yan, J. Chu, Q. J. Fan and Y. Yan
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 3–12, doi: 10.1680/geng.2009.162.1.3
This paper presents a case history of the construction of an offshore breakwater on soft clay using prefabricated, semicircular-shaped concrete caissons. The project was located near Shanghai Port, China. The breakwaters were designed as gravity-retaining structures, and used for deepening of the navigation channels along the Yangtze estuary. Some sections of the breakwaters were installed on a thicklayer of soft deposit. During the construction, the caissons in one section failed under a heavy storm. The causes of failure were investigated. The design of the breakwater and the soil improvement works are described in this paper. Surcharge preloading and prefabricated vertical drains were adopted to improve the soft soils below the caissons. Some other measures, such as the use of anti-sliding mats for the base of the caissons to increase the base friction, and the use of geotextile with sand-filled geotextile tubes and geotextile and concrete block composites to prevent scour, were also adopted. These measures were proven to be effective in maintaining the stability of the breakwaters against subsequent heavy storms.
Reclamation of a slurry pond in Singapore
J. Chu, M. W. Bo and A. Arulrajah
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 13–20, doi: 10.1680/geng.2009.162.1.13
A case study for the reclamation of a slurry pond as part of an offshore reclamation project in Singapore is presented in this paper. The slurry pond covered an area of 180 ha. The slurry in the pond was recently deposited ultra-soft high-plasticity clay. The water content of the slurry was more than 120% and the undrained shear strength was less than 8 kPa. The reclamation was first carried out by spreading sand fill in thin layers 20 cm thick using a specially designed sand spreader. The filling speed was carefully controlled to allow the slurry to be consolidated before more fill could be placed. Despite the precautions a failure occurred, in the form of mud bursting. As a remedial measure, geotextile sheets were used to cover a total area of 630 000 m2 before more sand fill was placed. After the completion of fill placement, fill surcharge and prefabricated vertical drains (PVDs) were used to improve and accelerate the consolidation of the slurry. As the performance of PVDs would deteriorate after they had undergone large deformation, they were installed in two passes. In the first pass PVDs were inserted with a square grid spacing of 2·0 m. After nearly 1·5 m of settlement had taken place, the second pass of PVDs with the same spacing was installed at the centre of the square grid of the PVDs installed in the first pass. After nearly 4 years of consolidation, the top of the slurry had settled more than 3 m. The undrained shear strength had also increased substantially. Therefore the use of PVDs for the improvement of the ultra-soft slurry was successful in this project.
Soil improvement works for an offshore land reclamation
J. Chu, M. W. Bo and A. Arulrajah
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 21–32, doi: 10.1680/geng.2009.162.1.21
The Changi East reclamation project was carried out in five phases along the foreshore of the east coast of Singapore. The water depths in the reclaimed area ranged from 5 to 15 m. The project involved hydraulic placement of 272 million m3 of sand onto soft seabed marine clay up to 50 m thick. A linear total of 170 000 km of prefabricated vertical drains (PVDs) were installed for accelerating the consolidation process of the underlying soft marine clay. The soil improvement works covered a total area of approximately 1200 ha. In this paper, the site conditions and the soil improvement works adopted are described. Pilot tests with full-scale field instrumentations as well as laboratory and in situ tests were carried out to verify the design, check the effectiveness of the soil improvement works using PVDs, and establish the most suitable drain spacing. Field monitoring data obtained from both the pilot tests and the reclamation works are presented and interpreted. Degree of consolidation was calculated based on both settlement and pore pressure data.
Instrumentation at Changiland reclamation project, Singapore
A. Arulrajah, M. W. Bo, J. Chu and H. Nikraz
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 33–40, doi: 10.1680/geng.2009.162.1.33
Ground improvement is often required in land reclamation projects on soft soil deposits to reduce future settlement under the projected future dead and live loads. In the case of thick deposits of marine clay, it is often necessary to accelerate the consolidation process of the clay using prefabricated vertical drains. In such projects, the degree of improvement has to be ascertained to confirm whether the soil has achieved the required degree of consolidation before the removal of surcharge. This analysis can be carried out by means of observational methods for which the ground behaviour is continuously monitored from the date of instrument installation. This paper provides a case study of the applications of geotechnical instrumentation for construction control in the Changi East reclamation project in the Republic of Singapore. Field instruments adopted in this project included settlement plates, deep settlement gauges, earth pressure cells, pneumatic piezometers, electric piezometers and water standpipes. Some of the field instruments were installed offshore. Special techniques were adopted to protect these offshore instruments before placement of sand fill by hydraulic pumping. Geotechnical instruments were also used in the construction control process to monitor the deformation of ground and the stability of the earth and retaining structure. Field instrumentation readings obtained from a pilot test area comprising a vertical drain area (horizontal vertical drain spacing 1·5 m × 1·5 m) and an adjacent control area (no vertical drains) are also presented in this paper.
Site investigation for disaster reconstruction in Aceh, Sumatra
J. S. Younger, R. A. Mattholie and B. Setiawan
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 41–47, doi: 10.1680/geng.2009.162.1.41
The tsunami that struck and devastated large sections of the coastline of Aceh in northern Sumatra, on 26 December 2004 was generated by a massive offshore earthquake of magnitude 9·3. As part of the reconstruction programme, UNICEF is engaged in the replacement of old schools and health clinics at over 580 locations throughout the province of Aceh and the island of Nias. The programme is urgent and tight, compounded frequently by difficult access. An approach to the examination of the ground conditions at the sites had to be established that would allow the use of light equipment and tools and a relatively unskilled workforce. In situ tests were conducted using a locally manufactured equivalent light Dutch cone apparatus, a dynamic cone penetrometer (DCP) and field vane equipment. Laboratory testing, apart from normal identification and classification tests, included undrained shear strength determinations. This paper sets out the modus operandi developed to suit the site conditions, and describes the types of soil and conditions encountered. The paper also presents a tentative correlation between the results of the in situ (DCP) testing and the undrained shear strength of the samples recovered for laboratory testing. The approach should have merit when responding to future disasters.
Recent UK experience of coastal cliff stabilisation
A. R. Clark and S. Fort
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 49–58, doi: 10.1680/geng.2009.162.1.49
There are many locations on the UK coastline where the soil and weak rock cliffs are subject to erosion, and the resulting instability threatens both public safety and infrastructure. With predicted climate change, the impact of this erosion and instability will be an ever-increasing adverse affect on the coast. A total of 14 cliff sites are reviewed. Each site described, which includes sites comprising glacial deposits, reactivated post-glacial landslides and Jurassic and Cretaceous weak rock strata, has been stabilised using a combination of different techniques. This paper is a review of the methods of investigation and the approaches to the selection of the stabilisation techniques that have been used at each of the sites. In some cases the significance of the environmental status of the sites and its influence on the selection of stabilisation techniques is described. Some of the techniques, including piles, buttresses, drainage and reinforcement are discussed further, although comprehensive details of the majority of the sites are provided in site-specific publications given in the list of references.
Examples of open caisson sinking in Scotland
D. Allenby, G. Waley and D. Kilburn
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 59–70, doi: 10.1680/geng.2009.162.1.59
Open caisson-sinking techniques permit a shaft structure to be progressively sunk, either under its own weight or with the aid of caisson jacks, in a controlled manner from the surface to a predetermined depth. The technique is suited to shaft construction through weak soils, high-to extremely high-plasticity clays, silts, and sands and gravels, particularly below the water table. In the tunnelling and public works engineering sectors, open caissons are typically circular in cross-section; those used in harbour works are commonly square or rectangular in plan. The paper briefly describes the components and method of sinking dry and wet open caissons, highlighting good practice. It then examines in detail four recent, highly successful caisson-sinking operations carried out under a framework agreement for Scottish Water Solutions in their wastewater treatment works at Crianlarich, Doune, Killearn and Drymen. Although modest in size, each contractor-designed caisson was sunk through a variety of ground and water conditions, illustrating the versatility of the technique and its advantages over more conventional methods of shallow-depth shaft construction. The importance of a comprehensive site investigation programme and the control measures required to satisfy verticality and structural integrity requirements are discussed. The paper concludes with a series of practical guidelines designed to assist those contemplating using the technique, and to remind practitioners of good caisson-sinking practice.
Observed increases in offshore pile driving resistance
S. Bhattacharya, T. Carrington and T. Aldridge
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 71–80, doi: 10.1680/geng.2009.162.1.71
This paper presents 53 well-documented cases of the short-term set-up of open-ended steel pipe piles driven at two fields in the North Sea. Increases in soil resistance during driving (set-up) were observed when pile driving was re-started after delays typically ranging between 24 h and 100 h. The soil encountered at the sites consisted of overconsolidated sands and clays, where the clays varied from very stiff clay (su of 250 kPa) to very hard clay (su of 800 kPa). The field records were analysed to derive correlations between the length of the delay and the increase in soil resistance, or set-up. The available data have been classified in terms of pile diameter, pile tip penetration and the undrained shear strength of the clay strata. The data indicate significant set-up at such sites, even in the short term, with the rate of increase following a logarithmic curve. The data relate to practical issues associated with installation of piles, and do not address the long-term load-carrying capacity of the piles.
Biological in situ reinforcement of sand in near-shore areas
M. Van der Ruyt and W. van der Zon
Proceedings of ICE, Geotechnical Engineering, 162, No. 1, February, 81–83, doi: 10.1680/geng.2009.162.1.81
BioGrout is the process by which bacteria control the precipitation of calcium carbonate while consuming supplied reagents. By controlling the amount of reagents, the amount of calcium carbonate and thus the strength of the reinforced sand body can be engineered. The main advantage of BioGrout is that soil (or sand) can be strengthened, without interfering with the hydraulics of the treated soil and without excavation or replacement. The result is a stiffer sand layer with a higher bearing capacity, higher resistance against erosion and higher resistance against shear forces. The higher strength enables can protect under water slopes against liquefaction and breaching but can also protect dunes or river bends against erosion, reduce costs for wave protection measures (stone protection) etc.
