Research Output
1 - 6 of 6
Publication
Geotechnical properties of Dublin boulder clay
2007-09, Long, Michael (Michael M.), Menkiti, Christopher O.
Geotechnical characteristics of Dublin Boulder Clay (DBC) based on detailed site
investigation and site experience from some recent large projects in Dublin are presented.
This paper attempts to synthesise available information in parallel with recent work by
Skipper et al (2005), who provide an updated understanding of the geology of the DBC.
Having assessed the effects of sampling disturbance, the paper characterises the various
formations and sub-units of the DBC. The interpreted material behaviour is related to
observed engineering performance. It was found from the behaviour of earth retained
structures that intact, clayey, DBC formations are 2 to 3 times stiffer than assessed from
high quality laboratory tests on block samples. DBC is shown to be significantly stiffer
than other well-characterised tills. Relatively inexpensive Multi Channel Surface Wave
techniques (MASW) can give very reliable estimates of in situ small strain stiffness. High
undrained triaxial compression strengths were measured and it appears that simple UU
tests on high quality specimens give good results. Significant strength anisotropy was
suggested by undrained triaxial extension strengths that were only 30% to 50% of the
triaxial compression strengths. Field horizontal permeability values of the intact clayey
till units have a representative mean of about 10-9 m/s and when compared to laboratory
values suggest that the material may exhibit some degree of anisotropy of permeability.
Overall, the measured engineering parameters for the DBC are favourable for many
construction projects. Further work is required in order to understand the in situ
horizontal stress profile and the stiffness anisotropy of the till.
KEYWORDS: Glacial soils, In situ testing, laboratory tests, site investigation, strength,
stiffness.
Publication
Retaining wall behaviour in Dublin's estuarine deposits, Ireland
2012-01, Long, Michael (Michael M.), Menkiti, Christopher O., Skipper, Jacqueline, Brangan, Carl, Looby, Michael
Practising engineers in the Dublin, Ireland, area have much experience in dealing with the boulder clay which underlies much of the city. However, significant deposits of estuarine soils, some of them soft, exist along the east side of the city and in particular in Dublin docklands. Some construction difficulties have previously been encountered in these materials and significant developments, including a large tunnel project, are planned in the area overlying these deposits. Few published data exist on retaining wall schemes in the deposits. Data from nine sites, including three detailed case histories, are presented which confirm that construction of deep propped excavations and cantilever walls up to 7·5 m are feasible in these deposits and can perform well. A key issue is the soil that is located at excavation level and competent deposits here are essential to prevent large displacements or possible instability. The resulting movement will also be sensitive to the overall system stiffness. There seems to be scope for more efficient future design, including more use of cantilever walls, particularly for temporary works purposes. Beam-on-spring type computer analyses tend to give conservative results for these deposits and more sophisticated finite-element analyses may be warranted for future schemes.
Publication
Performance of soil nails in Dublin glacial till
2008-12, Menkiti, Christopher O., Long, Michael (Michael M.)
Soil nailing is being used in many projects in glacial tills in Ireland, particularly to provide
temporary support to steep slopes. Little design guidance is available for such materials and it
is known that application of design procedures developed for other material is conservative.
Detailed nail instrumentation and field monitoring during large scale soil nailing works for the
Dublin Port tunnel project has been undertaken. It was found that the short-term behaviour of
nails was the reverse of that assumed in current design methods. Most load was induced due
to drilling and nailing the lift immediately below the nail being monitored, rather than due to
excavation induced stress relief. The highest forces were developed in the upper nails, where
the largest ground movements occur. This is the reverse of most current design methods
where the highest soil-nail bond is assigned to the deepest nails. It would seem that the
observed short-term, pre-failure behaviour of nailed slope is governed more by the
deformation pattern of the slope rather than by large scale development of failed wedges.
Current design procedures should be reviewed. Despite this the trial confirmed that the
currently used procedures are highly conservative for Dublin glacial till.
Publication
Soil Nailing in Dublin Boulder Clay
2013-07-28, Menkiti, Christopher O., Long, Michael (Michael M.), Milligan, G. W. E., et al.
Extensive areas of Dublin City are underlain by a very stiff to hard lodgement till, known locally as Dublin Boulder Clay. Over the past number of years there has been increasing use of soil nails mostly to support temporary slopes for excavations, road widening and cut and cover tunnels. Although guidance for nailing in stiff clays exists, the very high strength and stiffness of the Dublin soils mean that it is conservative to use these established techniques and further optimisation may be possible. This paper presents data from the Dublin Port Tunnel project for a large set of short term pullout tests carried out on nails from a slope some 12 m in height together with slope monitoring data. The significant influence of drilling technique is discussed. Optimisation of temporary soil nails in Dublin Boulder Clay is appraised. The relationship between bond strength and actual nail performance is discussed drawing on data from a heavily monitored slope. Measured data on the vertical and horizontal movements of the slope together with data from piezometers confirm the excellent behaviour of the soil nailed slopes.
Publication
Retaining walls in Dublin Boulder Clay, Ireland
2012-01, Long, Michael (Michael M.), Brangan, Carl, Menkiti, Christopher O., Looby, Michael, Casey, Patrick
A good number of deep excavations have been recently completed in Dublin Boulder Clay, Ireland. These have included propped walls up to 25 m deep and permanent cantilevers 7.5 m high. Experience elsewhere in the world was used to design and construct these walls. However, case history data have shown that the behaviour of the
walls in Dublin Boulder Clay is very rigid and much stiffer than comparable systems worldwide. It appears this behaviour is due to the inherent natural strength and stiffness of the soil and the slow dissipation of excavation induced depressed pore pressures or suctions. There appears to be scope for developing more efficient designs and
in particular for reducing propping requirements. For temporary works, the use of undrained parameters in serviceability limit state calculations together with implementation of the observational approach on site could be considered for future schemes.
Publication
Some experience in measuring pore water suction in Dublin glacial till
2004-09, Long, Michael (Michael M.), Menkiti, Christopher O., Follet, Ben
Several recent articles and discussions in GIN, by Penman (2002), Thomann et al. (2003),
Ridley (2003) and Sellers et al. (2003) highlight the importance of pore water pressure
and suctions and in particular the difficulties associated with measuring suction. This
article outlines some recent experience in measuring suctions, which were developed in
cut slopes in a very stiff glacial till during the Dublin Port Tunnel (DPT) project in
Ireland. A particular feature of the project was the execution of a fully instrumented 12m
deep trial excavation (Menkiti et al., 2004).
Initially some background to the project and to the problem will be given. Then some
details of the specification, installation technique and some examples of the
measurements obtained will be outlined. Finally some conclusions will be made on the lessons learned and recommendations will be made for future similar applications.