by Deryck Chan and Gopal Madabhushi, University of Cambridge; Duncan Nicholson and Tim Chapman, Arup; and Sergio Solera, Mott MacDonald
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An 11m deep basement structure in London SW1 was left vacant from 1968 to 1989. The basement heaved significantly during this period due to the lack of a superstructure, providing a unique opportunity to study the development of long-term heave in London Clay.
May (1975) presented the monitoring results from 1968 to 1973 and excerpts of site data collected after 1973 have also been circulating informally in the industry since the 1990s. However, the full set of monitoring data remains hitherto unpublished.
This paper was initially drafted in the early 1990s when three of the authors (Nicholson, Chapman, and Solera) were working together with Arup. The paper was never published as circumstances changed. More recently, Chan started postgraduate research on heave and pressure beneath slabs in excavations in over-consolidated clays, using the heave monitoring data from the draft paper to complement centrifuge test results. For this reason, it was decided that the draft paper should be revised and published for the benefit of the wider industry.
This paper provides a comprehensive case history of the site, publishing further heave data to June 1989, giving a total of 21 years of heave monitoring. Further site investigation data and calculations are included for comparison. The data show that the presence of a basement did not significantly reduce the shear strength of the clay beneath it. The coefficient of consolidation of the clay was 38 to 52m²/year and long-term heave was still ongoing 21 years after the end of excavation.
A basement was constructed at 124 Horseferry Road, London SW1 between 1966 and 1968, originally as part of a new sorting office for the Post Office. The average depth from ground level to the bottom of the base slab was 11m. Due to planning difficulties, construction work stopped after completion of the basement to ground floor level in May 1968 and the associated superstructure was never built. The site was left largely vacant until redevelopment of the site started in 1989. Following the insolvency of the first developer, the Channel 4 Television Company completed the scheme to provide a building for its new headquarters.
The heave movements of this empty basement were monitored between 1968 and 1989. May (1975) presented monitoring results from March 1968 to February 1973 and made predictions of future heave. Further excerpts of monitoring data up to 1989 were included in several publications about the design and monitoring of deep basements in London Clay such as Pumphrey (2001) and Chan and Madabhushi (2017), but the original monitoring data remains hitherto unpublished. The primary aim of this paper is to make public the full set of monitoring data for the benefit of the wider industry.
Table 1 lists significant events in the history of the site.
2.0 The structure
The site is approximately 70m by 90m in plan (see figure 1) and the ground level is at +5mOD. The original design of the structure was for a 20 storey high steel framed tower with a four storey high surrounding podium and a two-level basement with vehicular access. The construction was terminated at ground level due to planning difficulties.
There were two levels of in situ concrete basement over a concrete raft. The top of the raft was at -5.9mOD (10.9m below ground level) and the raft was generally 1.2m thick, thickened to 1.8m in the centre (beneath the proposed tower block) and reduced to 0.9m around the perimeter (see figures 2 and 7).
The raft was constructed from the centre outwards and provided support for internal raking temporary props to the diaphragm wall during excavation, which was initially supported by an earth berm and later by the raking props. The excavation was taken to a depth of 12m approximately. Permanent strutting was then provided at two levels by the upper basement and ground floor slab at -1.1mOD and +4.34mOD respectively. The walls were 0.6m thick diaphragm panels cast in 1.5m widths extending into the London Clay by a minimum of 1.5m (3m to 12m below the underside of the basement). The diaphragm wall was incorporated as the finished basement wall. The structural details are shown in figure 3.
The vertical effective stress at formation level before the construction of the basement was estimated to be 190kPa. The net unloading due to excavation was 184kPa, comprising 234 kPa of unloading due to excavation and 50 kPa of reloading by the new structure. The basement raft was only pinned down at the edges alongside the diaphragm wall so the centre of the basement was relatively free to heave. All sections of the diaphragm wall extended into the clay.
3.0 Site investigation
Two site investigations were carried out – the first in 1962 before construction of the basement and the second one in 1989, 21 years after completion of the basement structure. The locations of boreholes referred to in the text are shown in figure 1. Figure 2 shows the stratigraphy superimposed on an east-west section through the basement. Although the excavation had slabs at three levels (May, 1975), in figure 2 only the basement slab is shown for clarity.
The geological map of the area – Geological Survey of England and Wales, Sheets TQ 27 SE, TQ 27 NE, 1973 – shows a scour hollow to lie partially beneath the site. The origin and frequency of the scour hollows in London are discussed by Berry (1979). Contour of the surface of the London Clay as revealed by the site investigation are shown in figure 1 and show the hollow to be about 10m deep. The standard penetration test (SPT) shows that the Flood Plain Gravel towards the base of the hollow are loose to medium dense.
Groundwater levels during the 1962 and 1989 site investigation are indicated on figure 2. The 1989 groundwater level in the Flood Plain Gravel outside the site was found to be at about -2mOD, some 4m above the lowest floor level of the basement and similar to May’s prediction of -2.5mOD.
Inside the basement, a bleed test performed during the 1989 site investigation showed that the ground water level before bleeding was at -2.5mOD, similar to the external ground water level. The recovery of the water pressure after bleeding was very slow. These results show that the diaphragm wall was a reasonably effective cut-off to seepage flow, though sufficient time had passed since the basement’s construction for ground water equilibrium to re-establish under the slab. It should be noted that this paper does not consider the global changes in water table in London.
The piezometric profile in 1989 (figure 4) shows a near-hydrostatic distribution through most of the London Clay, followed by a sharp drop in head through the top of the Lambeth Group. The drop shows drainage into the lower aquifer and suggests that the Lambeth Group clay may have a lower vertical permeability than the London Clay locally. This finding is concordant with piezometric observations from nearby sites by Simpson et al (1989) and Nicholson and Harris (1994).
Figure 5 plots the undrained shear strength results from boreholes both outside the basement and inside the basement alongside results from a site investigation in 1962. The 1962 strength data appears to be from undisturbed sample testing while the 1989 strength data is based on unconsolidated undrained triaxial tests on 102mm diameter samples. The 1989 site investigation also included SPTs and this information is reproduced in the supplementary data. The results outside the basement were from 1962 and 1989 while those inside the basement were only from 1989.
A two-sample heteroscedastic t-test (Welch, 1947) was performed on undrained shear strength results at depths between -8mOD and -18mOD, to see whether the samples under the basement in 1989 were significantly weaker than the samples from 1962. This method involves calculating the mean and standard deviation of each set of strength measurements, then using the t-distribution to establish if the means of the two sets of data are different from each other in a statistically significant way.
The results showed that the difference in strength profile between samples under the excavation and samples from 1962 was not statistically significant (table 2). A similar analysis using the SPT values also showed that the results are not statistically significant. Nevertheless, it is noted that the recommendation by Padfield and Mair (1984) of 20% reduction in undrained shear strengths beneath an excavation is within the 95% confidence interval of the 1962 strength profile, which means the scatter of data at this site was too broad to confirm or disprove the 20% recommendation with statistical confidence. The dataset and calculations in this statistical analysis are available in the supplementary data.
4.0 Observations of heave
In total 12 survey stations were installed into the basement structure at the time when it was constructed. A temporary benchmark was established at the lift shaft in the form of a steel bolt screwed to the inner wall of the central lift shaft, which in turn was surveyed from a benchmark located beneath a stopcock cover on Monck Street, 50 m east of the boundary of the site. The 11 other survey stations were in the form of rounded brass studs located within the bases of selected columns and concealed beneath steel covers. Figure 7 shows the locations of the survey stations.
Monitoring of settlement and heave began in September 1967. Measurements were taken multiple times a year until 1977 and then in May 1978 and November 1980. Arup undertook two more surveys in February 1989 and June 1989.
The full set of monitoring data from 1967 to 1989 can be found in the supplementary data. Inevitably, some heave would have occurred between the start of excavation in June 1966 and the start of monitoring. The monitoring data showed negligible vertical movement between September 1967 and March 1968. May (1975) gave two possible explanations for this. Firstly, as substructure construction progressed, the settlement due to the increase of structural weight offset the heave due to earlier excavation. Secondly, there might have been an error in the September 1967 datum measurement.
Noting that construction ceased around the time of the March 1968 survey, the remainder of this paper will take March 1968 as the datum of further analysis.
Figure 6 shows the development of heave with time from 1968 to 1989, when monitoring ceased. The locations of the survey points referred to on these figures are given on figure 1. Figure 7 shows that as of 1989, the heave at the centre of the base slab was about 105mm, whereas the edges of the slab heaved less than 70mm over 21 years. This difference reflects the effects of the diaphragm wall around the perimeter of the basement.
The solid lines on figure 6 show that the development of heave with time was generally consistent with one-dimensional consolidation theory. By 1989, the evolution of heave had reached the exponential decay phase but heave was still noticeable. It is estimated that long-term heave at the centre of the slab would reach 110mm if the Channel 4 building was never built.
5.0 Analysis of heave
The overall thickness of the London Clay beneath the site is generally 32m, reducing to 21m beneath the centre of the scour hollow in the south-west corner of the site. The base of the London Clay layer was estimated as -38mOD, underlain by the Woolwich & Reading Beds (now part of the Lambeth Group). The net heave per net change in total stress is estimated as 110mm/184kPa = 0.6mm/kPa.
Engineers for the Channel 4 project analysed the heave data using the one-dimensional settlement program VDISP from Oasys to provide reference values for the ratio between drained stiffness and undrained shear strength (E’/cu). The analysis approximated the basement as a rectangular box with plan area 88.8m by 64.2m, underlain by 30.8m of London Clay followed by 16.5m of Woolwich and Reading Beds, the bottom of which was taken as a rigid boundary. Iterating between different linear distributions of drained London Clay stiffness, the analysis concluded that a strength profile of cu = 75 + 6z (kN/m2) (see figure 5), where z is depth below the surface of the London Clay (-6mOD), and E’/cu = 350 provided the best fit. Further details of this analysis are available in the supplementary data.
The time constant tref = d2/cv is extracted from the best-fit consolidation curves of each of the 11 monitoring points (figure 6 shows the best-fit curves of three monitoring points; calculations for all monitoring points can be found in the supplementary data), where d is the drainage distance and cv is the coefficient of consolidation. tref is the time (in years) at which the dimensionless time Tv = 1 in one-dimensional consolidation theory, corresponding to approximately 95% consolidation in terms of linear displacement. The values of tref range from 17 years to 21 years, with the lower values coming from locations where the top of the London Clay was the deepest according to figure 1. This is to be expected because a depressed clay-gravel boundary implies a thinner clay layer and thus shorter drainage distances.
Using these values for tref, it is estimated that consolidation was 96% – 98% complete as of 1989. Taking the values of tref and the estimated thickness of the London Clay layer beneath each monitoring point (27 – 31m), a single-drainage consolidation calculation gives the range of the coefficient of consolidation cv as 38 to 52m²/year.
The bottom of the London Clay is considered to be an impermeable boundary following the aforementioned discussion of ground water conditions which suggests the Lambeth Group clay was likely to have a lower vertical permeability than the London Clay locally. It should be noted that May (1975) considered the base of the London Clay as a drainage boundary, which would lead to a double drainage calculation giving cv = 10 to 13m²/year. These interpretations ignore the effect of horizontal drainage along silty layers in the London Clay. The values of cv from this site are significantly higher than the typical values (cv ≈ 3m²/year) for London Clay derived from field measurements of permeability or element testing of undisturbed block samples, which may be because the rate of consolidation in stiff clay is driven by preferential drainage along fissures (Nicholson and Harris, 1994; Ng, 1998).
- The evolution of heave displacement with time agrees with predictions by one-dimensional consolidation theory.
- A comparison of shear strength data from two site investigations 27 years apart did not show a statistically significant loss of shear strength due to heave of London Clay caused by the presence of the basement.
- The ratio of drained stiffness to undrained shear strength (E’/cu) was estimated as 350 for this site.
- The coefficient of consolidation (cv) was found to range from 38 to 52m²/year, assuming an impermeable boundary at the base of the London Clay. This value is much greater than the typical value of cv = 3m²/year from laboratory tests and field measurements of permeability.
- Long-term heave was still occurring 21 years after the end of excavation.
- The net heave per net change in total stress was estimated to be 0.6mm/kPa.
The authors would like to thank D C Shohet and P A Thompson for contributing to the unpublished manuscript which formed the basis of this paper; to Arup Geotechnics for making archive documents available for this publication, especially Vicki Hope and Laura Evelyn-Rahr for their previous work on cataloguing the archive content; and to the EPSRC Centre for Doctoral Training in Future Infrastructure and Built Environment at Cambridge University (EPSRC grant reference number EP/L016095/1) for supporting this research.
Berry, F G, (1979): Late quaternary scour hollows and related features in Central London, QJEG, Vol 12 pp. 9-29
Chan, D Y K and Madabhushi, S P G, (2017): Designing urban deep basements in South East England for future ground movement - Progress and opportunities for experimental simulation of long-term heave, International Symposium for Next Generation Infrastructure, London.
May J, (1975): Heave on a deep basement in the London Clay, Conference on the Settlement of Structures, Pentech, London.
Ng, C W W, (1998): Observed Performance of multipropped excavation in stiff clay, Journal of Geotechnical and Geoenvironmental Engineering, ASCE.
Nicholson, D P and Harris, S J, (1994): The use of gravity relief wells below basements in London Clay to control the rise in groundwater level, Groundwater problems in urban areas, Thomas Telford.
Padfield, C J and Mair, R J, (1984): Design of retaining walls embedded in stiff clay, Ciria Report 104.
Pumphrey, L, (2001): Heave at the Shell Centre, Imperial College London.
Simpson, B, Blower, T, Craig, R N, and Wilkinson, W B, (1989): The engineering implications of rising groundwater levels in the deep aquifer beneath London, Ciria Special Report SR 68
Welch, B L, (1947): The generalization of “Student’s” problem when several different population variances are involved. Biometrika. 34 (1–2): 28–35
Further details on the heave monitoring data, the site investigation data, and the calculations presented in this paper are available via the Cambridge University Data Repository