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Problems associated with building on a variable depth of fill

PAPER

Summary

The safe and economic redevelopment of many brownfield sites is dependent on an appropriate decision concerning the area of land which cannot be built on because the changes in depth of fill give the potential for unacceptable long term differential settlement. Building Research Establishment is currently carrying out a programme of research into this hazard and this paper presents a conceptual framework within which the severity of the difficulties which are likely to be encountered in different situations can be assessed.

Introduction

In reviewing the performance of an engineered opencast backfill at Hurst Industrial Park at Dudley, Waite and Knipe (1991) concluded that: 'In the vicinity of the high and side walls of the opencast excavation, the potential for long term unacceptable differential settlement exists due to the rate of change in fill thickness although the levelling surveys have shown a relatively smooth transition across these areas. Thus, a 'restriction zone' straddling the margins of the opencast pit was designated with no building development within this area, although it could be used for parking, storage or other non-sensitive use.'

With increasing pressure to locate building developments on brownfield sites, many of them containing significant depths of fill materials, the situation described by Waite and Knipe will become increasingly important. There is a need to define those areas where changes in fill thickness give the potential for unacceptable long term differential settlement and from which buildings should be excluded. The subject has major implications for the safe and economic redevelopment of many brownfield sites. If unsafe decisions are reached, building damage and consequent legal action can result. On the other hand, an over-conservative approach will needlessly sterilise large areas of land and could make many sites uneconomic to develop.

Figure1 shows a typical cross-section through a backfilled excavation. Significant changes in depth of fill are not confined to locations at the edges of the filled area and changes in the depth of the fill at any location constitute a potential hazard. The precise extent of the areas within which the variability in depth of fill is likely to cause serious differential settlement problems is not immediately obvious.

BRE is currently investigating the problem and this paper presents some preliminary findings. The problem lends itself to numerical analysis and parametric studies. However, the difficulties in predicting surface settlement profiles using finite elements have been well documented (Addenbrooke et al, 1997), and it is intended to validate the numerical studies by some experimental work under carefully controlled conditions in a test pit and also at full scale in the field. The initial analytical studies have provided a conceptual framework within which the severity of the difficulties which are likely to be encountered in different situations can be assessed.

Building on backfilled excavations

The major hazard for buildings founded on fill is generally associated with the settlement of the fill. Previous work by BRE has shown that volume reduction in the fill can occur due to a number of causes such as long term creep under self-weight (Charles, 1993), collapse compression on inundation (Charles et al, 1977, 1978, 1993; Burford and Charles, 1991; Charles and Watts, 1996, 1997) or biodegradation of organic material (Watts and Charles, 1990); damaging settlements are not usually attributable to the weight of the building. Collapse compression on inundation has been identified as the major hazard for many non-engineered fills. While heavy compaction of the fill during placement should reduce collapse potential, it may not completely eliminate it.

It is not the magnitude of the total settlement but rather the magnitude and distribution of the differential settlement which causes distortion and damage to buildings. In a situation where settlement is occurring, non-uniform settlement may be attributable to a number of factors including the variability of the fill and variations in depth of fill, as well as a non-uniform distribution of the loading.

There are particular problems associated with differential settlement at the edges of the filled area. The geometrical configuration of the boundary of a filled area will depend mainly on its past use. An old dock will have vertical sides and a quarry in hard rock may also have a near vertical face. Gravel and clay pits will have slopes that are not so steep. In opencast mining a steep excavation slope is termed a 'highwall' and Figure 2 shows fill placed against the highwall in a typical opencast operation. A highwall may emerge at ground surface or may be buried under a substantial depth of fill. The highwall is likely to be the location of significant differential settlement.

It should be emphasised that an abrupt variation in the depth of fill under a building is not only an important cause of differential settlement; it may also result in horizontal movements.

Although there is an extensive literature on the problems of building on fill (Charles, 1993), little has been written on the problems occasioned by changes in depth of fill. The lack of attention to the highwall problem is, perhaps, not surprising when the received wisdom has been not to build over it. In 1949, BRE (then the Building Research Station) published Digest no 9 on Building on made-up ground or filling which warned that: 'Special care should be taken with buildings sited near the edges of filled ground; in particular placing a building partly on the natural ground and partly on fill should be avoided. Instead of this, the foundations should be carried down to the natural ground by piers or piles.' (This Digest has been revised on several occasions and the latest version, Digest 427, was published in three parts in 1997/98).

A BRE investigation of a factory that had been built on fill led Meyerhof (1951) to recom- mend in a similar way that: 'In gen- eral, buildings should not be constructed partly on fill and partly on natural ground unless the former is compacted to the same den-sity as the latter, nor should they be constructed near the edge of made-up ground, especially where this ends in a free slope, which is generally in a loose state.'

Such advice presupposes that the edge of the fill has been identified and accurately located. A highwall which emerges at ground surface may be found using trial pits, but the identification and location of a buried highwall may be difficult.

The problems are not confined to the UK. Lange (1986) has described work which appears to have been principally associated with the construction of pipelines across brown coal opencast mining areas in the Rhineland. Ground deformations over steep opencast mine slopes were studied and the results presented in terms of influence values; both settlement and extension were examined. Problems due to varying depths of fill have occurred in California and Rogers (1992) recommended that the variation in fill depth under a building should not exceed 15% of the depth.

Criteria for acceptable deformations

There is little point in analysing ground deformations unless some criteria are available which define what is acceptable for building development. It is helpful, therefore, to identify the significant factors relating to deformation acceptability criteria before examining the influence of fill properties and fill geometry on ground deformations. For problems involving building damage, various criteria have been specified in terms of parameters such as angular distortion and relative rotation. The use of these parameters to provide criteria for acceptable deformations is relatively complex as soil-structure interaction has to be taken into account. Furthermore, horizontal extension movements can also cause serious damage (Boscardin and Cording, 1989).

Where small structures are built on poor ground, the buildings are likely to have stiff foundations. Such foundations can be provided relatively cheaply. In this situation, it can be assumed that the foundation is sufficiently stiff to ensure that the structure does not deform, but simply tilts, and that the superstructure is not subjected to damaging horizontal tensile forces. As a consequence, a simplified approach can be adopted and a suitable deformation criterion can be defined in terms of acceptable tilt (a). Such an approach would not be appropriate for cases in which a very large or flexible structure was being considered.

The problems caused by tilt will depend on the type of the building and its purpose, but it is suggested that the following represent reasonably typical values for low-rise buildings;

structural collapse at 1/20

structural concern/distress at 1/50

noticeability at 1/200

In the light of these values, it is suggested that a suitable limit for an acceptable tilt for design purposes in a typical case is 1/500. A tilt of this magnitude would not normally be noticed and would have a substantial margin against failure. The area where it is estimated that the tilt, , will be greater than 1/500 will be termed the building exclusion zone.

Fill properties and geometry

The most important fill properties are the stiffness and the volume reduction potential. Fill does not behave as a linear elastic material and it is difficult to characterise the stiffness in a simple way; the problem is made more difficult by the complex patterns of stress changes which will occur in the fill near to a highwall.

With a vertical, or near vertical highwall, the angle of friction () at the interface of the fill and the highwall will be significant. In most practical problems the interface will be relatively rough, but anold dock wall could be quite smooth. In general 0 where is the angle of shearing resistance of the fill.

The cross-section of the fill can be defined by the following three parameters as shown in figure 3:

height of the highwall (H)

depth of burial of top of highwall (D)

angle of highwall to horizontal ()

Another geometrical factor relates to the location of the building in relation to the highwall.

A wide variety of fill geometries may be met in practice, but it is helpful initially to examine the four cases which are illustrated in

figure 4:

case A: a vertical, or near vertical, highwall with a smooth interface with the fill and with the top of the highwall at ground level

case B: a vertical, or near vertical, highwall with a rough interface with the fill and with the top of the highwall at ground level

case C: a highwall which is buried by a depth of fill considerably greater than the height of the highwall

case D: a long shallow slope

In each case it is assumed that the volume reduction potential (v) in the fill is uniform.

Some preliminary parametric studies have been carried out for cases B to D to assess the relative importance of the geometrical factors in determining the shape of the settlement profile. The fill was taken to be uniform and was represented by isotropic linear elastic parameters in the finite element analyses. The stiffness parameters were chosen to represent a typical fill with some collapse potential. The undisturbed strata forming the highwall were taken to be at least 100 times as stiff as the surrounding fill and no slippage was permitted between the fill and wall. The effect of changing the limiting angle of friction at the fill-wall interface and of the fill properties very near to the wall will be investigated at a later stage in the project. A reduction in volume throughout the fill up to the level of the top of the highwall was induced by allowing an imposed uniform pseudo-excess pore pressure to dissipate. In a fill of uniform thickness this would have resulted in a constant vertical strain of 1% throughout the fill up to the level of the top of the highwall. The surface settlement profiles were differentiated to calculate the tilt as a function of distance from the highwall.

Case A: Smooth vertical highwall at edge of excavation

In this case, there is a perfectly smooth interface between the vertical, or near vertical, highwall and the fill; there is no fill placed over the top of the wall. The situation is characterised by the following:

is 90degrees or close to 90degrees

D = 0

= 0

No parametric studies were required for Case A as the analysis is trivial. When there is volume reduction in the fill, slippage will occur along the interface between the fill and the undisturbed ground and result in a step at ground level at the top of the wall. There will be uniform settlement elsewhere within the fill as illustrated in figure 5 for a uniform vertical compression of 3%. Thus the building exclusion zone is, in theory, simply a line. Clearly this situation is very serious if a building has been located over the highwall. Otherwise the smooth interface may seem beneficial as movements are concentrated at an identifiable location and the area from which building is excluded is reduced to a minimum. An illustration of such a situation at the top of a dock wall is shown in figure 6.

Case B: Rough vertical highwall at edge of excavation

With this more typical highwall, there is a rough interface between the vertical, or near vertical, high wall and the fill; the top of the wall is at ground level. This case is characterised by the following:

is 90degrees or close to 90degrees

D = 0

=

Friction at the fill/wall interface is sufficient to prevent a step in the ground surface, but differential movement is still likely to be severe over a limited area. Defining the area of land next to the highwall which must not be built on is not simple, although differential settlement will be relatively localised in the vicinity of the wall. For illustrative purposes, a typical settlement profile is shown in figure 7(a) corresponding to a uniform volume reduction potential in the fill of 1%. With no slippage at the interface, the deformation pattern will be largely a function of the shear modulus of the soil. From the point of view of building development, the key issue is the location of the high wall and the need to avoid building over it. For the case illustrated in figure 7(c), the width of the zone where the tilt will be greater than 1/500 is shown in figure 7(b) plotted against volume reduction potential.

Case C: Buried highwall

This is the more general case of a buried highwall with the following characteristics:

30degrees

D/H 1

In this situation, the area of ground affected by differential settlement is much increased but the severity of the differential movements is reduced.

The deformed profile of the ground is principally a function of the ratio D/H and the fill properties and does not depend too strongly on the angle of the highwall. A typical settlement profile is shown in figure 8(a) for a volume reduction potential of 1% throughout the fill up to the level of the top of the highwall, based on some preliminary finite element analyses.

Burial of the highwall ensures that differential movements are reduced in magnitude but they occur over a wide area. For the case shown in figure 8(c), preliminary finite element analyses suggest that if the reduction in volume of the fill is small and v 1% there is unlikely to be any problem, but, if v 2%, there may be serious problems and a wide exclusion zone.

Case D: Long shallow slope

In this case fill has been placed on an undisturbed stratum with a relativelyshallow slope ( 15degrees).

The proposed building is small compared to the length of the slope and far from either end of it. If the fill compresses uniformly, differential movement of the fill will be entirely due to the variation in depth of fill under the building and will cause tilt of the building but not distortion. It is necessary to

determine what variation in depth of fill under the building is acceptable.

If the tilt of the building is , then:

= v.z /L(1)

where v is the uniform vertical compressive strain in the fill

z is the variation in depth of fill under a building of length L

z = L.tan(2)

Combining equations (1) and (2).

= v tan(3)

Figure 9 shows limiting values of corresponding to different values of v for the acceptable design tilt, a = 1/500. These results indicate that if v 0.5% even relatively shallow slopes could cause majorproblems, but that if v 0.5% there should be few problems. This finding confirms the classification of fills based on vertical compression potential proposed by Charles and Burland (1982) in which 0.5% was identified as the value below which fill wouldgenerally form a good foundation material with few problems, and 2% was identified as the value above which problems would be severe.

Conclusions

The safe and economic redevelopment of many brownfield sites is dependent on an appropriate decision concerning the area of land which cannot be built on due to changes in depth of fill. Where an excessive area is sterilised, redevelopment may not be economically feasible. Where the area sterilised is too small, building damage could result. Currently there is a lack of appropriate guidance.

A programme of research of the problems has been commenced at BRE involving numerical analysis and parametric studies. Some experimental work under carefully controlled conditions in a test pit and also at full scale in the field is planned. Clearly, research results will only give broad indications of the seriousness of differential ground movements due to variations in the depth of fill. In practice, other factors such as fill variability will also lead to differential settlement.

The initial analytical studies using typical stiffness parameters for the fill have made it possible to reach certain broad conclusions:

With high values of and low values of D/H, the friction between the fill and the highwall has a controlling influence on deformations. Where the surface is rough, the shear modulus of the fill will largely govern the pattern of deformations. In this situation even small reductions in volume of the fill will cause potentially damaging ground movements, but the area affected will be very limited.

With the top of the highwall buried under a substantial depth of fill, the geometry of the highwall itself becomes much less important and the magnitude of D/H has a dominant effect on the settlement profile. Burial of the highwall ensures that differential movements are reduced in magnitude but they occur over a wide area. If the reduction in volume of the fillis small with v 1%, for many cases there is unlikely to be a problem, but, if v 2%, there may be serious difficulties.With a relatively shallow slope, problems are unlikely with v 0.5%.

Away from these bounding cases there are a large number of con- figurations for which the exclusion area is difficult to predict in practice.

Acknowledgements

The research work described in this paper has been carried out for DETR under the Building Regulations Framework Agreement and Environment Business Plan.

References

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Building Research Establishment (1997/1998). Low-rise buildings on fill. Digest 427 (3 parts).

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Charles JA (1993). Building on fill: geotechnical aspects. Building Research Establishment Report, BR 230, 163pp.

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Lange S (1986). Building on uncompacted dumps in the Rhenish brown coal area of the Federal Republic of Germany. Building on Marginal and Derelict Land. Proceedings of Conference held in Glasgow, May 1986, pp 137-153. Thomas Telford, London, 1987.

Meyerhof GG (1951). Building on fill with special reference to the settlement of a large factory. Structural Engineer, vol 29, no 2, pp 46-57. Discussion, vol 29, no 11, pp 297-305.

Rogers JD (1992). Long term behaviour of urban fill embankments. Stability and Performance of Slopes and Embankments II: Geotechnical Special Publication no 31, vol 2, pp 1258-1273. ASCE, New York.

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Watts KS and Charles JA (1990). Settlement of recently placed domestic refuse landfill. Proceedings of Institution of Civil Engineers, Part 1, vol 88, December, pp 971-993.

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