Ground movement The latest in our occasional series on geotechnics1 looks at how ground movements can affect new building
Ground movements can arise from two major sources: movements due to ground instability, caused, for example, by slope instability, geological voids, or subsidence due to the collapse of old mineworks; and movements due to a changing stress-state, leading to volume changes within the soil. Dewatering, tree problems or loading of foundations can change stress- states.
Of the two types, movements due to ground instability are potentially the most serious as they can lead to the collapse of a building. Movements due to changing soil volume tend to occur more gradually and normally result in damage rather than collapse. However, remedial measures such as underpinning and structural repair can be expensive for buildings subject to movement from either cause.
Some ground movement will always take place. In most cases its effects on buildings can be predicted and dealt with using routine design solutions. A thorough desk study is an important part of early feasibility work for new development. (Bonshor & Bonshor2 reviews the various causes of cracking in existing buildings.)
Mass movements of the ground: These are usually due to some form of slope instability. The potential for inherent slope instability should be identified during a preliminary desk study. However, re-grading works, such as cutting (particularly near the toe of the slope) and filling (particularly near the top of the slope) can trigger fresh problems.
As a rule, slope problems become more important for slopes greater than 10degrees in clay soils or 20degrees in granular deposits. In some ground conditions, gentler slopes can also cause problems.
Ground anchors or soil nails can rectify unstable existing slopes. However, these schemes can be very expensive to design and install and can lead to an ugly landscape that needs regular maintenance. In addition, using ground anchors may require a licence to pass under neighbouring land. New slopes needing a steep face angle are easier to engineer using reinforced soil (Jewell3) or gabions (Chapman4) - see diagram.
Mining and other voids: The uk has a legacy of old mineworkings beneath much of the country. Their extent can be gauged from regional reviews by Arup5. Subsidence is normally associated with the collapse of old shafts, although more extensive collapses of mines are also possible. Old mineworkings can also be a source of gases. Foundation types to mitigate the effects of mining subsidence are described by Atkinson6.
Particular geological conditions can also be a source of voids. A frequently encountered example is 'deep solution features' - the surface chalk has been dissolved by water, creating a void that has filled with very loose or collapsible material. Site investigation may not identify all such features.
Planning Policy Guidance, Note 147 identifies several other reviews of potential geotechnical hazards in the uk. These include natural underground cavities (Applied Geology8) and landsliding (Geomorphological Services9). Much of this work has been summarised for detr by Wimpey Environmental10.
Changing ground stress
Settlement of buildings: As foundations are loaded they settle, depending on the concentration of loading and stiffness of the soil. The magnitude of both absolute and differential settlements needs to be considered. A very good summary of possible building damage is given by Burland and others11. They identified angular distortion (variation in tilt across a building) and horizontal tensile strains as most damaging for structures, rather than overall tilts, which are the most commonly quoted.
For shallow foundation design, the allowable bearing pressure on the soil depends on the acceptable settlement. Foundations on granular soils seldom settle by more than 25mm; but settlements on clay can be greater. Settlements of rafts are often governed by disposition of structural loading or ongoing heave (upward movement of the ground).
Piled foundations are normally chosen where the shallow strata are too soft, or where the groundwater level is high, or foundation loads are particularly high, or for a combination of these reasons. Pile settlements depend on the type of pile, its loading and the ground conditions. Settlements of 0.5-1.0 per cent of the pile diameter for an individual pile are typical. Piles spaced at closer than 3 x pile-diameter centre-to-centre, tend to settle by larger amounts. A dense forest supporting a tall building can settle by much more than would the individual piles in isolation. Occasionally, piles are needed for low-rise buildings, at a relatively high foundation cost.
Changes due to water: Trees act as powerful pumps, drawing water up from their roots. In fine-grained soils the consolidation induced in the surrounding soil can be great and extend to some depth. However, the effects are relatively confined in plan, generally limited to a radius no greater than the height of the tree, so movements induced in structures can be quite local. One corner of a building can undergo significant movements while the opposite corner may be completely unaffected, resulting in significant, potentially damaging differential movements.
Maximum movements are likely when a new tree is planted and grows to maturity near a structure, especially a tree species with high water demand. The reverse process, felling a tree, causes movements in the opposite direction and can be equally damaging. See Chandler and others12 on soil desiccation. The previous article in this aj series (aj 27.5.99) and bre Digest 29813 focused on trees and new build. A future article will cover tree damage for existing buildings.
Dewatering (the lowering of the water table), either deliberately or by natural means, can have two major effects. In fine-grained soils such as clays it causes the soil to consolidate, which induces settlement. With softer, more compressible soils such as alluvial clays, settlements can become large. When using groundwater-control systems, high water-pressure gradients close to the pump can draw finer grains out of the soil. The resulting soil structure can be susceptible to collapse-settlements. The effects are usually but not always local.
Some soils are liable to reduce in volume on inundation by water. In the uk this is generally only a problem for fill materials. In particular, backfilled opencast-mining sites can be prone to 'collapse-compression' as the water level rises through the fill following cessation of groundwater pumping or due to downward penetration of surface water. See Charles14 on problems of building on fill.
Heave: This is the upward movement resulting from unloading the ground or, typically in clays, from the removal of mature trees. Heave will occur after the demolition of a building sitting on shallow foundations, although the largest heaves occur after the excavation of deep basements. Typically, excavating a 10m-deep basement would have the same effect as demoliting a 20-storey building.
For basements underlain by sand or gravel, the ground stiffness is higher so movements are less. More importantly, the heave is completed rapidly, usually before the building is complete. For basements underlain by clays, the movements can take decades to complete. Thus, long-term heave will be competing with the settlements due to the weight of the new building. Their combined effects at different times of a building's life have to be assessed.
Allowing for movements in new structures: Where the cause of potential movement is sufficiently localised, a changed site-location can be the best choice. Ground treatments such as consolidation or filling of old mineworkings might be possible. Otherwise, movements have to be addressed through design, matching structural forms and materials to the magnitudes of movement at an early stage in design.
Where piles are required they may need also to resist induced tension from heave. A heave-void below a basement slab may allow a more economical structural solution.
While movement joints may be needed above ground for a variety of structural reasons, below ground such joints can cause problems in waterproofing and are better avoided. (See Bussell and Cather15 on design of construction joints.).
Tim Chapman is an associate at Arup Geotechnics. Deborah Lazarus is an associate director of Arup Research and Development. Mike Crilly is a project manager at bre's Centre for Ground Engineering and Remediation.