Interim Report Section 3: Inquiry Issues and Recommendations

3.2 Geotechnical considerations

Characteristics of Canterbury soils in the area of the CBD

In the design of foundations on deep alluvial soils it is essential to allow for potential liquefaction, and the strength and stiffness of the soils. These deep alluvial gravels also affect the response spectra.

Complex inter-layered soil formations deposited by eastward flowing rivers from the Southern Alps underlie the CBD to a depth of up to 500m or more. In the top 20 to 25m these layers consist of recent deposits of gravels, sands, silts, peat and their mixtures. The soils are highly variable within relatively short distances both horizontally and vertically. These soils are subject to liquefaction and in some cases when deposited in a loose state exhibit very low resistance to liquefaction. As an example, the nature of soils along Hereford Street is depicted in Figure 4 below.

Figure 4: Subsurface cross section of Christchurch CBD along Hereford Street (reproduced and modified from Elder and McCahon, 1990)

The presence of near surface ground water increases the susceptibility to liquefaction. Depths to the water table vary from about 5m in the western suburbs to within 1.0 to 1.5m to the east.

The Royal Commission sought expert advice from Associate Professor Misko Cubrinovski from the University of Canterbury and Ian McCahon, Principal of Geotech Consulting Ltd. Their report entitled ‘Foundations on Deep Alluvial Soils’ provides information on the characteristics and behaviour of soils during the Canterbury earthquakes and has been published on the Royal Commission’s website.

The consequence of subjecting the variable soil structure when subjected to earthquake vibration is the creation of pronounced liquefaction that often, but not always, leads to discharge at the surface of sands, silts and water. The consequences of liquefaction include loss of soil strength, lateral spreading and adverse effects on the performance of foundations. Lateral spreading involves displacement of some areas of the surface layers and typically occurs in sloping ground or level ground close to waterways.

The deep alluvial deposit beneath Christchurch, when subjected to earthquakes, also increases the period of vibration of the subsoil mass, which in turn alters the surface accelerations to which buildings are subjected.

Liquefaction

Liquefaction of soils in the CBD occurred during three of the earthquakes but this effect was much greater in the 22 February earthquake than in the other two events, due to the greater horizontal accelerations experienced in that event. The areas within the CBD subjected to serious liquefaction are indicated in Figure 5. The figure has been constructed from on-site inspections 10 days after the 22 February earthquake (Cubrinovski and McCahon).

Figure 5: Preliminary liquefaction map indicating areas within the CBD affected by liquefaction in the 22 February earthquake. Legend: red = moderate to severe liquefaction; green = low to moderate liquefaction

When subject to shaking, fully saturated sand and silt soils experience a near instantaneous increase in ground water pressure. This increase in water pressure cancels the gravity loads which have held the particles together and transforms the soil into a heavy liquid state with corresponding loss of stiffness and strength. An upward flow of water to the surface relieves the pressure induced by the weight of the overburden and results in the soil/water mixture spurting from the surface in localised areas.

In its liquefied state the soil allows heavy objects to sink or settle into the ground while lighter buried objects such as empty pipes, tanks and manholes may float upwards.

Lateral Spreading

Lateral land movement is a possible consequence of liquefaction. Even on a gentle slope (2˚ to 3˚) the loss of strength of the soil coupled with the cyclic motion of the earthquake can cause a down-slope movement to occur. This is marked at the free edge of river banks and has occurred in many areas close to the banks of the Avon River. Horizontal movements in the CBD of the order of 50 to 70cm towards the river have occurred with some movements extending to a distance of 150m from the river.

Response Spectrum

The deep gravel silt and sand formations below the CBD amplify some of the periods of vibration generated by the earthquake and de-amplify others. The cyclic movement of the soils on which the structures are supported generates the forces to which the structures are subject. The amplification effect is an important feature of the Canterbury earthquake events.

An example of amplification and de-amplification is indicated in Figure 6.

The diagram shows acceleration occurring in the rock base (the red line). In this example, acceleration in the soft soil site (blue line) is reduced in periods below 0.5 seconds but is increased in longer periods – up to three times the acceleration is indicated.

Figure 6. Acceleration response spectra recorded on rock (LPCC) and soil (LPOC) in Lyttelton during the 22 February earthquake illustrating typical effects of alluvial soils on response spectra (5% damped, elastic spectra

Alpine Fault

The Royal Commission has noted that the Cubrinovski and McCahon report suggests that a magnitude (Mw) 8.0 Alpine Fault event can be expected to induce less intense liquefaction than the 22 February 2011 event. While this suggestion appears to be reasonable, it should also be noted that in such an event, areas of liquefaction can be expected to occur in other areas in Canterbury. It should also be acknowledged that there might be cases in which worse effects and poor building performance will result from the much more prolonged duration of shaking caused by an Alpine Fault event. Earthquakes generated by the Alpine Fault, or other major faults in or near the mountains and in North Canterbury, remain the most likely sources of damaging earthquakes in Christchurch once the aftershocks from the recent high stress earthquake swarm subside.

Performance of Foundations

It is obvious that the loss of strength of surface soils will have adverse effects on building foundations. Several buildings in the CBD have experienced serious consequences from the ground movement.

Table 3 below indicates a range of foundation types that have been used in the Christchurch CBD.

Table 3: Typical foundation types used within the CBD

Foundation Type	Building Type	Foundation Soils
Shallow foundations (isolated spread footings with tie beams)	Multi-storey buildings Low-rise apartment buildings	Shallow alluvial gravel Shallow sands, silty sands
Shallow foundations (raft foundations)	Multi-storey buildings Low-rise apartment buildings with basement	Shallow alluvial gravel Shallow sands, silty sands
Deep foundations (shallow piles)	Low-rise apartment buildings	Medium dense sands (soft silts and peat at shallow depths)
Deep foundations (deep piles)	Multi-storey buildings	Medium dense to dense sands (areas of deep soft soils or liquefiable sands underlain by dense sands)
Hybrid foundations (combined shallow and deep foundations or combined shallow and deep piles)	Multi-storey buildings	Highly variable foundation soils including shallow gravels and deep silty or sandy soils beneath the footprint of the building

 

 

Cubrinovski and McCahon report that the liquefaction in the CBD adversely affected the performance of many buildings resulting in differential settlements, lateral movement of foundations, the tilting of buildings and some bearing failures. The following conclusions are noted:

• buildings on shallow foundations, on loose-medium dense sands and silty sands that liquefied suffered differential settlements and residual tilts. Several buildings sank into the soil;
• pile-supported structures, when the piles reached competent soils at depth, generally showed less differential and residual movements than shallow foundations, even in areas of severe liquefaction;
• multi-storey and high-rise buildings supported on shallow foundations sitting on shallow gravels showed mixed performance. Variability in thickness of gravel and underlying soil layers resulted in some differential settlements, tilt and permanent lateral displacements. These adverse effects were especially pronounced in transition zones where ground conditions change substantially over short distances;
• hybrid building foundations (shallow and deep foundations or piles of different lengths) performed relatively poorly;
• within the CBD, zones of ground weakness (either localised over a relatively small area or sometimes continuous over several blocks) exhibited pronounced ground distortion and liquefaction that adversely affected a number of buildings. Buildings only 20 to 30m apart behaved differently, according to the condition of the ground; and
• the effects of lateral spreading within the CBD were localised but quite damaging to buildings causing sliding and stretching of the foundations and the structures.

Structure-soil-structure interaction of adjacent (multi-storey) buildings was another response feature that influenced the performance of the foundations of buildings in the CBD to some extent.

Although pile-supported structures typically suffered less damage, piles can lose support when supported in or above soils that liquefy.

There is no single foundation system that will be used to support the buildings of the future. Each structure will need foundations chosen with careful consideration of the soils beneath. A number of factors need to be considered in choosing the optimum foundation. Factors will include the size and cost of the building – a lower rise building will be of lower weight. A foundation that spreads building loads over soils in a wider area of a variable nature may be suitable (for example, raft foundations). Piled foundations for higher-rise buildings can penetrate to stronger layers at depth.

Site Investigation

One clear conclusion for the design of buildings in the CBD is the need for a very comprehensive site investigation in which the layering of soils and the soil properties of each layer are clearly understood. In addition, the rectification of damage to the ground and subsurface due to liquefaction from the 2010 and 2011 earthquakes will need to be addressed.

The variable characteristics of the CBD soils and the extent of foundation damage as a result of the recent earthquakes have highlighted the importance of foundation design in the CBD. Knowledge of the soil layers and their characteristics to a depth of 25m is required. This knowledge may, in part, be ascertained by bores on adjacent sites if these are available. Soil parameters derived from the standard penetration (hammer) test and cone penetration tests add to the knowledge required for assessment of liquefaction and choice of foundation type and should form part of specific site investigations.

Cubrinovski and McCahon conclude that it is necessary to understand the behaviour of the soil-structure system during strong ground shaking and the contribution to this behaviour made by the foundation soils and the foundations themselves. Best practice internationally is for the issues associated with foundations on deep alluvial soils to be addressed by either:

• comprehensive geotechnical investigations of the site and robust design methodology considering the soil-foundation-superstructure system including use of in-depth analysis to scrutinise the performance of the system; or
• avoiding locations with difficult soil conditions.

After considering the discussion in the Cubrinovski and McCahon report the Royal Commission is of the view that the Christchurch City Council should require thorough foundation soils investigations to be carried out as a prerequisite to foundation design.

Piled Foundations

The choice of foundation will be specific to the building proposed. However, in the choice of pile type, the Royal Commission has been informed that driven piles have fallen out of favour. One reason given has been the noise and vibration during construction of these foundations. Piled foundations in which the load capacity of the pile can be inferred from driving records may offer advantages and should again be considered as a possible foundation.

Geotechnical Parameters

The Royal Commission acknowledges the progress that has been made in geotechnical engineering. There is clearly a need to maintain progress in this way. The Canterbury earthquakes have shown the need for this effort to continue particularly in light of new evidence about foundation performance. Two matters have been highlighted. One is the need for better knowledge of soil profiles and properties. These demand enhanced site investigation.Secondly, the interaction of soils and structures during earthquakes requires further consideration by geotechnical and structural engineers. These matters will be further developed and included in the Royal Commission’s Final Report.

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