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Impact of fire on tunnels in Hawkesbury sandstone

A.G. Smith a,*, P.J.N. Pells b

a Pells Sullivan Meynink Pty Ltd., 28 Prospect Street, Fortitude Valley, QLD 4006, Australiab Pells Sullivan Meynink Pty Ltd., P.O. Box 173, Terrigal, NSW 2260, Australia

Received 15 May 2006; received in revised form 26 October 2006; accepted 12 November 2006Available online 5 January 2007

Abstract

The majority of tunnels in Sydney, Australia are within near-saturated Hawkesbury sandstone. Crown support in these tunnels typ-ically comprises permanent rockbolts, and shotcrete ranging in thickness from about 75 mm to 250 mm. Sidewalls are mostly exposedsandstone with occasional rockbolts, and, in places, a thin shotcrete skin for surface protection.

Rock will quickly be exposed to high temperature in a tunnel fire where no, or a thin layer of shotcrete exists. Rock with thicker shot-crete may also be exposed where spalling of the shotcrete occurs.

The phenomenon of spalling in fire has been widely researched for concrete and, to a lesser extent, shotcrete [e.g. Tatnall, P.C.2002. Shotcrete in fires: effects of fibers on explosive spalling. Shotcrete, 1012]. It is assessed as being primarily associated with steampressures produced by evaporation of water in pores [Hertz, K.D. 2002. Limits of spalling of fire-exposed concrete. Fire Safety Jour-nal 38, 103116].

This paper assesses, by means of laboratory and field tests, how exposed sandstone is likely to respond in a tunnel fire.It is concluded that substantial explosive spalling will occur early and at relatively low temperatures. This spalling could create dan-

gerous conditions for rescue, and escaping personnel. However, beyond the zone of spalled sandstone there would be only minor struc-

tural impact on the rock mass. 2006 Elsevier Ltd. All rights reserved.

Keywords: Hawkesbury sandstone; Fire; Tunnel; Spalling

1. Introduction

The majority of tunnels in Sydney are within Hawkes-bury sandstone. Road tunnels include the Eastern Distrib-utor, The Cross City Tunnel, Lane Cove Tunnel, and theNorfolk Tunnel on the M2. In these tunnels crown support

comprises permanent rockbolts and shotcrete ranging inthickness from about 75 mm to 250 mm. Being usuallybelow the ground-water table, the sandstone around mostof these tunnels is saturated or close to saturation. Side-walls are mostly exposed sandstone with occasional rock-bolts and in places, a thin shotcrete skin for surfaceprotection.

Where the shotcrete is thin or non-existent, rock willquickly be exposed to high temperature in a tunnel fire.Rock with thicker shotcrete may also be exposed wherespalling of the shotcrete occurs.

The question addressed by this paper is, How mightexposed sandstone in the Sydney tunnels respond to a high

temperature tunnel fire? This question has been investi-gated through field fire tests conducted on an exposure ofHawkesbury sandstone, and a program of laboratory tests.

2. Review of previous research

There is little literature which addresses the behaviour ofrock, and specifically sandstone in fire. Dorn (2003) dis-cusses spalling of sandstone boulders during wildfires inthe Sierra Ancha Mountains, Arizona. Erosion and nutri-ent cycles in a natural environment are this papers focus;

0886-7798/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.tust.2006.11.003

* Corresponding author. Tel.: +61 400673553; fax: +61 731358203.E-mail addresses: [email protected] (A.G. Smith), pells@

psmtoo.com.au (P.J.N. Pells).

www.elsevier.com/locate/tust

Available online at www.sciencedirect.com

Tunnelling and Underground Space Technology 23 (2008) 6574

Tunnelling and

Underground Space

Technologyincorporating Trenchless

Technology Research
mailto:[email protected]:pells@mailto:pells@mailto:[email protected]

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it does draw unsubstantiated conclusions about the effectsof moisture and heating rates on the level of spallingdamage.

A few authors have done work on the effects of fires onhistoric sandstone buildings. In a number of papers (Haj-pal and Torok, 2004, 1998; Hajpal, 1999, 2002) data is pre-

sented from samples of 10 sandstone types from Germanyand Hungary. Samples were subject to temperatures up to900 C. Testing after heating and cooling included uncon-fined compression and indirect tensile strength as well aschange in colour, density, porosity, water adsorption andultrasonic sound velocity. Thin section analysis, X-ray dif-fractometry and scanning electron microscope photogra-phy was performed. Different sandstone types displayeddistinctly different trends of behaviour. The rock matrixmaterial was assessed to be the primary reason for the dif-ferences. Spalling was not considered.

For the rebuilding of a historic sandstone building inGermany, Pohle and Jager (2003) performed tests on cored

sandstone samples to determine the loss of strength inunconfined compression after being subject to high temper-ature. Uniaxial compressive strength on cubes showed acontinuous drop in strength. begin[ing] to be particularlypronounced at temperatures above 300 C. It was notedthat samples heated to 1000 C had 35% less strength thansamples which had not been heated.

A few larger blocks were put under vertical loads duringa fire test on the basis of the so called standard tempera-ture time curve. No data is available from the test on thelarger blocks, only that the duration of fire resis-tance. . .was proven. Photographs of after this test show

a large amount of spalled rock, shaped predominately inthin slabs.

In 1985, the Technical Research Centre of Finland con-ducted two full scale tests in a 6 m wide by 5 m high tunnelin a limestone quarry, 45 m below the ground surface. Therock surface was wet and was unreinforced and unpro-tected. The first experiment was designed to simulate a firein a subway car stalled in a tunnel. The second experimentsimulated the case when one car in a queue of cars in a tun-nel catches fire. Forced ventilation of fresh air at the rate of7 m3/s was used. The fire load was made of wood cribs thatallowed an air space of 50% of the total volume. Tempera-tures of air, rock surfaces of the walls and the ceilings andtemperatures of the steel and concrete columns placed onthe floor were recorded at several locations.

In the first test, the fire load temperature reached about800 C after about 40 min, the power being computed atabout 1.8 MW. However, 25 min after the fire started thefirst strong hollow sound was heard caused by fallingrock. At the end of the experiment there was a largeamount of spalled rock on the floor of the tunnel, typicallyin the form of large flakes between 50 mm and 150 mmthick, the largest estimated to weigh about 400 kg. It isinteresting that the investigators record unfortunatelythe inspection of the tunnel immediately after the fire was

impossible because of the danger of falling rocks.

The maximum recorded surface temperatures of therock in the crown ranged between about 120 C and210 C. Unfortunately, due to falling rocks the maximumtemperatures at the centre of the ceiling were not measured.

The second test comprised 8 cribs of timber but the firstfire failed to jump to crib 3. Cribs 7 and 8 were then ignited.

In the first partial fire the maximum ceiling temperaturewas 450 C, and the upper sidewall 210 C. In this experi-ment an approximately 5 m 5 m area of the crown wasprotected by a 100 mm layer of mineral wool. Spalling ofrock occurred to a lesser extent than in the first test, withthe flakes being thinner. The mineral wool was effectivein protecting the ceiling over a 5 m 5 m patch.

The investigators give no views as to the mechanisms ofspalling and as to the lesser degree of rockfall in the secondtest. They simply note that this spalling is highly hazard-ous for both the instrumentation and the people working inthe tunnel.

The phenomenon of spalling in fire has been widely

researched for concrete (e.g. Hull and Ingberg, 1925) andto a lesser extent, shotcrete (e.g. Tatnall, 2002). It isassessed as being primarily associated with steam pressuresproduced by evaporation of water in pores.

The Handbook of Tunnel Fire Safety (Beard and Car-vel, 2005) published in 2004 has the objective of coveringthe state of practice of all facets of tunnel fires. The hand-book includes a detailed discussion of the behaviour ofconcrete linings under high temperature, but provides verylittle information in regard to the corresponding behaviourof exposed rock. It mentions the 1985 tests at Lappeen-ranta discussed above but otherwise there is no technical

discussion of this facet of tunnel fire behaviour in theHandbook. The only other mention of this topic is in thechapter on Fire and Rescue Operations (by specialists fromSweden), wherein it is stated:

It is difficult, too, to access what will be the effect of

flames and heat on the tunnel walls and roof, and how they

will be affected by the shock of extinguishing with cold

water. . . .. The lives of several of the fire-fighters in the

Tauern Tunnel were probably saved as a result of experi-

ence from the mistakes made in the Mont Blanc fire about

two months earlier. Practical fire trails carried out in

Sweeden by the Telia telephone company also showed

the considerable risks due to the collapse of tunnels as aresult of fire when large blocks of rock collapsed.

Based on the above it is reasonable to conclude that thematter of rock behaviour during and after high tempera-ture tunnel fires is an important but poorly understoodissue.

3. Objectives

Laboratory tests were conducted to investigate the effectof heating on intact strength and stiffness of Hawkesburysandstone after cooling. Spalling effects were specifically

removed.

66 A.G. Smith, P.J.N. Pells / Tunnelling and Underground Space Technology 23 (2008) 6574

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Field testing was undertaken with two main objectives:

1. To provide heat transfer data.2. To observe physical changes and spalling.

4. Study program

4.1. Laboratory testing

A suite of testing was undertaken on samples cored fromthree blocks of Hawkesbury sandstone sourced from theGosford Quarry. Samples were baked at temperaturesvarying from 250 C to 950 C for 2 and 4 h as summarisedin Table 1.

The specimens were cut to a length/diameter ratio of atleast 2:1. The ends were prepared in accordance with theguidelines of Pells (1993). The specimens were dried andthen subjected to different temperatures in an electric fur-

nace for periods of 2 or 4 h. Table 1 is a summary of thenominal temperatures and period of heating of the speci-men. An additional nine samples were subject to normaldrying at 105 C for 24 h and nine samples retained fortesting at a relative humidity of between 70% and 80%.

After removal from the oven, the specimens were cooledinside desiccators to preserve the moisture condition.

The specimens were then subjected to unconfined com-pressive strength (UCS) testing in accordance with ISRMguidelines. Axial deformation measurements were madeby means of a transducer system attached to the end plat-ens. This system is known to give slightly higher axial strain

measurements than strain gauges due to end effects. How-ever, since the object of this study was mainly to obtaincomparative behaviour, this small loss in absolute accuracyof strain measurements is not an issue for concern.

4.2. Field testing

The apparatus for the field fire test comprised a bricklined charcoal furnace against the rock cutting (see Fig. 1).

Air was blown through the furnace from the bottom toachieve the high temperatures required. Some control of

temperatures could be achieved by slight changes in airquantity. The heated area measured 0.9 by 0.9 m. Fig. 2is a plan showing the position of thermocouples installedin the rock adjacent to the furnace, using small diameterholes drilled from the top of the cutting. The thermocou-ples were installed at 500 mm vertical height above the fur-nace base. These holes were cement grouted afterinstallation of the thermocouples.

The furnace was operated for approximately 4 h prior tothe forced air being removed, and no further charcoal fuelbeing added. Following cooling of the furnace, the bricks

Table 1Summary of samples

Nominaltemperature

Time (h) Number of samples Sample numbers

250 2 6 16250 4 6 712500 2 6 1318500 4 6 1924750 2 6 2530750 4 6 3136950 2 6 3742950 4 6 4348Unheated, undried 0 9 4957

Unheated, dried 0 9 5866

Fig. 1. Site photo.

Fig. 2. Plan of test setup.

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were removed, photographs were taken and the depth ofspalling was measured on a 250 mm grid, allowing the vol-ume of spalling to be calculated.

A second fire test was conducted on a second sandstoneexposure to assess the impact of small diameter holesdrilled into the rock with the intent of relieving steam pres-

sures. The fire-exposed face was divided into two equalhalves; 12 mm diameter, 150 mm long holes were drilledinto the rock on a 150 mm grid in one half and on a75 mm grid in the other half. Each hole was filled with ver-miculite, providing a highly porous conduit for steam pres-sures to escape from the rock, whilst reducing the effect ofincreased thermal radiation into open holes.

5. Results

5.1. Laboratory testing

The only visual changes to the specimens caused by the

baking process were subtle colour changes at the highertemperatures, the brown sample became a red/brown,white samples became a duller white.

Samples with the same treatment (maximum tempera-ture and duration of heating) showed close similarity intheir behaviour under unconfined compression. Figs. 311 are plots of stress versus strain for each UCS test, withsamples undergoing the same treatment grouped into a sin-gle graph.

For comparison purposes a representative test resultfrom each of the groups presented in Figs. 311 is plottedin Fig. 12.

The data summarised in Fig. 12 show:

Decreasing strength with increase in the baketemperature.

Increasing strain to failure and decreasing stiffness withincrease in the bake temperature.

Fig. 3. Stressstrain plots, samples heated at 250 C for 2 h.





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Fig. 11. Stressstrain plots, samples at ambient humidity.

Fig. 12. Selected stressstrain plots.


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A secondary magnification of the above effects withincreasing duration of the bake.

These primary effects are shown in the correlation plotsin Figs. 1316.

5.2. Field testing

The timetemperature curve achieved during operationof the furnace is plotted alongside a number of standardfire curves in Fig. 18. It can be seen that the field testwas a reasonable approximation to a hydrocarbon fire.The ISO834 curve shown is that adopted by Australianstandards including AS3600 for reinforced concrete andAS4100 for steel structures. Fig. 17 is a photograph ofthe furnace in operation.

Explosive spalling of the rock commenced in the firsttest as the furnace temperature approached 900 C,approximately 22 min after application of heat. Rock tem-

perature at this time at a depth of 0.1 m was still less thanabout 30 C (see Fig. 20). Each spalling event is shown inFig. 18. The first spalling event produced a loud explosion,knocking over the furnace and requiring reinforcing of thefurnace and restarting of the test (see Fig. 19). A thin(25 mm) slab of rock was spalled during this event. The fur-nace was reinforced as shown in Fig. 17.

Fig. 20 shows the distribution of temperature into therock at selected times during the test. The temperature axisis shown logarithmic for clarity, given the high thermalgradient.

Spalled material inside the fire was characteristically,thin sheets of rock parallel to the face varying from 10 to30 mm thick. Outside the heated zone, thicker blocks wereformed by extension of cracking initialised inside thefurnace.

It is inferred from the abrupt change in temperature in

thermocouple 1 (Fig. 20), noted audible spalling and

Fig. 13. Strain to failure verses subjected temperature.

Fig. 14. Unconfined compressive strength verses subjected temperature.

Fig. 15. Tangent modulus at 50% of UCS verses subjected temperature.

Fig. 16. Initial secant modulus at 5 MPa verses subjected temperature.


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post-test observations, that spalling at approximately 4 h11 min exposed the grouted drill hole of thermocouple 1.Prior to this, at this thermocouple less than 100 mmfrom the heated face, the temperature had not exceeded110 C. Spalling may therefore be triggered bytemperatures not much exceeding the boiling point of

water.The following observations were made in the first test inrelation to steam and steam pressures:

steam was visible from all spalled surfaces, extending upto 1 m outside the fire,

steam was forced out of the grouted hole that carried thethermocouple 100 mm from the furnace face.

The second fire test was proposed based on the linkbetween steam pressures and spalling postulated fromobservations made during the first test. Holes (12 mm)were drilled into the rock at spacings of 75 and

150 mm, with the aim of releasing steam pressure and lim-iting spalling.

The results of the second test were almost opposite towhat were expected in that:

There was substantial increase in generated steam in theinitial hour of the test compared to the first test.

Lower temperatures were achieved in the first hour dueto the high amount of steam from the rock that inhibitedbuild up of the furnace temperature.

Fig. 18. Timetemperature curve and published fire timetemperature curves.

Fig. 17. Operation of the furnace.


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Spalling was recorded in the first twenty minutes of thetest when the temperature measured in the middle of thefurnace was only 150 C.

The nature of the spalled material was different, beinglarger pieces rather than thin slabs.

It is postulated that the vermiculite filled holes allowedincreased transfer of heat into the rock, increasing thequantity of spalling and changing the nature of the spallingto thicker blocks.

In a tunnel situation spalled material would mostly fallaway from the crown and sidewalls, completely revealingthe fresh surfaces beneath it. By the nature of the test set

up, the charcoal and furnace provided support to spalled

material. Newly exposed rock was therefore partiallyshielded and was not exposed directly to the fire to theextent that would occur in a tunnel.

Fig. 19. Timetemperature curves of furnace and thermocouples installed into the rock.

Fig. 20. Curves of temperature verses distance into rock at selected times.

Table 2Volumes of spalling

Test Spalled volume (m3)

Test 1, total 0.115Test 2, total 0.155Test 2, (holes at 75 mm centres) 0.20Test 2, (holes at 150 mm centres) 0.135

Fig. 21. Fire test 1 location.


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With cognisance to the limitations described above, the

volumes of spalling were recorded for each test. These aresummarised in Table 2.

It can be seen that the 12 mm diameter holes drilled intothe rock for the second test, supposedly to relieve steampressures, actually exacerbated spalling.

Figs. 2124 are photographs of before and after fire test-ing showing the rock, which spalled during the tests.

6. Conclusions

6.1. Spalling

Spalling of Hawkesbury sandstone may occur after onlya few minutes of exposure to fire. The extreme tempera-tures given in standard fire curves are not required to causeexplosive spalling. Only a few hundred degrees may be suf-ficient. The mechanism causing spalling appears to be pri-marily the generation of steam pressure.

Many tunnels in Sydney have a relatively thin shotcretelining and the sandstone is saturated with water. Thepotential for significant explosive spalling in a tunnel fire

in Sydney should be considered.

6.2. Rehabilitation of a tunnel after fire

The impact of a fire on rock properties is unlikely toextend much beyond the spalled material. Sandstone hasa low thermal conductivity (Fig. 20) and high tempera-tures must be experienced prior to significant loss ofstrength (Fig. 14). It is possible that rehabilitation of atunnel in Hawkesbury sandstone may require only rein-statement of pre-fire tunnel support after scaling ofspalled rock. Scaling and work procedures similar to

those during tunnel construction may be sufficient to pro-vide safety during work.

Acknowledgements

The authors thank Mr. Greg Cook and Gosford Quar-ries for permission to conduct work and assistance withinstrument installation. Financial assistance was providedby PSM Research Pty Ltd. Adrian would also thank Terry,Warwick, Mark Smith and his wife Anna for practicalassistance.

References

Beard, A., Carvel, R., 2005. The Handbook of Tunnel Fire Safety.

Thomas and Telford.

Dorn, R.I., 2003. Boulder weathering and erosion associated with a

wildfire, Sierra Ancha Mountains, Arizona. Geomorphology 55, 155

171.

Hajpal, M., 1999. Burning effect on sandstones of historic buildings and

their petrophysical and mineralogical studies. Periodica Polytechnica:

Civil Engineering 43, 207218.

Hajpal, M., 2002. Changes in sandstones of historical monuments

exposed to fire or high temperature. Fire Technology 38 (4), 373

382.

Hajpal, M. and Torok, A. 1998. Petrophysical and mineralogical studies

of burnt sandstones 2nd Int. PhD Symposium in Civil Engineering,

1998. Budapest.

Fig. 22. Effect of fire test 1.

Fig. 23. Fire test 2 location.

Fig. 24. After fire test 2.


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Hajpal, M., Torok, A., 2004. Mineralogical and colour changes

of quartz sandstones by heat. Environmental Geology 46, 311

322.

Hertz, K.D., 2002. Limits of spalling of fire-exposed concrete. Fire Safety

Journal 38, 103116.

Hull, W.A., Ingberg, S.H., 1925. Fire resistance of concrete columns.

Journal of the Franklin Institute 200, 379381.

Pells, P.J.N., 1993. Uniaxial strength testing. In: Comprehensive Rock

Engineering, vol. 3. Pergamon (Chapter 3).

Pohle, F., Jager, W., 2003. Material properties of historical masonry of the

Frauenkirche and the masonry guideline for reconstruction. Construc-

tion and Building Materials 17, 651667.

Tatnall, P.C., 2002. Shotcrete in fires: effects of fibers on explosive spalling.

Shotcrete, 1012.


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