Editor:
Re: Ministry Fact Sheet: Can a bridge be built on a river delta?
I read with interest the Massey Tunnel replacement bridge fact sheet, originating with Ministry of Transportation and Infrastructure and published on your newspaper's website. It suffers from a certain lack of precision which this letter will attempt to remedy.
The logs of the two boreholes drilled to 335 metres from surface for the Massey Tunnel replacement bridge show only sand and silt, with decomposed organic material as far as 280 metres from surface, ranging from 20 per cent to 50 per cent water content. While the boreholes did encounter layers of stiff, dense material, core recovery ranged erratically with depth from 100 per cent down to as little as 0 per cent, the missing material being so soft that it was not recovered in the coring tube. The question that I posed some time ago: “Given the foundation conditions, can a bridge be built at reasonable cost, if at all?” remains unanswered, beyond the obvious fact that you can build (almost) anything, provided you spend enough (taxpayer) money doing so.
Before the Massey Tunnel was built, Crippen Wright Engineering wrote the following report:
Crippen Wright Engineering Ltd. Comparative Report on Fraser River Bridge and Tunnel Crossings at Deas Island. December 1955
Page 5: “Surface and subsurface investigations show that the site is well suited to the construction of a tunnel.”
Page 6: “Subsurface investigations disclose soil with relatively low load bearing characteristics, and there is no bed rock at practicable depths; the foundations for the main piers will require an extensive pile driving program.”
Page 8: “Bridge piers and anchors will require very expensive foundations since there is no bed rock or other good bearing material at any practicable depth.”
The report did not say that a bridge could not be built, just that it would be very expensive to do so. The foundation conditions were one reason, among many, why a tunnel was chosen instead of a bridge.
The fact sheet points to other bridges built nearby:
The Alex Fraser bridge.
Bazett, D.J., McCammon, N.R. Foundations of the Annacis cable-stayed bridge. Canadian Geotechnical Journal, Volume 23, No. 4, 1986, reads as follows:
Page 461: “On the south side, the stratigraphic sequence consists of glacial and interglacial sediments at least l00 m thick, which have been overridden by at least one of the major
glaciers. They are hard or very dense and form good foundation bearing materials.”
“In sharp contrast, the north bank subsurface deposits consist of approximately 65 m of postglacial sediments resting unconformally on late glacial and older glacial marine sediments.”
Figure 3 shows the north tower as built on piles bottoming in layered material described as:
“Glaciomarine and marine sediments. Stiff to hard. Grey clayey silt interbedded with stony equivalents up to 4 m thick and layers of gravelly sand.”
And
“Subaqueous glaciofluvial sediments. Very stiff to hard, dense grey silt, sandy silt, clayey silt, and silty sand grading into medium to coarse sand with thin layers of gravel at depth.”
This contrasts with the 335+ metres of sand and silt encountered at the Massey Tunnel replacement bridge site.
The first Port Mann bridge.
The first Port Mann bridge was a four-lane structure, opened in 1964.
See Golder, H. Q., Willeumier, G. C. Design of the Main Foundations of the Port Mann Bridge. Engineering Institute of Canada, 1964.
Page 1: “On the south side of the river the soil conditions were worse than on the north side and consisted of a layer of soft peat to a depth of 15 ft. overlying soft organic silts and clay silts down to about 40 ft., below this again was a compact peat underlain by clay to a depth of from 45 to50 ft. Layers of sand of varying density, with occasional layers of silt extended down to a depth of about 110 ft. and below this was gravel and sand to 120 ft. depth. From a depth of 120 ft. to 190 ft. the soil consisted of soft to firm sensitive clays and silts with occasional sand partings. Below this was compact granular material, some of which was till or till-like and some of which was probably waterlaid sands and silts which had been loaded by ice in the past.
“Whatever the actual geological history of this material, for the purposes of this paper it is referred to as “the till” or “the till-like material.” Artesian water pressure existed in some of the lower gravel layers. The foundation problem for the bridge stopped when the till-like material was reached.
“On the north side of the river the conditions were simpler consisting of a thick sand layer overlain by some compressible material and overlying a clay layer of some 60 ft. thick. Below this was the till-like material.”
Till, also known as glacial till or boulder clay, is defined (McGraw Hill Dictionary of Architecture and Construction, 2003.) as:
“An unstratified glacial deposit which consists of pockets of clay, gravel, sand, silt, and boulders; has not been subject to the sorting action of water; usually has good load-sustaining properties.”
This contrasts with the 1,100 ft. of sand and silt encountered at the Massey Tunnel replacement bridge site, from which till is notably absent.
The new Port Mann bridge.
The new, 10-lane Port Mann bridge opened in 2012.
A web note by International Bridge Technologies, Inc. has this to say:
“Foundations for the new Port Mann Bridge are generally 1.8-m (5.9-ft) steel piles or drilled shafts, supported on a firm ground till layer under the loose sand deposits at a depth below the river.”
The Pitt River bridge.
The six-lane Pitt River bridge was opened in 2009.
International Bridge Technologies, Inc. The Pitt River Bridge. 2011, reports:
Page 5: “The geotechnical conditions at the site were not favorable. As expected in and around the river, deep layers of soft soil were present. The firm till layer existed some 30m below the mudline. While it could be shown that skin friction had the ability to carry the vertical loads of the bridge, the Owner stipulated that the piles be embedded into the till.”
This contrasts with the 335+ metres of sand and silt encountered at the Massey Tunnel replacement bridge site, from which till is notably absent.
Also:
Sorenson, J. New Pitt River bridge pier pilings push envelope. Journal of Commerce, August 15, 2007.
“The construction of the pilings supporting the piers for the new Pitt River bridge will push the envelope for British Columbia bridge construction, says project manager Ross Gilmour of Peter Kiewet Sons Ltd.
“Pilings are being driven to a depth of 100 metres to support the piers for the new bridge. By comparison, the depth of piers driven for the existing bridge was 60 metres.
“The construction of the pilings supporting the piers for the new Pitt River bridge will be pushing the envelope from what is normally seen in B.C. bridge construction, says Peter Kiewet Sons Ltd. project manager Ross Gilmour. ‘For piles of this size and the depth to which they are being driven, for all intensive purposes, we are pushing the envelope of what has been done. It is not the biggest pipe or the deepest in the world but it is on the edge of the envelope,’ he says.”
“Gilmour says pilings for the new bridge are being driven to a depth of 100 metres to support the piers for the new bridge. By comparison, the depth of piers driven for the existing bridge was 60 metres. (The new Golden Ears Bridge connecting Langley to Maple Ridge has piers driven to a 90 metre depth across the larger Fraser River).
“ ‘I wasn’t here then,’ he says, when the existing Pitt River bridge was constructed, but, he guesses that technology had not advanced to drive piles deeper during the 1970s. (Over the years, there has been some noted sinking of the existing bridge structure.) Gilmour says that the area in which the Pitt River bridge sits is mainly clays and silts, which vary in depths throughout the Fraser Valley. “What it means is that there is nothing solid to get a foundation on until we get to that (100 metre) depth,” he says. Exploratory drilling has been done to ensure the foundation material exists at that level and is suitable.”
No such foundation material is evident in the 335-metre boreholes drilled on the site of the George Massey Tunnel replacement bridge.
The Golden Ears bridge.
The 6-lane Golden Ears bridge was opened in 2009.
See Yang, D., Naesgaard, E., Byrne, P. M. Soil-Structure Interaction Considerations In Seismic Design For Deep Bridge Foundations. 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August, 2008.
Page 2: “The subsoil conditions at the main river crossing consist of loose to medium dense sands, up to 35m thick on the south bank of the Fraser River and typically 20m thick within the river channel, resting upon normally consolidated to lightly over-consolidated clays and silts extending to the bottom of the deepest test holes drilled up to 120m below the ground surface.”
No such foundation material is evident in the 335-metre boreholes drilled on the site of the George Massey Tunnel replacement bridge.
The fact sheet refers to the six-lane Sutong bridge in China, opened in 2008.
See Bittner, R. B., Safaqah, O., Zhang, X., Jensen, O. J. Design and Construction of the Sutong Bridge Foundations. DFI Journal, Volume 1, No. 1, November, 2007.
Page 4: “The soils at the pylon site consist mainly of firm to stiff CL clay extending to elevation -45m followed by layers of medium to very dense fine to coarse sands and silty sands with occasional loam layers. Bedrock is located at approximately 240 m below riverbed.”
This contrasts with the 335+ metres of sand and silt encountered at the Massey Tunnel replacement bridge site.
The fact sheet refers to the four-lane Rion Antirion bridge in Greece, opened in 2004.
See Biesiadecki, G. L., Dobry, R., Leventis, G. E., Peck, R. B. Rion – Antirion Bridge Foundations: a Blend of Design and Construction Innovation. Fifth International Conference on Case Histories in Geotechnical Engineering, New York, April, 2004.
Page 4: Figure 5. Generalized Soil Profile, shows borings going to 160 metres below sea level intersecting 30-80 per cent clay layers, the balance being sand and silt. This material may be more favourable to foundation construction than the 335+ metres of sand and silt encountered at the Massey tunnel replacement bridge.
The fact sheet refers to the Jamuna River bridge, Bangladesh – four lanes plus railroad, opened 1998.
See Barr, J. M., Farooq, A., Guest, S. Foundations of the Jamuna Bridge: design and construction. ETH, Zurich, 1999.
Page 250: “The site lies in the Bengal geosyncline which is continually subsiding, leading to the deposition of sediments brought down from the upper reaches. At Sirajganj the depth to basement rock is as much as 6km.”
However:
“Soil investigations undertaken between 1986 and 1988 during Phases I and II of the Feasibility Studies approximately 1 km from the final alignment showed recent alluvial silty sands, loose at the surface becoming medium dense with gravelly layers below a depth of about 50m extending to about 100m where hard silty clay overlies a dense mica silt.”
This contrasts with the 335+ metres of sand and silt encountered at the Massey Tunnel replacement bridge site.
The fact sheet cites: “Numerous major bridges over the Mississippi River in the United States.”
Taking one at random, let us look at the I-70 bridge at St. Louis, MO.
Geotechnical Report, I-70 Mississippi River Bridge, Volume I – Engineering Report, St. Louis, Missouri – East St. Louis, Illinois.Missouri Dept. of Transportation, Job No. J6i0984, Missouri Dept. of Transportation Bridge No. A6500.
Page 6. “While the bedrock is exposed in the Illinois bluffs several miles away, none outcrops
in the project area. The bedrock surface ranges from 10 to 40 feet below the surface on the Missouri upper bank to 70 feet at the west bank, then slopes downward eastward along the project to a depth of 130 feet near Illinois Route 3.”
This bridge site is underlain by shallow bedrock.
The six-lane Biloxi Bay replacement bridge was built in the more challenging conditions of the Mississippi delta in 2007.
See Thompson, W. R., Held, L., Saye, S. Test Pile Program to Determine Axial Capacity and Pile Setup for the Biloxi Bay Bridge. DFI Journal, Vol. 3 No. 1, May 2009
Page 14: “In general, the soils at the site consist of sands and clays of Pleistocene or early Recent age. The surface deposits are typically early Recent sands and soft clays. Beneath the sands are Pleistocene deposits of very stiff to stiff clays and medium dense to dense sands.”
Borings went to 160 feet from surface (Figure 1), encountering stronger material than that underlying the George Massey Tunnel replacement bridge site.
The fact sheet states: “Thousands of hours of professional geotechnical and bridge structural engineering have been dedicated to ensuring that the new George Massey replacement bridge and its supports are appropriately designed for the conditions at the crossing site and for a major seismic event.” The ministry will doubtless have no objection to sharing the reports that this work must have generated.
The bridges cited by the ministry fact sheet are four- and six-lane structures, all founded – ultimately – on a firm bearing layer capable of supporting the weight of the bridge. If a bearing layer, capable of supporting the heavier 10-lane Massey Tunnel replacement bridge, exists within the 335-metre depth from surface exposed by the two boreholes, it is not obvious. The ministry is planning a heavier bridge than those cited on apparently weaker foundation material.
Assuming that the planned bridge can, in fact, be built, the question remains: “Can it be built for any reasonable cost?” Time will tell.
Tom Morrison
