You could blame it on glaciers.

The Port Authority of Allegheny County will need to spend an additional $3 million to $4 million on cement for its transit tunnel project to the North Shore, and the problem stems from a layer of earth laid down by retreating glaciers more than 10,000 years ago.

Contractors are using cement grout to stiffen the soil on both ends of the twin-tunnel project, on the North Shore west of PNC Park and on Stanwix Street, Downtown.

Test borings done before the work started overestimated how dense the glacial layer of earth was, 35 to 60 feet below the surface, and so contractors will now need up to four times as much cement as originally estimated to fill in the spaces in the gravel and silt mixture, said Henry Nutbrown, the authority's manager of engineering and construction.

Workers are injecting the grout to create two underground walls of fortified soil, one about 20 feet wide and 60 feet deep on the North Side, and the other about 20 feet by 40 feet on Stanwix Street, Mr. Nutbrown said.

The walls will form firm vertical slabs for a $10 million tunnel boring machine to dig through at the beginnings and ends of the tunnels, he said.

The extra money needed for the cement and some other additional project costs will come from a $9 million contingency fund established for this leg of the work, and so should not raise the project's total price tag of $435 million, he said.

Mr. Nutbrown doesn't expect the faulty test borings, which were overseen by general contractor DMJM Harris, to affect the critical underwater portions of the tunnels.

The tunnels will burrow under the Allegheny River about 20 feet beneath the riverbed, and the borings taken from the river surface showed sedimentary rock at that level.

Test borings that show rock are generally more reliable than those that show a gravel-soil mixture, Mr. Nutbrown said.

There may be ways to lower the additional cement costs, he added.

Under its contract, the Port Authority will be able to renegotiate the price of most of the new cement it will need. However, because of rising demand for cement in China and other fast-growing nations, and higher fuel costs to produce cement, it's hard to know how much of a savings that might produce.

A more reliable way of economizing, Mr. Nutbrown said, will be to use fly ash -- the residue left after coal is burned at power plants -- to extend the volume of the cement grout.

The Port Authority is testing two different mixtures of cement and fly ash right now, he said, and the new formulation could shave up to $1 million off the bill.

Because some of the soil borings were inaccurate, the Port Authority is planning to talk to DMJM Harris about sharing in the additional costs, Mr. Nutbrown said.

Paul Gennaro, a DMJM Harris spokesman in New York, said the contractor "is conducting further analysis" of the test boring results and would have no further comment until that work is done.

The borings were carried out by Geomechanics Inc. of Elizabeth. Javaid Alvi, president of the firm, said yesterday that test borings can't guarantee the consistency of every square foot of an underground layer of soil because they are done at intervals.

He said he expected to participate in discussions about the work site with other contractors soon, but did not know anything about the possibility of helping to share the additional costs.

The authority also has run into about $300,000 in extra expenses for utility line relocation work so far, he said. In one case, drawings did not show a steel beam near a highway overpass on the North Shore, and in another case, several utility lines beneath Stanwix Street were at different depths than anticipated.

The Port Authority board will be asked to approve an initial allocation of $2.8 million from the project's contingency fund at its meeting tomorrow.

The 22-foot-diameter tunnel boring machine made by Herrenknecht AG in southern Germany is being shipped across the Atlantic right now.

It is expected to arrive in Pittsburgh the week of Aug. 6, and then will be assembled on the North Shore to begin drilling the first tunnel sometime in late September or early October.