Back Bay on the rise; High-rise construction fills the sky in fashionable Boston community

About the color of cocoa powder before you add milk, a bulky panel of precast concrete is hoisted slowly off its berth on a flatbed trailer. On slender steel cables it rises higher and higher into the afternoon fog until, finally, it reaches the 38th floor. Hands reach out to guide it into place on the western face of the Marriott Hotel, like a missing piece of a huge jigsaw puzzle.

Thus does the Marriott, the next-to-last major section of Copley Place, take shape. Only a glass-enclosed, climate-controlled pedestrian bridge over the street linking Copley Place with the neighboring Prudential Center remains to be built.

It's all an impressive feat of engineering, but hardly anyone stops to watch anymore. The Marriott is 70 percent finished, and for most Bostonians the novelty of new construction in their midst has long since worn off. They wait now for the glittering grand opening of Copley Place next February.

If Copley Place were alone, it would be enough to change the face of this city. At 91/2 acres along the southern edge of Back Bay, it will feature the Marriott and one other high-rise hotel, the Westin, a shopping center, office complex, 100 apartment units, and three parking garages.

But Copley Place isn't alone. It is only one of five major new construction projects rising in Back Bay, with still more on the drawing boards. Most of these are - or will be - on the main thoroughfare, Boylston Street. And for some here, that is enough to raise fears that their once-placid neighborhood of mostly small businesses and residences is steadily becoming another Manhattan.

''We don't think Copley Place is too much,'' says Elliott Laffer, president of the Neighborhood Association of the Back Bay, believed to be the largest residents group in the city. He pauses to spread butter on a breakfast bagel and adds: ''There is subsequent development, though, that may be. We may wake up one day, five years from now, and say, 'What . . . did we do?' We may not recognize the Back Bay.''

The neighborhood is bounded east and west by the Public Garden and the Fenway and north and south by the Charles River and Huntington Avenue. Visitors to Boston know it primarily for its large concentration of quality hotels and ethnic restaurants, its chic Newbury Street shops, and for such landmarks as the John Hancock tower, Public Library, Symphony Hall, and the Prudential and Christian Science centers.

Into that space, however, are also being wedged the massive $100 million state transportation building, the 280-room Four Seasons Hotel, and 399 Boylston Street and One Exeter Place, both large mixed office and retail projects.

That doesn't leave much in the way of open space, which an umbrella group known as the Back Bay Federation particularly laments. In its statement of development goals issued earlier this year, the federation maintains that ''available open space should be extended and enhanced by . . . broad sidewalks and landscaped pedestrian walkways.'' Residential sections, it adds, ''must be protected from further commercial encroachment.''

Back Bay isn't the only section of Boston that is being built up with the blessing of development-minded Mayor Kevin H. White, who calls this a ''world-class city.'' But the neighborhood has special concerns to be taken into consideration that most of the other sections don't.

Once a tidal estuary, Back Bay was landfilled in the 1800s. Pilings must be driven down to bedrock before anything can be built here. Early buildings rest atop wooden pilings that, as long as they remain in contact with wet soil, should remain solid. But in some cases a lowered water table under the neighborhood has allowed air to reach the pilings, causing rot. Parts of some historically important buildings have cracked or sunk.

Constant monitoring of the water table now is standard procedure in new developments. When too much water is displaced by new construction, more must be pumped back onto the affected site.

Gordon Hyslop, manager of development for the Chicago-based Urban Investment & Development Company, the driving force behind Copley Place, says that the land itself was basically worthless. ''We had to create a whole new piece of real estate that was buildable,'' he adds. ''Just putting in foundations was a nightmare.''

Every time a new piling was driven, Mr. Hyslop says, the one beside it would ''bounce.''

There are some who also worry about the extra burden that new construction may be placing on the city's aging water-delivery and sewer systems. And the neighborhood has experienced some significant electrical blackouts in recent years.

Other concerns: maintaining compatibility with the current mix of building sizes and architectural styles.

Two decades ago, no building in the city was more than 26 stories high. Now, the John Hancock and Prudential towers are both well in excess of 50 stories. The two hotels in Copley Place - Marriott and Westin - are 39 and 36 stories, respectively. The Marriott will be the tallest hotel in Boston.

Still another major project is the expansion of Hynes Auditorium, the city's principal - but undersized - convention facility. In keeping with the city's bid to become a major convention center, plans call for the floor space of ''the Hynes'' to be more than doubled by adding an extra story and by building out over what is now a frontal access road.

And there's more. New England Life Insurance Company wants to put up a $200 million office complex with twin 23-story towers. Another parcel, known as Hadassah Way, is the subject of development proposals for mixed housing and commercial use.

The New England Life proposal, particularly, is a matter of raging controversy.

''The sheer bulk . . . is out of proportion with the business and residential neighborhood and would dominate the area to an unacceptable degree,'' a neighborhood citizens advisory committee complained in a report to the city last August. It cited such potentially deleterious effects as increased traffic, additional shadows over a significant area, added pressure on parking, and the unwillingness to face ''another five years of construction.''

But for all those who fear a possible Manhattan-izing of the Back Bay, there are others who are just as convinced that it will never happen.

One is John King, editor of the Boston Ledger, a free-circulation weekly that focuses on Back Bay.

''I don't buy it,'' he says. ''I think that most of the concerns of the neighborhood group are valid, but overstated. I don't think the development that is taking place now is going to drastically affect Back Bay.''

Adds Kevin Cartwright, executive director of the Back Bay Association, which represents 250 commercial establishments in the neighborhood: ''The view of my organization is that those developments are only strengthening the Back Bay. They're going to make this area only that much more viable.''

Boylston Street, Ms. Cartwright adds, ''has always been very unspecial,'' but now ''it's really going to come into its own.''

Mitchell Fishman, senior project coordinator for the Boston Redevelopment Authority (BRA), speaks of ''the tight lid that we try to keep on the Back Bay.''

The projects approved by the BRA in the past three or four years, he says, ''show a particular amount of restraint. The developers also are sensitive.''

With a few minor exceptions, Mr. Fishman says, the BRA supports the Back Bay Federation's goals and guidelines for development and has said so in writing.

Besides, he adds, the neighborhood ''doesn't have what I'd say are any more 'soft' areas left'' that could easily lend themselves to redevelopment.

As for Copley Place, it's hard to quarrel with new development after a stroll through the Westin Hotel. The gleaming glass pedestrian entrance, pungent fragrance of hundreds of potted marigolds, and muted roar of its indoor waterfall play on the senses. And that's before arriving at the mezzanine lobby, which features plush furnishings, custom-designed carpeting imported from England, Italian marble, and elegant brass columns.

Open since August, it is doing a brisk business and has convention bookings as far in advance as 1995. Westin wanted to build alone on the site - a former interchange on the Massachusetts Turnpike - 10 years ago. But it settled for a place in the $500 million project.

Elliott Laffer himself says it's better to have Copley Place than not have it. And he expects to be present at the ribbon-cutting next February.

''To me,'' he says, ''open space is a park, not a highway. Rubble-strewn vacant lots are open space, too.''