Share this story
Close X
Switch to Desktop Site

Soil mechanics; Ask them before you tinker with the terrain

Soil mechanics and foundation engineering sound dull as dirt.

But there's a lot more to this science than shoving around mounds of earth and driving piles, as the people of Rissa, Norway, found out. What they learned the hair-raising way is that many problems this relatively new branch of civil engineering deals with are awesome, life-or-death matters.

About these ads

In 1978 a farmer in Rissa excavated enough soil to lay the foundation for a new wing on his barn. He piled the earth on the shore of a nearby lake. This slight additional weight caused a strip of shoreline about 300 feet long to slide suddenly into the lake and disappear.

That was the beginning of a massive quick-clay landslide.

The chief characteristic of this ultrasensitive marine clay is that in an instant its consistency can change from solid to liquid. In its undisturbed state, it is strong. If overloaded, it collapses and flows away, as it did that day.

Without warning, another, much bigger chunk of farmland broke off and dropped into the drink. A photographer, who happened to be on the scene with his movie camera, rushed to a point of land on the lakeshore. What he captured is the first movie of a quick-clay landslide in progress.

With each successive slippage, clay that for centuries had supported farms, barns, houses, roads, and people liquefied, racing down the now steep scarp until the slide area extended over seven acres.

Then the real disaster began: An enormous wall of clay sank down and started moving toward the lake, carrying the schoolhouse with it. The photographer ran for his life as a wave of earth came rolling behind him. People on the farm behind the slide escaped moments before their whole farm slid out.

A second photographer up on the hillside filmed the final shots of the catastrophe, as waves, caused by the landslide, raced across the lake, damaging a village nearly five miles away. In 40 minutes, 81 acres of farmland vanished into the lake. One person was killed.

About these ads

Quick-clay landslides are a serious problem in Norway. About 5,000 square miles of the country are overlain with deposits of this treacherous material.

When such natural disasters occur, the ''firemen'' who rush to the scene are geotechnical engineers. Their art is to apply the science of soil mechanics, rock mechanics, and other geosciences to the design and construction of buildings, bridges, dams, tunnels, highways, etc.

What they have learned about quick clay, which was originally under the ocean , is that when its natural salt content drops below a certain level and the clay is overloaded, it collapses.

Unknown to Rissa's inhabitants, over the centuries an upward flow of fresh ground water had leached salt out of the salt water trapped in the pores of the quick clay. The slight addition of the farmer's excavated soil heaped on the shore was all it took to trigger the catastrophic failure. When salt is restored to it, quick clay regains its strength.

With this knowledge, the zone around the Rissa slide was re-stabilized in less than a year. Families received compensation, and the incident resulted in a nationwide mapping of quick-clay deposits.

Such is the drama of soil mechanics, the science that is adding to mankind's dominion over the earth in all its strange and varied manifestations.

Until 1925, when soil mechanics got its start here in Cambridge, foundation engineering was based largely on experience, judgment, a knowledge of building materials, and guesswork. When structures were erected, builders hoped for the best. How much they might settle, nobody knew.

Nothing illustrates this better than the central domed building of the Massachusetts Institute of Technology, focal point of the school's central courtyard facing the Charles River. In 1916 MIT moved from Boston's Back Bay to a new campus on the Cambridge side of the river. In the process, it became a laboratory for development of the new technology of soil mechanics.

John R. Freeman, an illustrious civil engineer and insurance executive, played a prominent part in the planning and preliminary design of the new buildings. His studies revealed that nearby buildings had settled 14 inches in 15 years. Freeman correctly identified the cause of the settling: As buildings were built and landfill added, their weight squeezed water out of the underlying clay - Boston blue clay, to be precise. There was no reason to believe the proposed MIT buildings would not settle just as much, he warned. He proposed foundation designs to offset such subsidence, but they were rejected.

Large parts of Greater Boston are underlain by a deep layer of Boston blue clay, a marine sediment entirely different from quick clay. In its dry state the clay is very hard. In its normal, wet condition, it is fairly soft but does not share the quick-change characteristic of quick clay. Because of all the academic studies made of it in this capital of soil mechanics, Boston blue clay is world famous.

Engineers expected settlement to be less than a quarter of an inch. But within a year of opening day in 1916, the heaviest building of the group, the great domed, columned central building, settled more than 2 inches and an adjacent building 1.5 inches.

Subsidence continued year after year. This renowned center of technology could do nothing but watch it happen. Soil mechanics had not yet been born. There were stories that MIT students would someday enter their buildings on the second floor.

Most of the settlement occurred in the first 10 or 15 years. Today, though the main building has settled slightly more than 10 inches, the rate of settlement is nearly zero. Some cracks in floors and plaster had to be repaired , but no significant structural damage occurred.

Meanwhile at Robert College in Istanbul, Karl Terzaghi, an Austrian professor of civil enginering, was developing procedures for testing how various sands, gravels, and clays behave under the load of structures, and how to predict their bearing capacity. In 1925 he brought out his now famous - to geotechnicians - ''Erdbaumechanik'' (''Soil Mechanics'').

MIT at once invited him to lecture here and to size up the subsidence problems of its main buildings. With Freeman's support, Terzaghi's new ideas were accepted. He soon became universally recognized as the father of soil mechanics.

HarlP. Aldrich Jr., president and senior principal of Haley & Aldrich Inc., geotechnical engineers in Cambridge, is another of soil mechanics' most prominent pioneers. In 1951 he became the third person in MIT's history to receive a doctorate in the field. A former professor at his alma mater, he is an expert in the design and construction of foundations for buildings, earth dams, and waterfront structures. His firm has extensive experience on important projects in the United States and abroad.

Dr. Aldrich points out that one of the earliest soil mechanics problems in Boston showed up while Terzaghi was still in Cambridge: The front steps of the Boston Public Library began cracking. This aroused great concern for the foundations of this architecturally important public building.

Tests showed that the wooden piles supporting the library were deteriorating. Water draining into a broken sewer main had lowered the ground water table at that point to below the tops of the piles. Air was reaching them, and rot had set in.

That incident, Dr. Aldrich says, ''created an awareness on the part of the engineering community and public officials in the Boston area of the importance of preserving ground water levels, particularly in Back Bay.'' (Boston's Back Bay is a land-filled portion of the city which was once a tidal estuary.) Most of the earliest buildings in Back Bay rest on wooden piles.

Since then, extreme precautions have been taken, especially during periods of construction, to protect all buildings in the area from the danger of lowered ground water levels. Constant monitoring of water levels under major wooden pile-supported buildings in Back Bay is standard practice.

Soil mechanics sprang into being just in time to facilitate immense construction projects such as airfields during the 1940s and the vast network of federal Interstate highways and bridges built during the '50s and '60s.

Geotechnical engineering has evolved in response to changing public needs, especially for energy-related industries. First the demand was for hydropower; then for foundations under fuel-oil storage tanks; next for liquefied natural gas tanks and nuclear plants; now for offshore drilling platforms in very deep water.

Marvels of engineering were achieved in construction of tunnels before 1925. But Dr. Aldrich points out that they often involved great loss of life, in part because of ignorance of rock and soil characteristics. Now much tunneling and underground construction are going on in cities for high buildings, rapid-transit lines, and water supply systems. But all are safer for the builders, he says, because of the growing knowledge of soil and how it reacts to loading.

Charles Cushing Ladd, a professor of civil engineering at MIT, combines teaching with private practice as an independent geophysical consultant.

A specialist in soft-ground construction, he is currently working on several poor-site projects: a cement plant on Alabama clay; a sewage-disposal plant in Bombay's tidal mud flats; a new international airport Hong Kong is pushing out into the sea; and an extraordinarily daring project in James Bay, Canada -- building earthwork dams on a 60-foot layer of quick clay!

Dr. Ladd likes being involved in a project from initial planning and design through construction and performance. ''It's like the weatherman. You make predictions of what is going to happen. Then you observe what actually happens. It keeps you humble,'' he says, grinning.

In his running battle with the elements, he says, what he enjoys most is saving clients millions of dollars they would otherwise have had to spend sinking piles. The cheaper alternative is almost always to use the soil that is already there, however soft.

This can be done by placing earth or some other weight on it to squeeze out the water and make it stronger and less compressible. The trick is to load on the maximum weight possible without the soil's collapsing or oozing out sideways.

Dr. Ladd doesn't mind in the least living on the brink of disaster. In fact, the most most fun he ever had on a project, he says, was at the Portsmouth, N.H. , interchange of US Route I-95, where five bridges were to be built, each having ramps up embankments.

He was called in as a consultant by Haley & Aldrich in 1967 when it was found that the whole area was underlain by a 30-foot layer of quick clay. ''That clay was awful, but boy, it was exciting!'' he recalls.

This was his chance to put into practice research he had done in the laboratory, MIT's elaborate torture chamber for all kinds of soils. ''For the first time in the US,'' he says, ''we built a 500-foot-long test embankment on top of the clay and purposely took it to failure to see what warning signs there would be.''

Sand drains had been devised in the US as a means of speeding up the dewatering of a site to stabilize ordinary soil. But ramming a steel casing in the ground, filling it with sand, and then yanking it out would turn this sensitive clay into soup. So Dr. Ladd used a Dutch-invented method for gently inserting these columns of sand into the clay.

''It worked magnificently. Five bridges and 10 miles of pavement - and if you drive over them, you won't see a ripple in the road. Nobody in this country had ever built anything this big on a soil deposit that was so poor.'' This project won Haley & Aldrich an honor award in the Engineering Excellence Competition of the American Consulting Engineers Council.

Every part of the US has soil peculiarities. Florida has its limestone sinkholes. California its mud slides. In parts of Texas the problem in building new houses on hardened clay is that sometimes the water which such habitation adds will cause clay to swell, lift a house up, and crack it apart.

Soil mechanics gets more sophisticated all the time. In the '60s one of Professor Ladd's doctoral students built a high-vacuum device that correctly predicted that the soil on the moon would be strong enough to support a landing craft.

Today, X-rays, laser beams, computer technology, and new tools for getting soil samples (from the ground as well as from the ocean bed) are giving engineers improved capability to predict whether soil will fail and how much it will settle.

''With these new testing devices,'' Dr. Ladd says, ''in five to 10 years we will be able to predict routinely the amount of settlement in the field from small loads right up to failure. This is going to revolutionize how offshore exploration is done as well as make things a lot easier on land.''

Follow Stories Like This
Get the Monitor stories you care about delivered to your inbox.