SEDIMENTS AND GEOGRAPHY

4.2.1 Location, Location

The fact that many towns and cities were founded near rivers or seacoasts emphasizes the relevance of sedimentary soils in engineering. The reason of course is accessibility for transportation and trade. Sediments in alluvial and coastal areas often are recently deposited and relatively soft, having settled to an equilib-rium density under their own weight. This means that if an additional weight such as a foundation is added, it will compress the soil and settle. The amount of settlement that can be allowed is the most common criterion used in foundation design. The soil density and stiffness tend to increase with depth, a factor that is utilized when founding structures on piles.

4.2.2 Settlement

Sediments that are in equilibrium with their own weight are said to be normally consolidated. Venice is an example of a city that is situated on a soil that still is normally consolidating, which explains why Venice is sinking. Both the amount and rate of settlement are important considerations for any design.

The position of a groundwater table also affects settlement because of the buoyant reduction in soil weight—the lower the weight, the less a soil will tend to con-solidate under its own weight. For example, consider the weight of a bucket of sand both in and out of the water. According to a principle first put forth by Archimedes, the weight of the submerged object is reduced by the weight of the water that it displaces. This difference is readily calculated by knowing the density of the soil and of the water.

Mechanism Sediment name (sorting) Characteristics Table 4.1 Common sedimentary soils and their transporting agents

Gravity Talus or scree (poor) Rockfalls: steep cone-shaped deposits of loose rock fragments at the base of cliffs or mountains

Landslides (poor) Downslope sliding of a mass of soil.

Usually caused by erosion and removal of soil at the toe and triggered by wet conditions

Water and gravity

Slope wash or colluvium (moderate)

Loose mix of soils collecting near the base of gentle hillslopes

Water Alluvium (excellent) Wide variety of soil materials from boulders to clay, deposited by streams and rivers

Beach (excellent) Loose gravel or sand deposits reworked by wave action

Offshore deposits (moderate)

Soft silts and clay deposited in relatively quiet water

Ice Glacial drift

(variable)

Wide variety of deposits from glacial melting, including glacial till and glacio-fluvial deposits

Till (poor) Heterogeneous mix of all sizes of soil materials, sometimes firmly compacted under the weight of the ice

Water and ice

Glacio-fluvial deposit (good)

Sand and gravel deposited from glacial meltwater. Deposits extend down river valleys far from the glacial source Wind Dune sand (excellent) Loose fine-grained sand

Loess (good) Thick deposits of silt that may collapse if wet, transitional to thinner deposits of silty clay with increasing distance from a source area

Water and plants

Peat (poor) Fibrous plant material. Generally considered the worst soil for engineering purposes but good for potting plants

Man Fill (poor) The most random and potentially

dangerous of all sediments. We know who is to blame

Engineered fill (selected)

Selected soils that have been compacted under strict guidelines for use as foundation soils, earth embankments, and lagoon linings

Buoyancy therefore is an important consideration in geotechnical engineering, and simply lowering the groundwater table, for example by pumping from wells, will increase the weight of the emerging soil by approximately a factor of two.

The additional weight leads to additional compression of the underlying soil.

This is dramatically illustrated in Mexico City, where pumping from wells has resulted in settlement that in some areas is measured in tens of meters.

Another option in areas of soft soils is to support structures on piles that extend down to underlying hard strata or bedrock. In this case settlement of adjacent unsupported areas can create the illusion that buildings are rising up out of the ground. In places like Mexico City a common countermeasure is to support a structure on hydraulic jacks that can be lowered to keep pace with the settlement.

4.2.3 Medieval Construction

Many medieval structures have survived not because the builders were better engineers, but because manpower was many times slower than machines, and centuries often were required to complete a castle or cathedral. Typically, the foundation was simply large stones laid flat, and structures settled as they were being built. Corrections then were made during building to compensate for uneven settlement and tilting.

For example, courses of masonry in the famous Leaning Tower indicate that as tilting occurred during construction and was compensated by increasing the height on the low side. Tilting then pursued in a different direction because, as soil under the low side was compressed, it became stronger and less compressible.

Had the corrections not been made, the increasingly eccentric loading would have caused the tower to topple, and after each correction was made a new cycle of tilting proceeded in a different direction. It was only after the tower was completed that there was no further correction and an increasing danger of falling over. The latest correction devised by English and Italian geotechnical engineers involved adding a temporary surcharge load to the high side, which arrested further tilting, and then augering soil out from under the high side to bring it back to a safer position. What makes this tower so unique is because it survived;

most medieval towers collapsed.

4.2.4 Surcharging

Fourteenth century construction scheduling does not fit a modern mold, and a simple procedure that can be used to reduce or control settlement is to preload the soil with a weight and pressure that are equal to that of a proposed structure, allow time for the soil to compress, and then remove the weight and build the structure. Extra weight may be used to hurry things along, and normally does no harm so long as it does not punch down into the soil sufficiently to cause a shear failure. This method of construction is called surcharging.

Surcharge loads usually are piles of soil that can be moved to successive building sites in a complex. Another option is to store all building materials on a site footprint slab; then as the structure is built the weight stays the same. Surcharging is common where a new highway must cross soft, compressible terrain. Surcharging can save money but requires time, and owners see time as money. It therefore is important to be able to predict the time that will be required and monitor the progress of settlement. A method described later in this book shows how to predict future settlement from measurements made over a period of time.