UNDERGROUND CONSTRUCTION

GROUND SETTLEMENT
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GROUND SETTLEMENT DUE TO EXCAVATION
Ground settlement is a major consequence of any excavation work and its control necessarily becomes one
of the most important objectives of the support system to be adopted during excavation. Any deep excavation
whatever the support system- inevitably leads to some movement of the surrounding ground and any
structure, lying within the zone of influence, is likely to be affected.
EXCAVATION IN SAND :
The settlement due to excavation does not extend beyond a distance equal to the depth of the cut in sand. In
properly braced large cuts, the settlement of the adjacent ground surface does not usually exceed about 0.5
percent of the depth of the cut.
When large cuts are made in sand below the water table, it is advisable to lower the water table before
construction. The excavation can be carried out with no more subsidence than would be associated with a
similar cut in moist sand. If ground water has not been brought under complete control, sands may flow into
the excavation and give rise to large erratic and damaging settlements in the immediate surrounding area.
The location and magnitude of such settlements unfortunately can not be predicted.
EXCAVATION IN CLAY
When large cuts are excavated in soft clay, the weight of the soil surrounding the edge of the excavation acts
like a surcharge on the soil at the level of the bottom of the cut and develops lateral forces in the sub-soil. If
the depth of the cut becomes so great that the bearing capacity of the soil beneath the sides is approached,
large movements are inevitable , irrespective of the care with which the sides of the cut may be braced.
If the bottom of the excavations is underlain at a shallow depth by a firm stratum, the tendency for such a
bearing capacity failure is greatly reduced. The movement can also be decreased by driving piles around the
boundary of the cut until they are firmly embedded in the underlying stiff stratum. These piles are braced by
struts as excavation progresses.
Field measurements have demonstrated that the volume of soft clay that moves inward at the sides of a cut
plus that which moves upward beneath its base is represented by an equivalent volume of subsidence
surrounding the excavation. Therefore any measures that can be taken to prevent lateral movements or heave
will be reflected in a decrease in settlement. Hence it is advisable to keep such cuts braced tightly at all times
and to insert braces as soon as possible as the excavation progresses.
A variety of factors influence the magnitude and pattern of ground movement adjacent to deep excavations
in soft clay. Some of the important factors are
1. Sub soil profile and engineering properties of different strata.
2. Shape and depth of excavation
3. Type and stiffness of support system
4. Sequence and method of construction
5. Time period of construction
6. Surcharge and surrounding structures, if any.
SETTLEMENT DUE TO LOWERING THE WATER TABLE
Whenever the water table is lowered the effective weight of the material between the original and final
position of the water table is increased from that of submerged to that of moist or saturated soil. The increase
of the effective overburden pressure causes additional compression.
EFFECT OF LOWERING THE WATER TABLE IN SANDY STRATA
Pumping from sand increases the effective pressure but the corresponding settlement is usually small unless
the sand is very loose. However, it the water table is raised and lowered periodically, the settlement may
become important, because every temporary increase of effective pressure increases the settlement by a
certain amount. The magnitude of the increment of settlement decreases with increasing number of cycles
and approaches zero, but the final settlement is many times greater than the settlement produced by the first
cycle.
UNDERGROUND CONSTRUCTION
GROUND SETTLEMENT
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In some instances, large subsidence have occurred near drained excavations in sand because springs have
been permitted to form at or near the bottom of the excavations. Water flowing into springs from the
surrounding sand may carry the material into the excavation grain by grain and a tunnel may be formed.
When the tunnel becomes sufficiently large, its roof may collapse and the ground surface above it subside.
The subsidence may take the form of a sink hole that may be located at a considerable distance from the edge
of the excavation.
EFFECT OF LOWERING THE WATER TABLE IN CLAY STRATA
Lowering the water table within or above a stratum of clay ultimately increases the inter granular pressure
precisely as in sand. However, the process of consolidation initiated by lowering the water table may require
considerable time for the development of the ultimate amount of settlement. The time depends primarily on
the permeability of the material. The settlement of more impervious clays may not be excessive during the
relatively short construction period. If the compressible layers are underlain by a layer of sand in which the
water table is lowered, the pierometric levels may be decreased for a very large distance from the excavation.
The corresponding settlements of the overlying materials may extend for from the site of construction and
may lead to serious structural defects.
There are several methods of analyzing cantilever and anchored sheet-pile walls. Two of the early methods
were (a) the free earth support and the (b) fixed earth support along with the simplified assumption of
active (from filled side) and passive pressure on the free side below the dredge line. The design was based
primarily on taking moments about the anchor rod, increasing the depth of embedment D until F
h
was
satisfied, and then computing the resulting bending moments in the piling. A safety factor was incorporated
by using a reduce K
P
for passive pressure or by increasing the embedment depth D some arbitrary amount
such as 20 or 30 percent. Two of the simplifications could result in errors:
Cantilever sheet-pile walls were analyzed similarly to anchored walls, except the soil pressure profiles were
slightly different and moments were usually taken about the base since there was no anchor rod.