based on judgment and experience. Borings should always be extended through unsuitable foundation
bearing material, such as uncompacted fill, peat, soft clays and organic soil, and loose sands,
and into dense soil or hard rock of adequate bearing capacity. In a general sense, the depth of subsurface exploration will depend on the size and loading of the proposed foundation, the sensitivity
of the proposed structure to settlements, and the stiffness and coefficient of compressibility of the
strata that will underlie the foundation.
In terms of additional general rules, Hvorslev (1949) states:
The borings should be extended to strata of adequate bearing capacity and should penetrate all
deposits which are unsuitable for foundation purposes, such as unconsolidated fill, peat, organic silt, and very soft and compressible clay. The soft strata should be penetrated even when they are covered with a surface layer of higher bearing capacity.
When structures are to be founded on clay and other materials with adequate strength to support the
structure but subject to considerable consolidation by an increase in the load, the borings should penetrate the compressible strata or be extended to such a depth that the stress increase for still deeper strata is reduced to values so small that the corresponding consolidation of these strata will not materially influence the settlement of the proposed structure.
Except in cases of very heavy loads or when seepage or other considerations are governing, the borings may be stopped when rock is encountered or after a short penetration into strata of exceptional bearing capacity and stiffness, provided it is known from explorations in the vicinity or the general stratigraphy of the area that these strata have adequate thickness or are underlain by still stronger formations.
When these conditions are not fulfilled, some of the borings must be extended until it has been
established that the stiff strata have adequate thickness irrespective of the character of the underlying
material.
When the structure is to be founded on rock, it must be verified that bedrock and not boulders have
been encountered, and it is advisable to extend one or more borings from 10 to 20 ft (3 to 6 m) into sound rock in order to determine the extent and character of the weathered zone of the rock.
For localized structures, such as commercial or industrial buildings, it is common practice to
carry explorations to a depth beneath the loaded area of 1.5 to 2.0 times the least dimension of the
building (Lowe and Zaccheo, 1975). Table 2.8 presents additional guidelines for different types of
geotechnical and foundation projects.
Another commonly used rule of thumb is that for isolated square footings, the depth of subsurface
exploration should be two times the width of the footing. For isolated strip footings, the depth of subsurface exploration should be four times the width of the footing. These recommendations are based on the knowledge that the pressure of surface loads dissipates with depth. Thus, at a certain depth, the effect of the surface load is very low. For example, a common guideline is to perform subsurface exploration to a depth where the increase in vertical pressure from the foundation is less than 10 percent of the applied pressure from the foundation. There could be problems with this approach because as will be discussed in Chap. 7, there could be settlement of the structure that is independent of its
weight or depth of influence. Settlement due to secondary influences, such as collapsible soil, is often
unrelated to the weight of the structure. Especially when geologic conditions are not well established,
it is always desirable to extend at least one boring into bedrock to guard against the possibility of a
deeply buried soil strata having poor support characteristics.
For some projects, there may be specifications on the required depth of borings. For example, the
Standard Specifications for Highway Bridges (AASHTO, 1996) states:
When substructure units will be supported on deep foundations, the depth of subsurface exploration
shall extend a minimum of 20 ft (6 m) below the anticipated pile or shaft tip elevation. Where pile or shaft groups will be used, the subsurface exploration shall extend at least two times the maximum pile group dimension below the anticipated tip elevation, unless the foundation will be end bearing on or in rock. For piles bearing on rock, a minimum of 10 ft (3 m) of rock core shall be obtained at each exploration location to insure the exploration has not been terminated on a boulder.
At the completion of each boring, it should immediately be backfilled with on-site soil and compacted by using the drill rig equipment. In certain cases, the holes may need to be filled with a
cement slurry or grout. For example, if the borehole is to be converted to an inclinometer (slope monitoring device), then it should be filled with weak cement slurry.
Likewise, if the hole is to be converted
to a piezometer (pore water pressure monitoring device), then special backfill materials, such
as a bentonite seal, will be required. It may also be necessary to seal the hole with grout or bentonite
if there is the possibility of water movement from one stratum to another. For example, holes may
need to be filled with cement or grout if they are excavated at the proposed locations of dams, levees,
or reservoirs.
About Unknown
0 comments:
Post a Comment