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Building Design and Construction Handbook - Merritt - Ventech!

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6.100 SECTION SIX<br />

TABLE 6.16 General Factors That Control the Stability of the Excavation Slopes<br />

<strong>Construction</strong><br />

activity Objectives Comments<br />

Dewatering In order to prevent boiling,<br />

softening, or heave of the<br />

excavation bottom, reduce<br />

lateral pressures on sheeting,<br />

reduce seepage pressures on<br />

face of open cut, <strong>and</strong><br />

eliminate piping of fines<br />

through sheeting.<br />

Excavation<br />

<strong>and</strong> grading<br />

(also see<br />

Art. 6.10)<br />

Excavation<br />

wall<br />

construction<br />

Utility trenches, basement<br />

excavations, <strong>and</strong> site<br />

grading.<br />

To support vertical excavation<br />

walls, <strong>and</strong> to stabilize<br />

trenching in limited space.<br />

Blasting To remove or to facilitate the<br />

removal of rock in the<br />

excavation.<br />

Anchor or<br />

strut<br />

installation<br />

To obtain support system<br />

stiffness <strong>and</strong> interaction.<br />

Investigate soil compressibility <strong>and</strong><br />

effect of dewatering on settlement of<br />

nearby structures; consider<br />

recharging or slurry wall cutoff.<br />

Examine for presence of lower<br />

aquifer <strong>and</strong> need to dewater. Install<br />

piezometers if needed. Consider<br />

effects of dewatering in cavity-laden<br />

limestone. Dewater in advance of<br />

excavation.<br />

Analyze safe slopes or bracing<br />

requirements, <strong>and</strong> effects of stress<br />

reduction on overconsolidated, soft,<br />

or swelling soils <strong>and</strong> shales.<br />

Consider horizontal <strong>and</strong> vertical<br />

movements in adjacent areas due to<br />

excavation <strong>and</strong> effect on nearby<br />

structures. Keep equipment <strong>and</strong><br />

stockpiles a safe distance from the<br />

top of the excavation.<br />

See Art. 6.7 for retaining wall design.<br />

Reduce earth movements <strong>and</strong> bracing<br />

stresses, where necessary, by<br />

installing lagging on front flange of<br />

soldier pile. Consider effect of<br />

vibrations due to driving sheet piles<br />

or soldier piles. Consider dewatering<br />

requirements as well as wall stability<br />

in calculating sheeting depth.<br />

Movement monitoring may be<br />

warranted.<br />

Consider the effect of vibrations on<br />

settlement or damage to adjacent<br />

areas. <strong>Design</strong> <strong>and</strong> monitor or require<br />

the contractor to design <strong>and</strong> monitor<br />

blasting in critical areas, <strong>and</strong> require<br />

a pre-construction survey of nearby<br />

structures.<br />

Major excavations require careful<br />

installation <strong>and</strong> monitoring, e.g., case<br />

anchor holes in collapsible soil,<br />

measure stress in ties <strong>and</strong> struts, etc.<br />

Sources: NAVFAC DM-7.2, 1982, Clough <strong>and</strong> Davidson 1977, <strong>and</strong> Departments of the Army <strong>and</strong> the<br />

Air Force 1979. G. W. Clough <strong>and</strong> R. R. Davidson, ‘‘Effects of <strong>Construction</strong> on Geotechnical Performance,’’<br />

<strong>and</strong> Department of the Army <strong>and</strong> the Air Force, ‘‘Soils <strong>and</strong> Geology, Procedures for Foundation <strong>Design</strong>.’’

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