Soil strength
F all the inherent dangers in
construction work, one of the biggest, and one of the most prevent- able. is the trench or excavation wall cave-in. Each year scores of workers are killed or seriously injured throughout the world by the failure of someone to estimate properly the stability of excavation walls.
The key to trench and excavation safety is soil testing. Testing can aid in determining how deep a trench or open cut may be made in a given soil without support. But to the engineer in the field, often miles from adequate testing facilities, these words and prin- ciples are not often easy to follow,
The normal test procedure in this case (the unconfined compression test of soil) takes too long and delays operations, even when test apparatus is available in the field. Decisions about trench depth, trench wall sloping or trench
by
tests in
in excavation
Charles W. Borden
Engineering Vice-Chairman, Construction Section, U. S. National Safety Council.
bracing have to be made on the spot so that safety measures, if required, can be planned immediately for the efficiency and economy of the excavation opera- tion
A recent development in field testing allows field engineers to maintain com plete safety in trench excavation with- out sacrificing the time to conduct the tests with laboratory equipment. This development is a four ounce instrument
ROOF GOES ON FIRST
NEW construction method for a 17-storey block of flats was announced in London recently. The roof will be built. after the comple- tion of foundations. and raised by forty 220-ton hydraulic jacks. The top storey and the other storeys, reversing the usual order, will then be built and raised by the jacks at the rate of one storey per fortnight.
This is believed to be the first time this method. devised by Felix Adler. Chief Structural Engineer. of Richard Costain Ltd. has been used for a multi-storey public or domestic building. The system is claimed to bring factory technique to building: in fact, a "vertical" production line.
The structural arrangement of the building resembles the structure of a
tree.
Its trunk is formed by the hollow reinforced concrete core from which the floors. branch-like. are cantilevered out. This central core will contain circulation areas and staircases, and house the lifts and refuse chutes serving the flats. It is under the walls of this core that the main lifting jacks are situated.
At ground level building opera- tions are carried out in a heated polythene-covered workshop. unaf- fected by weather conditions. The finishing trades operate concurrently
72
with the formation of the structural frame. By the time each floor reaches the fifth storey level it will be completely finished. including the installation of services and final decorations. The workshop area is heated so that all building operations can be carried out in dry conditions and in equable working temperatures. so loss of working time through rain and frost will be virtually eliminated.
Jacking operations for the 190 foot 7.500-ton building takes place every other week. During the “rest” week the floor is constructed, parti- tions fixed and cladding fixed in position. each operation being carried out at the factory level. Some jacks are withdrawn when a floor is being built into position and then they are used to push this floor up to the appropriate level.
The building rises slowly at the rate of one inch every three minutes and the operation is extremely ac curate with a high degree of control. The jack layout is such that some of the jacks act as compensating jacks giving perfect stability during the lifting operation, and vertical alignment is ensured even in high winds. The risk of accidents is re- duced, because heavy components are not lifted by cranes or hoists, and men do not work at great heights.
called the Soiltest Pocket Penetrometer, an instrument small enough to be car- ried around in the pocket by anyone working in the field. In a matter of seconds it produces results to within 15 per cent of those from a conventional unconfined compression test using laboratory equipment.
the
A pocket penetrometer, developed by а firm in the United State: consists of а calibrated spring The contractor or and reading scale engineer obtains
unconfined soil the compression strength reading on penetrometer by pushing the instrument shaft into the undisturbed trench wall to a depth of 1/4 inch. This point on The pres the shaft is clearly marked, sure involved in making the penetration is the reading which appears on the scale in pounds per square foot.
For example, take a problem that often confronts the engineer in the field: How deep can a trench be dug in given soil condition without involving bracing or sloping? There is a formula that can be used here:
H = I.2 x C
W
Where H is the safe unsupported height of the trench wall in feet: Uk the unconfined compressive strength in pounds per square foot as determined by the pocket penetrometer: W is the wet weight of the scil in pounds per cubic foot.
Assume that the recommended three or more penetrometer readings of th soil average 1,000 pounds per cubic foot and that the soil weight is 130 lbs. per cubic foot. In this case, the safe trench wall height in feet is equal to 1.2 times 1,000 divided by 130, or about 91 feet.
Another common type of problem quickly solved by the penetrometer is determining the stability of a wall in a trench that has already been excavat ed. Here the formula is inverted to: 1.2 times the safe unconfined compres sive strength in pounds per square foot is equal to the trench wall height ir feet times the weight of the soil per cubic foot. By comparing the resulting safe compressive strength with the existing strength of the trench wall, the engineer can easily determine whether or not bracing or stoping is called for.
The calculations, of course, apply only to cohesive soils. The cohesive condition is normally determined in the field by submerging a 3-in cube of the soil in a pail of water. If the sample' tends to lose its shape. it is considered cohesionless and trenches with walls of such materials usually will not stand unsupported. If the sample does tend to hold its shape, it is considered cohe sive and can then be considered safe as a trench wall depending on its compres
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THE HONG KONG & FAR EAST BUILDER—VOLUME 17, NUMBER 1
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