FOUNDATION FOR HIGH BUILDINGS
in Hong Kong
"Attempts to arrest settlement by grouting the soft clay layer are quite useless and should not be considered". This simple statement con- tained in the conclusions to a "Report on the Settlement of Buildings in the Monk Kok District of Kowloon, Hong Kong", caused a furore among practicing civil engineers in the Colony.
The report was prepared by Mr. P. Lumb, M.Sc. (Eng.), AMICE, FGS, senior lecturer in civil engineering, University of Hong Kong. Fol- lowing its publication in July, it has been the subject of a lengthy corres- pondence in the Hong Kong daily press. Engineers have critisized some of the recommendations made regarding grouting and choice of founda- tions.
Some form of explanation of local foundation conditions, to serve as a guide for improving foundation engineering practice in the Colony, would therefore seem appropriate at this time. Far East Architect & Builder, with the approval of the University of Hong Kong, takes the opportunity now to publish extracts from a Paper presented in 1961 at the Univer- sity's Symposium on the Design of High Buildings.
The Paper, "The Influence of Ground Conditions on the Choice of Foundations for High Buildings in Hong Kong", was written by Mr. M.W. Leonard, B.Sc. (Eng.), MICE, AMIMechE, and Mr. I.K. Nixon, ACGI, AMICE.
"HERE are a number of causes for
THE
the movement which takes place under load. Initially the ground mass adjusts itself elastically to the new set of conditions. In the case of solid bedrock this is generally the sole rea- son for settlement. Stability is main- tained however under conditions when the individual particles in the soil move closer together.
Normally water is present between the grains and is also under stress initially. However as it is free to move, it tends to migrate, giving rise to a condition known as consolida- tion. Fine grained soils with very small connections between the voids can only drain slowly and hence in these circumstances we get what is known as long term settlement. Sands and the other more coarse grained soils on the other hand drain quickly and usually the adjustment in the grain structure is complete almost immediately after the application of the load.
Whilst the total settlement allowed for tall buildings must necessarily be small, there is also a limit to the dif- ference in settlement which can be tolerated between the various parts of the structure, in order to prevent ex- cessive stresses within the framing and not to disfigure the panelling. This limit on angular distortion is usually the more important and is referred to as the maximum dif- ferential settlement.
Even under uniform ground condi-
tions the settlement beneath spread foundations is greater in the centre than at the edges and this forms an important consideration in the design of rafts.
move-
When there is movement under load of the grain structure beyond the stage of local readjustments then the ground is said to be overstressed. In this case settlements are usually great- er than can be tolerated, particularly for buildings. Catastrophic ment occurs with the rupture of the ground as a result of overall shear failure. The consequences attendant in both these cases, overstressing and rupture of the ground, cannot be over-emphasised, as the resultant failure of the foundation not only means loss of the building, but denial of the site for other uses if the build- ing is seriously damaged. Under- pinning may very occasionally be car- ried out with much delay and ex- pense assuming this is permitted by law.
Occasionally horizontal movement of the soil particles in the mass may take place giving rise to settlement. This can occur from beneath the edge of a foundation, either when it is too shallow or if an inadequately timber- ed trench is close by. Another case is when there is a very weak layer of soil at depth sandwiched between stronger material. Movement in this condition is known as 'squeeze'.
Settlement can also take place in the ground for a number of reasons
Far East Architect & Builder September, 1965
other than the increase in stress due
to the building loads. Made ground in itself may become more compact with time due to natural consolidation for which reason it is not normally considered a very reliable bearing stratum. Where such ground forms an additional load on the natural ground it is also necessary to take into account its effect on the underly- ing strata.
Very recent natural deposits may be subject to settlement within them- selves and this can give rise to nega- tive skin friction on piles. Lowering of the water table, whether temporary or permanent, can cause settlement due to the increase of load on the ground at lower levels as a result of the change in the weight of the soil above the level of the new water table.
Whereas previously the ground be- tween the old and new water table was submerged, after the lowering the full weight becomes operative. In sands and clays the change in weight may be about twice the submerged value, but for peat it is much greater as such soil is largely water, most of which is retained under the new con- ditions.
There are other reasons for move- ment in the ground which also need to be considered sometimes. Occa- sionally an allowance has to be made. in the level of a building for the heave which takes place when a deep excavation is made for a foundation. The removal of the ground allows the soil beneath to expand by virtue of the decrease in load.
Local Ground Conditions
Confining ourselves to those areas of Hong Kong where high buildings are more likely to be built, the ground conditions, broadly speaking, are re- latively simple from a geological viewpoint. The principal formation in the central part of the Colony is granite, and this occurs at the sur- face in many places in Kowloon and on the north side of Hong Kong Is- land.
Tropical weathering of this granite rock has given rise to two main cate- gories of soils: namely insitu decom- posed granite and colluvial decompos- ed granite. Around the shores of both the Kowloon peninsula and the Island, as well as beneath the sea, there are marine and river deposits. Practically everywhere however, as is
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