ber tyre roller, but it was proved to be unsuitable for this purpose. There- after, the rockfill was compacted by D8 bulldozers.
The inability of the contractor to quarry sufficient quantities of rock for the rockfill zones and for the produc- tion of filter materials seriously affect- ed progress. Placing of each of the horizontal drainage blankets took a long time, varying from 3 to 6 weeks. At one time the upstream rockfill was 10 to 12 ft. below the level of the shoulder.
It was the rate of production of rock and filter rather than overall quantity which caused the hold up. As a result, he had to supplement his supply by bringing rock from quar- ries near Kowloon, some 8 to
10 miles away, to catch up on placing of the rockfill upstream.
The original scheduled completion date of the embankment was Decem- ber 31. 1964. However, the con- tractor was asked in November 1963 to accelerate his progress SO as to finish the dam by July 10, 1964. De-
SHEAR STRESS LB/SQ. IN.
SHEAR STRESS LB/ SQ. IN.
120
[00
80
60
40
20
80.
SHOULDER MATERIAL
LIMITS OF SHEAR PARAMETERS
Cd = 9 LB/SQ. IN.
Od - 37/2°
Cd = 1 LB/SQ. IN. Ød
332°
spite the extra personnel and plant which the contractor brought on site to expedite the completion of the dam some 51⁄2 months ahead of the ori- bad ginal schedule. exceptionally weather and unusually frequent typhoons during the summer in 1964 seriously hampered his efforts to meet the revised targets. The em- bankment was completed in Decem- ber 1964.
Pore Water Pressure Measurements
In order to compare the behaviour of the dam during and after construc- tion with its design assumptions, piezometers and settlement devices were installed in both the embank- ment and the foundations.
A total number of 42 piezometer cells were embedded in the embank- ment and a further number of 22 cells were placed in the foundations. Each piczometer system consists of a Bishop-type cel! (3) with a fine porous pot, two nylon leads and a Bourdon
20
40
60
80
100
NORMAL EFFECTIVE STRESS
CORE
MATERIAL
+
LIMITS OF SHEAR PARAMETERS
Cd = 7 LB/SQ. IN.
Ød =
Cd - 3 LB/SQ. IN.
Ød
===
34/2 31°
60
40
20
20
40
60
مع
120
140
LB/SQ. IN.
100
160
120
140
160
NORMAL EFFECTIVE STRESS LB/SQ. IN.
Fig. 6. Limits of drained test envelopes on compacted shoulder and core materials
76
gauge which is installed in the gauge house located underground in the Each system is downstream slope.
led to the de-airing apparatus in such a manner that de-aired distilled water
may
be circulated through each piezometer.
In addition to measuring the pore water pressures during construction so that the stability of the embank- ment could be checked, the pore
water
pressure readings will also provide valuable data necessary to calculate the stability of the down- stream slope under conditions of steady seepage when the reservoir is full, and that of the upstream slope under conditions of rapid draw-down. The piezometers were installed in a vertical section of the dam and their positions are shown in Fig. 2.
Negative and negligible positive pore pressures were recorded in the shoulders because of the dry place- ment moisture content of the materi-
al. These pore pressures did not seem to be influenced by the rate of construction. On the other hand, the initial pore pressures in the core were high, due to the high placement moisture content of the material. These pore pressures dissipated rapid- ly as the fill consolidated with time.
Typical readings of pore pressures of two piezometers in the core are shown in Fig. 7. It will be seen that these pore pressures responded to the rate of construction. Since the em- banking progress was slow during the summer in 1964, the pore pressure ratios continued to decrease until the rate of embanking was much acceler- ated during November and December 1964, near completion, when the pore pressure ratios began to rise again.
Provision was made in the speci- fication to restrict the rate of raising the embankment if the constructional pore pressurers became high and the pore pressure ratios exceeded the de- sign value of 65 per cent. It was not found necessary, however, to regulate the rate of embanking on account of this.
During the frequent short periods of suspension of embanking opera- tions, partial dissipation of pore pres- sures took place. The effect of this partial dissipation of pore pressures was to retard the subsequent rate of development of pore pressures when embanking was resumed, i.e., the ratio of change of pore pressure to change of total stress became less after each stage of dissipation.
This has been shown by Bishop (5) to be due to the decrease in compres- sibility of the soil as the effective stress increases. Since the total stress was constant during shut-down, the increase in effective stress was brought about by the drop in pore pressure.
Settlement Measurements
Two U.S.B.R. type internal vertical movement devices (4) were installed in the embankment. No. I was found- ed in bedrock foundations, and set-
Far East Architect & Builder December, 1965
Page 40Page 41
No comments yet.
Private notes are available after approval.