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Technical Note (National Research Council of Canada. Division of Building Research), 1971-07-01
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Contact Pressures Under a Raft Foundation
Eden, W. J.
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DIVISION OF BUILDING RESEARCH
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1HI N ][ CAlL
NOTE
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LIMITED DISTRIBUTION
PREPARED BY W. J. Eden CHECKED BY LWG APPROVED BY
CBC
ｾ July 1971
PREPARED FOR Ontario Housing Corporation
SUBJECT CONTACT PRESSURES UNDER A RAFT FOUNDATION
This Note has been prepared to inform the several interested parties of results obtained to date, from an experiment to measure contact pressures under a foundation raft. The experiment is being conducted under a IS-storey apartment building at the corner of
Rideau and Wurtemburg Streets, Ottawa. The building was erected for the Ontario Housing Corporation by Catkey Construction Ottawa Limited. Architects for the job were Miska and Gale, structural engineers \vere
J. S. Hall and Associates, Ottawa, and soil engineers McRostie, Seto, Genest and Associates, Ottawa. The building is operated by the
Ottawa Housing Authority. Purpose
If a rigid plate is placed on an elastic medium, the maximum contact pressures will occur at the edge of the plate. Rigid raft foundations of stiff structures founded on clays may approximate this mathematical concept, in which case bending moments in the slab will be large and the amount of reinforcing required to maintain the required stiffness of the raft will be correspondingly large. Any tendency for plastic behaviour in the clay will reduce the high edge pressure.
2
-This experimental program had three basic objectives;
(l) to measure the distribution and magnitude of contact pressures from the edge to the centre of the foundation raft;
(2) to measure the variation of the contact pressures with time and set points;
(3) to measure the progress of total and differential settlement of the structure and the influence of such settlements on contact pressure.
The building being studied is an ideal case for such an experiment. It is nearly rectangular in plan, sYmmetrical, and very stiff due to five rein-forced concrete shear walls carried the full height of the structure. The structure is founded on a stiff ＳＭｦｴＭｾｨｩ｣ｫ foundation raft cast directly over 35 ft of over consolidated clay.
T est Installations
The test installations consisted of 10 earth pressure cells, 11 survey reference points, and a deep bench mark.
The earth pressure cells are Gloetzl cells (Lauffer and Schober, 1964)
I
approximately 8 by 12 inches. They were installed at the interface of the clay and the concrete raft to measure the contact pressures. Nine of the cells were placed under one quadrant of the building in an attempt to gain an impression of the distribution of the pressure. Figure 1 shows the location of the cells. The cells consist of a sealed flat jack full ofhydraulic oil. Earth pressures are balanced by the flow of hydraulic oil through a special orifice at the end of the inlet line. This orifice is located beside a membrane and opens only when the pressure in the inlet line exceeds the pressure within the cell acting on the other side of the membrane. Each cell is connected to the reading terminal by means of copper tubing. The circuit is completed by a return lead system common to all cells.
The cells were installed after a "mud" slab of lean concrete was cast directly on the clay subsoil. Planks left in the "mud" slab were lifted _ for cell installation (Figu:re 2). Each cell was installed on a prepared surf3.ce
•
of undisturbed clay and connected to the lead system. Figure 3 shows a pressure cell and the copper tubing system. After all cells had beeninstalled. the cells and leads were covered with lean concrete. The leads were assembled at a common point and brought up through the foundation slab in a short length of transite pipe (Figure 4). Later a terminal panel with control valves was built into a concrete block wall.
The settlement reference points were installed at the ends of the shear walls near ground level (see Figure 1) from which total and differential settlement of the structure could be measured by precise level surveys.
With the exception of No. 50. all points are installed in the end of the reinforced concrete shear walls about 2 ft above the ground floor level. Each reference point consists of a stainless steel plug 3/4 in. diam by 2 in. long set in a drilled hole with cement grout. Each plug has a
numbered, removable cap. At the time of the surveys, the cap is removed and a special insert is screwed in to serve as support for the level rod. Level readings are taken with references to a deep bench mark
(Bozozuk, Johnston and Hamilton. 1962), installed at the north edge of the property. Levels are taken with a precise level and a level rod with an invar tape. Readings are expected to be accurate to O. 004 in.
Progress to Date
On 27 May 1970 a deep bench mark was installed along the north property line. The bench mark was driven to a depth of 52 ft. below the surface. On 25 August 1970 the ten earth pressure cells were installed
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in the mud slab. The foundation raft was placed on 28 August. The terminal valve panel for reading the earth pressure cells was installed on 1 and 2 September and the first set of readings taken. Readings haveI
been made at frequent intervals since then and are presented in Table 1.On 16 October 1970 the ends of the shear walls at the first floor level wer e accessible and the survey reference points were installed. The first
level survey was made on 19 October at which time form work was proceeding for the fifth floor. Table 2 lists the settlement measured at the survey
reference points up to the present.
Readings were taken at the following dates along with notes on the progress of building: 19 October 1970 26 ｏ｣ｾｯ｢･ｲ 1970 5 November 1970 13 November 1970 27 November 1970
work proceeding on 5th floor work proceeding on 8th floor slab for 11th floor in place slab for 13th floor in place
building frame completed - masonry work started
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22 December 1970 11 February 19719 June 1971
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-masonry complete to 7th floor masonry almost complete - wall
board being installed. Dead load more than 95% applied. building practically completed
-finish and decorating in progress
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On 11 and 12 May 1971 a bore hole was made from which 2-in. -diam. piston samples were obtained continuously from a depth of 15 ft 0 in. to 50 ft 4 in•• (the bottom of the clay layer). Results of tests on these samples will provide reasonably complete information on the properties of the clay at the site. This testing program has not been started at the date of writing. Concluding Statement
The foregoing note describes the instrumentation and the work conducted by the Division of Building Research on this experimental _ program. To complete the study. a soil testing and analysis program remains to be done. In the next few weeks. it is planned to meet with both the soil engineers and structural'engineers to assess the significance of results obtained to date. Ultimately it is hoped that with the cooperation of the design engineers the results of this program can be presented to the profession at large in form of one or more technical papers.
References
1.
Lauffer. H. and W. Schober. 1964. The Gepatsch Rockfill Dam in the Kauner Valley. Proc. Eighth Congress on Large Danis. Vol. 3.p. 635-660.
2. Bozozuk, M •• G. H. Johnston and J. J. Hamilton. 1962. Deep Bench Marks in Clay and Permafrost Areas. ASTM Spec. Tech. Publication No. 322. p. 265 -279.
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TABLE 1
-CONTACT PRESSURE MEASUREMENTS
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Measured pressure at position -psf
Date Remarks
1 2 3 4 5 6 7 8 9 10
25. 8.70. 0 0 0 0 0 0 0 0 0 0 Date installed 2. 9.70. 43 -130 201 288 -29 86 144 58 346 58 Floor slab i.n place 23. 9.70. 503 446 288 374 -14 101 331 590 528 86 Work on 1at floor 19.10.70. 893 893 705 562 58 130 618 1005 805 100 Work on 5th floor 26. 10. 70. 1190 1110 720 505 58 72 648 1150 922 86 Work on 8th floor 5.11. 70. 1540 1540 1140 720 201 274 936 1670 1165 230 Work on 11 th floor 18. 11. 70. 1890 2020 1510 805 288 . 360 1150 1940 1380 288 Work on 13th floor 27.11. 70. 2160 2390 1650 865 274 375 1195 2160 1510 302 Frame complete 22. 12. 70. 2300 2760 2060 1040 432 575 1500 2520 1790 475 Masonry to 7th floor .
11.
2.71. 2690 2980 2260 1080 575 660 1930 2740 2020 660 Masonry complete 15. 3.71. 2920 3160 2230 1090 675 634 1610 2740 2190 79022. 4.71. 2830 3050 2450 1225 790 855 1860 2860 2300 1095 Buildi.ng nearly completed 9. 6.71. 3010 3190 2590 1300 850 895 1970 2900 2450 1170
1_ i_:;:::=
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TABLE 2
SETTLEMENT OF REFERENCE POINTS
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Settlement at reference points
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As inches. Date 50 51 52 53 54 55 56 57 58 59 60 19.10.70. 0 0 0 0 0 0 0 0 0 0 0 26. 10. 70. 0.062-
0.058 0.030 0.021 0.062 0.014 0.034 0.029 0.019 0.027 4. 11. 70. O. 101 0.062 0.065 0.047 0.065 0.068 0.008 0.056 0.076 0.067 0.064 17.11.70. 0.122 0.094 O. 121 0.129 0.139 0.135 0.039 0.087 0.106 0.104 0.085 26. 11. 70. 0.168 0.138 0.200 0.199 0.192 0.181 0.099 0.143 O. 161 0.154 0.130 23. 12. 70. 0.233 0.178 0.209 0.213-
0.229-
-
-
-
O. 185 11. 2.71. 0.295 O. 251 0.286 0.322-
0.357 0.314-
-
-
0.266 19. 4.71. 0.347 O. 297-
0.416-
0.452 0.443 0.464 0.480 0.417 O. 327 8. 6.71. 0.361 0.322 0.414 0.455 0.521 0.583 0.541 0.550 0.511 0.437 0.35058 R WALL 59 60 51 LEGEND:
o
EARTH PRESSURE CELLｾ SURVEY POINT ;SHEA ｾ - STAIRWEll
