geotechnical assessment report on zone-a settlement (2013!07!08) - rev.03-1 correction

7/29/2019 Geotechnical Assessment Report on Zone-A Settlement (2013!07!08) - Rev.03-1 Correction

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Brunei 4000-Unit National Housing Scheme Projectat Menkabau, Kampong Mentiri, Bandar Seri Begawan

GEOTECHNICAL ASSESSMENT REPORT

ON

CRACKS ON HOUSES IN ZONE A

(Preliminary)

Submitted on

8th

July 2013

by


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Brunei 4000-Unit National Housing Scheme ProjectAt Menkabau, Kampong Mentiri, Bandar Seri Begawan

GEOTECHNICAL ASSESSMENT REPORTON CRACKS ON HOUSES IN ZONE A

Table of Contents

Executive Summary ............................................................................................................. i

1 Introduction .................................................................................................................. 1

2 Original Ground Level of Affected Area ..................................................................... 5

3 Soil Condition of Affected Area .................................................................................. 6

3.1 Borehole data ..................................................................................................................... 6

3.2 Possible localised loose soil materials .............................................................................. 14

3.3 Possible Fault Lines ......................................................................................................... 16

4 Preliminary Conclusions & Recommendations ....................................................... 17

4.1 Preliminary Conclusions ................................................................................................... 17

4.2 Recommendations ........................................................................................................... 18

Appendix-1: Original topography of affected housing platform

Appendix-2: Photos of cracked houses

a) ZA-B528 unit no.2 Ground Floor ...................................................................................... 1

b) ZA-B528 unit no.2 1st

Floor .............................................................................................. 3

c) ZA-B553 unit no.2 Ground Floor ...................................................................................... 3

d) ZA-B553 unit no.2 1st

Floor .............................................................................................. 5

e) ZA-B554 unit no.3 Ground Floor ...................................................................................... 6

f) ZA-B555 unit no.3 Ground Floor Back Yard ..................................................................... 9

g) ZA-B555 unit no.4 Ground Floor Back Yard ................................................................... 10

h) ZA-B555 unit no.4 Ground Floor .................................................................................... 11

i) ZA-B555 unit no.4 1st

Floor .............................................................................................. 7

j) ZA-B556 unit no.4 Ground Floor Apron Slab .................................................................... 9


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i

Executive Summary

This report will describe the process of observation, preliminary assessment of geotechnical

condition and recommendation for further investigation on the ground behaviour in order to

suggest possible remedial works.

Sometime in November 2012, SIC was notified that several houses in zone A were affected

with cracks on the wall. The houses were constructed on cut platform. Based on the existing

available borehole data, topographical survey plan, geological survey map and ground

settlement monitoring data, it is found that the soil condition of the cut platform where the

affected houses are located is considerably good and our opinion was unlikely that the

settlement is due to poor soil quality.

However, more houses were found with cracks on the wall and other places and based on

the alignment of the affected houses; it could be possible that localised loose soil materialexist along that axis. This could only be verified if further soil investigation is carried out.

This condition may pose a potential for temporary ground subsidence when excavation is

carried out in the vicinity. The temporary ground subsidence could be aggravated when the

groundwater washes the localised loose soil material during excavation works. However it

does not seem to be the case for other areas which may suggest the absence of loose

material. The cracks on the houses could be due to temporary aggravated soil subsidence

when this unforeseen localised loose soil material was being washed out from the soil layer

during excavation in the vicinity.

As the nearest fault line indicated in the Geological Survey Map is more than 1 km from the

affected houses, it is unlikely that the settlement is due to the fault lines.


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Page 1 of 20

1 Introduction

In November 2012, some cracks were observed on 5 rows of terrace houses in Zone-A,

platforms ZA-B526, ZA-B528, ZA-B553, ZA-B554 and ZA-B557 (refer to Figure 1).

Figure 1:Platforms with cracks observed on houses (Nov 2012)

As these platforms were constructed on cut grounds of substantial depth between 10 and

25m, and the borehole data showed that the soil condition there was good (with SPT > 50), it

was suspected then that the cracks on the houses may be caused by structural movement.This was especially so as the cracks initially appeared on the brick walls and precast joints

and not on the ground slabs. A structural investigation was carried out and reported

separately.

However, as the cracks reappeared after repair works were carried out, and more cracks

were observed in the neighbouring platforms, it was decided that more extensive settlement

markers monitoring was to be carried out.

Settlement markers were also installed on the affected platforms to monitor whether there

was any ground settlement. In total, 8 sets of readings were taken between 9

th

Nov and 29

th

Dec 2012, and there seemed to be insignificant ground settlement except for some


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Page 2 of 20

Ta

ble1:Settlementreadingsfrom9

Novto29

anomalies (refer to Table-1). As such, a fresh set of level monitoring will be initiated prior to

further analysis.


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Page 3 of 20

The proposed additional settlement monitoring shall be carried out on the 14 platforms as

shown in Figure 2.

Figure 2:Platforms for further settlement monitoring


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For each platform, 12 settlement markers will be installed at the locations as shown in Figure

3.

Figure

3:Locationsofsettle

mentmarkersforeachhousingplatform


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Page 5 of 20

2 Original Ground Level of Affected Area

The affected housing platforms are between PL.53.0m to PL.56.0m (refer to Figure 4),whereas the original ground level in that area ranges from about 54 to 100m as shown inAppendix-1. With this 15 to 40m thick layer of earth removed and the overburden pressure

relieved, it is unlikely that the load from the houses would cause any further consolidationsettlement.

Figure 4:Platform levels of cracked houses


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3 Soil Condition of Affected Area

3.1 Borehole data

The soil condition of the affected area is analysed from the nearby 4 boreholes, i.e., BH-02,

BH-04, ABH5 and ABH6 as shown in Figure 5.

Figure 5:Boreholes around affected area


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BH-02 was carried out at original ground level of 74.58m and terminated at 63.85m (refer toFigure 6). At depth 67.11m and below, the SPT N value reaches 50, and the soil isdescribed as Hard dark grey CLAY with traces of silt. Therefore, it is very possible that thecut ground at about PL.55m to be of similar good soil condition. The groundwater tablevaries between 1.24 to 2.29m below the original ground level, i.e. WL = 72.29 to 73.34m.The relatively high groundwater table will be lowered with time after the original ground hasbeen cut down to the proposed platform level. The rate of groundwater table loweringdepends on factors such as soil porosity, rainfall intensity, site conditions, etc.

Figure 6:Borehole data BH-02


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ABH6 is another borehole nearest to the affected area. It was carried out at a partially filledground level of 49.5m (although the original ground level was 46.2m) and terminated at level37.365m with a borehole depth of 12.135m (refer to Figure 7). The original soil condition

shows a minimum SPT N value of 7. It increases to a value of 16 at level 43.5m, and closeto 50 at 42.0m and below. Again, it confirms that the surrounding soil condition of theaffected area is of reasonable quality and strength. The recorded groundwater depth is0.51m from the borehole top level, i.e. WL = 49.0m, which is below the proposed platformlevel.

Figure 7:Borehole data ABH6


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ABH5 was carried out at original ground level of 96.2m and terminated at 46.87m. At depth85m and below, the SPT N value reaches 50, and the soil layers close to the housingplatform level are described as Hard dark grey CLAY with traces of silt and Very densebrownish grey silty fine SAND (refer to Figures 8a-8d). The recorded groundwater tablevaries from 23.35 to 25.68m below the original ground level, i.e. WL = 70.52 to 72.85m. Therelatively high groundwater table will be lowered with time after the original ground has beencut down to the proposed platform level. The rate of groundwater table lowering depends onfactors such as soil porosity, rainfall intensity, site conditions, etc.

Figu re 8(a):Borehole data ABH5


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Figu re 8(b):Borehole data ABH5


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Figu re 8(c):Borehole data ABH5


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Figu re 8(d):Borehole data ABH5


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Page 13 of 20

BH-04 was carried out at original ground level of 67.04m and terminated at 56.14m. At depthof about 59m and below, the SPT N value reaches 50, and the soil layer is descri bed asHard dark grey CLAY with traces of silt (refer to Figure 9). The recorded groundwater table

varies from 2.88 to 3.67m below the original round level, i.e. WL = 63.37 to 64.16m. Therelatively high groundwater table will be lowered with time after the original ground has beencut down to the proposed platform level. The rate of groundwater table lowering depends onfactors such as soil porosity, rainfall intensity, site conditions, etc.

Figure 9:Borehole data BH-04


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3.2 Possible localised loose soil materials

From the alignment of the affected houses (refer to Figure 11), it seems that although theground condition in that area is generally good, there could be a possibility that localisedloose soil materials exist along that alignment. During excavation, such localised loose soilmaterials could be washed out from the soil layer by the groundwater, thus aggravating thetemporary subsidence of the surrounding ground, which may have led to the cracks on thehouses. The existence of the localised loose soil materials could be verified by further soilinvestigation along the alignment of the affected houses.

The borehole data shows that the original groundwater table was higher than the cutplatform. The hill to the northwest of the affected houses is about 40m above the platformlevel as shown in Figure 10.

Figure 10:Hill northwest of affected houses

Borehole ABH5 shows that the groundwater table there is about 20m higher than theaffected housing platform level. In normal circumstance, the higher groundwater table will belowered with time to reach equilibrium with the groundwater table at the cut platform. This

natural process will take a long time and is often unnoticeable. However, it is inevitable thatduring construction when excavation is carried out groundwater will be released from theexcavation, causing the surrounding groundwater table to lower and possible temporaryminor subsidence to the surrounding ground.


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Figure 11:Alignment of affected houses


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Page 16 of 20

3.3 Possible Fault Lines

The Geological Survey Map from the Geological Survey Department, British Territories inBorneo shows 3 fault lines in the Muara Area close to the development site (refer to Figure12). As the fault lines are far from the affected houses, it is highly unlikely that the cracks

were caused by this condition.

Figure

12:FaultlinesinMuara

area


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4 Preliminary Conclusions & Recommendations

4.1 Preliminary Conclusions

Based on the existing available borehole data, topographical survey plan, geological survey

map and ground settlement monitoring data, the following preliminary conclusions are drawn:

The soil condition of the cut platform where the affected houses are located isconsiderably good. It is Hard dark grey CLAY with traces of silt with SPT N value of50 at the cut platform level. Therefore, it is unlikely that the settlement is due to poor soilquality.

However, based on the alignment of the affected houses, it could be possible thatlocalised loose soil material exist along that axis. This could only be verified if furthersoil investigation is carried out.

The groundwater table around the cut platform where the affected houses are located is

significantly higher than the house platform, especially at the hill to the northwest of theaffected houses. This poses a potential for temporary ground subsidence whenexcavation is carried out in the vicinity. The temporary ground subsidence could beaggravated when the groundwater washes the localised loose soil material into theexcavated ground.

The cracks on the houses could be due to temporary aggravated soil subsidence whenthe unforeseen localised loose soil material was being washed out from the soil layerduring excavation in the vicinity.

As the nearest fault line indicated in the Geological Survey Map is more than 1 km fromthe affected houses, it is unlikely that the settlement is due to the fault lines.


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4.2 Recommendations

Based on the preliminary conclusions, the following course of actions is recommended:

If the cracks on the houses were due to temporary soil subsidence caused by localised loosesoil material being washed out from the surrounding soil layer by the groundwater duringexcavation in the vicinity, the settlement will diminish with time after the excavation has beenbackfilled. To determine when the settlement has stabilised and is safe to carry out thestructural repair works, the following monitoring programme should be implemented:

Ground settlement monitoring of the 28 affected house platforms at an interval of 3days for a reasonable duration until no significant settlement is observed. If thesettlement readings show incremental settlement of less than 2mm in 3 consecutivereadings, it is likely that the temporary soil subsidence has stabilised and issufficiently safe to carry out the structural repair works. Ground settlement monitoringshould continue during the repair works, but reduced to only weekly monitoring.

Install 15 nos. of water standpipes at the proposed locations as indicated in Figure14. This is to monitor the surrounding groundwater table to detect if the drawdownrate is significant enough to cause further soil subsidence.

Carry out further soil investigation along the alignment of the cracked houses todetermine if there are localised pockets of loose soil material which could have beenwashed out by the groundwater during excavation, thus causing the groundsubsidence along that axis. 9 nos. of additional boreholes are proposed, and thelocations are as indicated in Figure 15.

To monitor the length and width of the existing cracks on the houses and see if theyare propagating further before carrying out any structural repair works.


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Figure 14:Indicative locations of proposed water standpipes


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Figure 15:Indicative locations of proposed additional boreholes


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Appendix-1

Appendix-1: Original topography of affected housing platform


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Appendix 2-1

Appendix-2: Photos of cracked houses

a) ZA-B528 unit no.2 Ground Floor


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Appendix 2-2


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Appendix 2-3

b) ZA-B528 unit no.2 1st Floor

c) ZA-B553 unit no.3 Ground Floor


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Appendix 2-4


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Appendix 2-5

d) ZA-B553 unit no.3 1st Floor


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Appendix 2-6

e) ZA-B554 unit no.3 Ground Floor


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Appendix 2-7

f) ZA-B555 unit no.4 1st Floor


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Appendix 2-8


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Appendix 2-9

g) ZA-B556 unit no.4 Ground Floor Apron Slab

h) ZA-B556 unit no.2 Ground Floor Back Yard


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Appendix 2-10

i) ZA-B557 unit no.4 Ground Floor Back Yard


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j) ZA-B557 unit no.4 Ground Floor

geotechnical assessment report on zone-a settlement (2013!07!08) - rev.03-1 correction

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