b4000 - preliminary geotechnical report (draft) 2010-11-1

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

PRELIMINARY GEOTECHNICAL REPORT

Submitted on

4th November 2010

by

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

PRELIMINARY GEOTECHNICAL REPORT

Table of Contents

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

1.1 Background............................................................................................................. 1

1.2 Objectives ............................................................................................................... 1

2. Existing Site Conditions .............................................................................................. 2

2.1 Site Location ........................................................................................................... 2

2.2 Climatic Condition ................................................................................................... 2

2.3 Existing Topography ............................................................................................... 3

2.4 Existing Drainage .................................................................................................... 4

2.5 Site Geology ........................................................................................................... 5

2.6 Existing Borehole Data ........................................................................................... 5

2.7 Indicative soil strata profile and soil properties across the site ................................ 7

3. Preliminary Geotechnical Assessment .................................................................... 11

3.1 SPT N-value versus Soil Depth ............................................................................. 11

3.2 Bulk Density versus Soil Depth ............................................................................. 12

3.3 Water Content versus Soil Depth .......................................................................... 12

3.4 Fine-grained Soil Classification ............................................................................. 13

3.5 Suitability of existing soil as earth filling material ................................................... 13

3.6 Use of high plasticity clays .................................................................................... 16

4. Total Consolidation Settlement ................................................................................ 17

4.1 Factors affecting ground settlement ...................................................................... 17

4.2 Estimation of total settlement ................................................................................ 17

4.3 Proposed Mitigation Measures for Long Term Settlements ................................... 18

5. Slope Stability ............................................................................................................ 19

5.1 Major slopes along Site boundary ......................................................................... 19

5.2 Proposed Slope Monitoring Instrumentation Plan ................................................. 19

6. Recommendations & Conclusion ............................................................................. 21


6.1 Recommendation of additional soil investigation ................................................... 21

6.2 Conclusion ............................................................................................................ 23


Pg. 1 of 23

1. Introduction

1.1 Background

The Brunei Economic Development Board (BEDB) has appointed United Engineers, Malaysia (UEM) as the Contractor for the pilot scheme to design and build 4000 houses together with the necessary supporting infrastructure for the National Housing Scheme of Brunei Darussalam on a green-field site of approximately 320 hectares at Mengkubau, Kampong Mentiri, Bandar Seri Begawan in Brunei Darussalam (hereinafter refers to as the Site). Surbana International Consultants (SIC), UEM’s appointed consultant for this project, is preparing this Preliminary Geotechnical Report as part of the earthworks design, and in accordance with the submission requirement of JKR (Geotechnical and Geological Section).

1.2 Objectives

The purpose of this report is to provide preliminary geotechnical assessment and considerations for the proposed earthworks and subsequent residential development. The information provided herein is primarily based on Report No. PF1174/09-7 “Soil Investigation Works for Pilot Scheme to Build 4000 Houses within 48 Months for the National Housing Scheme of Brunei Darussalam, Negara Brunei Darussalam” prepared by Teca (B) Sdn. Bhd. on behalf of BEDB, and is not intended for detailed engineering design purpose. The scope of this report includes:

• Characterisation of subsurface materials based on the available geotechnical data and assessment on the suitability of the existing soil as fill materials

• Evaluation on the adequacy of the earthworks and foundation design in terms of ground settlement and slope stability

• Recommendation of additional soil investigation for detailed geotechnical analysis


Pg. 2 of 23

2. Existing Site Conditions

2.1 Site Location

The Site is approximately 320 hectares of green-field site within the Brunei-Muara District and is located at Mengkubau in Kampung Menteri. It is approximately 12 km from Bandar Seri Begawan and bounded by both private and public buildings on Jalan Muara Road and Jalan Kota Batu Road on the North-West and East of the Site respectively. The Menteri-Subok Highway forms the southern boundary of the Site (refer to Figure 1)

Figure 1: Site Location Plan. Source: BEDB

2.2 Climatic Condition

Located in the north-western region of the Borneo Island, Brunei enjoys a tropical/equatorial climate with high rainfall and warm temperature throughout the year. The climatic changes are from the influences of the monsoon winds. The northeast monsoon occurs from December to March, while the southeast monsoon occurs from June to October. Brunei receives a high annual rainfall of about 2300mm. One of the two rainy periods is from September to January with the wettest month being December. The second wet season is from May to July. The mean temperature is about 23.8 to 32.1 ºC.


Pg. 3 of 23

2.3 Existing Topography

The Site topography ranges in elevation from RL15m at the northern boundary along Jalan Muara Road to RL100m at the southern boundary. This varies across the Site with broad ridges and gullies to the north and steep slopes with deep valleys to the south (refer to Figure 2). The general vegetation of the Site is a mixture of secondary forest of over 30 years with generic vegetation.

Figure 2: Existing Topography. Source: BEDB


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2.4 Existing Drainage

The natural drainage through the Site is illustrated in Figure 3. The Site is situated at the headwaters of four principal catchments as follows: · Sungai Orok river and Sungai Salimbigar river falling to the East · Sungai Mengkubau river falling to the north west crossing Jalan Kota Batu road · Three tributaries of the Sungai Tanah Jambu river falling in a northerly direction

which joins immediately after crossing Jalan Muara road

Figure 3: Existing Drainage. Source: BEDB


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2.5 Site Geology

Based on existing geological map, the formation of the Site is inter-bedded sandstones, mudstones, shale and occasional beds of lignite (refer to Figure 4)

Figure 4: Site Geology. Source: BEDB

2.6 Existing Borehole Data

Preliminary soil investigation works had been carried out by TECA (B) SDN. BHD. on 12th August to 15th September 2009. This included 13 numbers of boreholes to determine the subsoil conditions for preliminary design and cost estimation by the tenderers. Figure 5 shows the layout of the borehole locations with respect to the Site. The spacing between the boreholes varies from 500 to 900 m. The ground elevation at various borehole locations also varies significantly, with a maximum difference of about 40 m. Based on the borehole data, the water table ranges from 1 to 5 m below ground.


Pg. 6 of 23

Xxx

Figure 5: Borehole Location Plan in the Preliminary Site Investigation. Source: BEDB & Surbana

8 9 7

6

13

5 10

1

2

4

11 3

12


Pg. 7 of 23

2.7 Indicative soil strata profile and soil properties across the site

Figures 6 to 13 show the indicative soil profile across the project site. The subsoil generally

comprises very soft to very stiff clay and silt, and occasionally inter-bedded with very loose

to dense silty sand at some locations.

Figure 6: Indicative soil strata profile along BH 1 to BH 2

Figure 7: Indicative soil strata profile along BH 1 to BH 4


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Figure 8: Indicative soil strata along BH 2 to BH 4.

Figure 9: Indicative soil strata profile along BH 4 to BH 3.


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Pg. 10 of 23




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3. Preliminary Geotechnical Assessment

3.1 SPT N-value versus Soil Depth

Figure 14 shows the relationship of SPT N-values versus soil depth across the site, and it is

observed that the soil profile is highly heterogeneous across the site. The underlying hard

clay strata also varies considerably between the boreholes and occurs between the

shallowest of 1.65m and the deepest of 13m as recorded at BH 3 and BH 8 respectively.

The clay layer thickness varies from 1m to 13m.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

0 5 10 15 20 25 30 35 40 45 50 55

De

pth

(m

)

SPT (N)

BH1 BH2

BH3 BH4

BH5 BH6

BH7 BH8

BH9 BH10

BH11 BH12

BH13

Figure 14: Relationship between SPT N values versus depth of the soil across the site.


Pg. 12 of 23

3.2 Bulk Density versus Soil Depth

Figure 15 shows the bulk density versus soil depth across the site. The average bulk density

of soil is about 18 kN/m3.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

16 17 18 19 20

De

pth

(m

)

Bulk Density (kN/m3)

BH1 BH3

BH5 BH6

BH7 BH8

BH9 BH10

BH12

Figure 15: Relationship between bulk density and soil depth across the site

3.3 Water Content versus Soil Depth

Figure 16 shows the water content versus depth of soil across the site. The water content

varies from 20 to 30%.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

0 20 40 60 80 100

De

pth

(m

)

Water content (%)

PL

wc

LL

Figure 16: Water Content versus depth of the soil across the site


Pg. 13 of 23

3.4 Fine-grained Soil Classification

Figure 17 shows the fine-grained soil classification based on the plasticity chart, and it is

observed that all the data points fall above the A-line on the plasticity chart, and the sub-

surface soil materials primarily comprises very soft to very stiff clays (CV, CH, CI and CL)

with low to high plasticity characteristics.

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

Pla

stic

ity

Ind

ex

(%)

Liquid Limit (%)

CE

CV

CH

CI

CL

Figure 17: Fine grained soil classification based on Plasticity Chart

Note: CL = Clay of low plasticity (liquid limit <50, inorganic), CI = Clay of Intermediate plasticity, CH = Clay of high plasticity

(liquid limit >50, inorganic, fat clay), CV = Clay of very high plasticity, CE = Clay of extreme high plasticity (liquid limit > 90)

3.5 Suitability of existing soil as earth filling material

As mentioned earlier, the site topography is very undulating with a maximum ground elevation difference of about 85 m. Of the 13 boreholes that were carried out during the preliminary site investigation, two of them (BH-1 & 8) fall outside the Site boundary. Five of them (BH-2, 3, 4, 9, & 13) are located at the proposed cutting zones, while the remaining six (BH-5, 6, 7, 10, 11 & 12) at the proposed filling zones. Figure 18 shows the locations of the boreholes in the earth-cutting zones. The proposed cutting depth varies from place to place with a maximum of about 50 m. And Figure 19 shows the locations of the boreholes in the earth-filling zones. The filling depth also varies from place to place with a maximum of about 34m.


Pg. 14 of 23

Figure 18: Major Earth Cutting Zones. Source: Surbana

BH-3

BH-2

BH-4

BH-9

BH-13

Min. Cut-depth (m) Max. Cut-depth (m)


Pg. 15 of 23

Figure 19: Major Earth Filling Zones. Source: Surbana

BH-11

BH-12

BH-5

BH-6

BH-7

BH-10

Max. Fill-depth (m) Min. Fill-depth (m)


Pg. 16 of 23

Three number of laboratory compaction tests were conducted for sandy-silt soils as part of the preliminary soil investigation. These test results show that the optimum moisture content of the soil is about 12 % and the maximum dry density is about 18.8 kN/m3. Based on this information, the suitability of the soil in the earth-cutting zone as earth-filling materials will be evaluated. The principal geotechnical consideration regarding earth-filling is to achieve the design bearing capacity of 75 kN/m2 to support the raft foundation for the terrace houses and semi-detached houses. The soils across the site consist primarily of clays which are observed to possess low to medium plasticity characteristics, with some rare high plasticity characteristics. The low to medium plasticity clays (CI, CL) are considered suitable for use as earth-filling materials as they show reasonably good compaction characteristics which can be compacted by the sheepsfoot roller and rubber-tyre roller. Cohesive soils are stiff and tightly bound together by molecular attraction. They are plastic when wet and can be moulded, but become very hard when dry. Appropriate water content, evenly distributed, is critical for proper compaction with adequate impact or pressure.

3.6 Use of high plasticity clays

The use of the high plasticity clays as fill materials should be avoided due to their potential to change volume (swell or contract) in response to changes in water content. The subsurface soil which generally consists of stiff to hard clays (CH, CV) have a moderate to high potential expansive nature for shrinkage and swelling when subjected to fluctuation in moisture content. These types of clayey soils may not be suitable for earth-filling for building and pavement areas but could be used as general earth-fill materials for open spaces and public green. Removal of potentially expansive soils will need to be considered in foundation design. Alternatively, in order to optimise the use of earth-cutting materials, general soil mixing / blending of various suitable and unsuitable soils could be allowed with proper handling. Mixing unsuitable soils with suitable soils requires an evaluation of the engineering properties of the mixed soil in order to determine the right mix ratios for the various types of materials. If the volume of unsuitable soils added to suitable soils is limited to about 25 to 40%, the final product is generally acceptable. During excavation, high plasticity clays may also form lumps of too high or too low water content. Therefore, the earthworks contractor will have to adjust the water content on site in order to achieve the optimum level to allow proper compaction. The compaction characteristics of a soil can be assessed by means of standard laboratory tests. It is recommended that suitable numbers of standard laboratory compaction tests be conducted to obtain the range of optimum moisture content and dry density of soil. However, the results of laboratory compaction tests are not directly applicable to field compaction because the compacting efforts in laboratory tests are different from that produced by field equipment. The results of laboratory compaction tests are used only as a guide for the preparation of earth works in the field. Field density tests can be carried out to verify the standard of compaction in earthworks, where dry density is being calculated from the measured values of bulk density and water content. A number of methods of measuring bulk density in the field are detailed in BS 1377 (Part 4). The desired level of compaction is best achieved by matching the soil type with its proper compaction method (refer to BS6031: 1981). Other factors, such as compactor specification and job site conditions, must be considered as well.


Pg. 17 of 23

S.NO BH No Soil Type

Thickenss

of Clay (m) SPT N Value

Compression

Modulus

(MPa)

Coefficient Vol

Compressibility

(m2/kN)

1 12 Clay 3 12 12 0.00008333

2 11 Clay 3 11 11 0.00009091

3 7 Clay 4.5 3 3 0.00033333

4 5 clay 3 3 3 0.00033333

5 6 clay 3 10 10 0.00010000

6 10 Clay 3 11 11 0.00009091

Overburden

pressure

(kPa)

Settlement

(m)

Overburden

pressure (kPa)

Settlement

(m)

Overburden

pressure

(kPa)

Settlement

(m)

90 0.023 270 0.068 630 0.158

90 0.025 270 0.074 630 0.172

90 0.135 270 0.405 630 0.945

90 0.090 270 0.270 630 0.630

90 0.027 270 0.081 630 0.189

90 0.025 270 0.074 630 0.172

Fill Height = 5m Fill Height = 15m Fill Height = 34m

S.NO BH No

1 12

2 11

3 7

4 5

5 6

6 10

4. Total Consolidation Settlement

4.1 Factors affecting ground settlement

The type of foundation proposed has taken into consideration bearing capacity failure and excessive settlement. For foundations on cohesive soils, the principal design criterion is typically the latter, which will include the control of expected settlements within the allowable tolerance. The estimation of total settlement over the service of life of the structure is also a major factor in the choice of foundation design.

Settlement of soils (consolidation) is a complicated natural phenomenon, which is influenced by a number of factors, including the nature and mineralogy of the soil, the soil particle arrangement, whether the soil is undisturbed or remoulded, its past stress history, the drainage conditions affecting the particular circumstances, etc. For land development works, the pre-development soil investigation should identify areas of risk such as organic soils, swampy areas, etc.

Settlement will also occur within earth fills due to the self-weight of the fill. The consolidation settlement and elastic compression of fill are a function of time, albeit of long or short duration, thus in some cases it may be necessary to allow a period of time to elapse from the placement of fill to the commencement of building construction.

4.2 Estimation of total settlement

As stated earlier, 6 nos. of boreholes (BH 5, 6, 7, 10, 11 & 12) are located within the proposed earth-filling zones. The consolidation settlement of clay layer in the earth-filling zones is estimated for fill heights of 5m, 15m and 34m. The estimated total consolidated settlements are shown in Table 1. The maximum estimated consolidation settlement of about 0.945m is found in BH-7 location for a fill height of 34m. A major part of the total settlement could take place as immediate settlement during the construction stage, leaving only an allowable amount of long term residual settlement.

Table 1: Estimated total consolidation settlement across the project site

Note: Assumed fill density as 18 kN/m3


Pg. 18 of 23

4.3 Proposed Mitigation Measures for Long Term Settlements

a) Proper Site Clearance and Compaction

Prior to carrying out earthworks, the areas to be cut, to receive fill, or to receive stockpile materials should be cleared and stripped of all debris, deleterious materials, organics and vegetation, and remnants resulting from site demolition. All trees and root system should be removed from their entirety. If unsuitable materials are encountered during the excavation / cutting, these materials should be removed in its entirety and replaced with well compacted engineered materials. All fill materials should be soil that is free of organic matter and other deleterious substance. Materials for use as engineered fill should not contain rocks or hard lumps greater than 150 mm in maximum dimension and should have at least 80 percent passing the 9.5 mm sieve and at least 5 percent passing the 0.075 mm sieve.

Fill material should be spread in uniform lifts not exceeding 300mm in un-compacted thickness where heavy equipment is used and not more than 100mm where light hand-operated compactors are used. Before compaction begins, the fill should be brought to a moisture content that will permit proper compaction by either aerating the material if it is too wet, or spraying the material with water if it is too dry. Each lift should be thoroughly mixed before compaction to provide a uniform distribution of water content. The fill material should be at water contents above the optimum water content prior to being placed on the fill surface for compaction. The compacted fill should achieve dry density of not less than 95 % of the maximum dry density at optimum moisture content as determined in the standard proctor laboratory tests. In general, the higher the degree compaction the higher will be shear strength and the lower will be the compressibility of the soil.

b) Preloading of filled ground

The most important factor in ensuring satisfactory performance of stable earth-filling is to limit post-construction differential settlements. The design and construction of fills should be such that the long term residual settlements are kept within acceptable limits. The degree of short term consolidation of the subsoil should be more than 90%, leaving less than 10% of the total settlement to take place over the long term. For raft foundation, it is recommended that post-construction settlement should not exceed 80 mm. Long term consolidation may take place over many years depending on the drainage conditions of the embankment or foundation. If necessary, preloading could be adopted as a ground improvement technique to accelerate the consolidation process of subsoil. It involves loading on the filled ground surface to induce a greater part of the ultimate settlement that the ground is expected to experience after construction.


Pg. 19 of 23

5. Slope Stability

The assessment of slope stability by consideration of geological features, measurement of soil strengths, groundwater conditions, and calculations of theoretical factors of safety, is a specialised task which requires adequate soil data and relevant design parameters. The problem is exacerbated by the need to consider the following:

i. The range of parameters assumed to be applicable, given the present state of stability of a slope.

ii. Presence of unique geological features iii. Present and future groundwater levels iv. Consequences of limitations on future site development. v. High intensity rainfall events.

Hence, it is important to identify possible indications of the future slope failure locations in the proposed development boundary.

5.1 Major slopes along Site boundary

Major slopes along the Site boundary are identified in Figure 20. The heights of the slopes vary from about 5 to 30m. They will be terraced at height intervals of not more than 4.5m according to the guidelines provided by the Town & Country Planning (TCP) department. A detailed analysis of these slopes is not possible at this stage due to the limited soil information within reasonable proximity of the slopes to give an accurate representation of the soil condition nearby.

5.2 Proposed Slope Monitoring Instrumentation Plan

Slope stability and landslide monitoring involves selecting certain parameters and observing how they change with time. The two most important parameters are groundwater levels and displacement. Slope displacement can be characterised by depth of failure plane(s), direction, magnitude, and rate. One or all of these variables may be monitored. Conventional slope monitoring utilises a single or combination of the following methods.

i. Water standpipes allow the determination of water levels; ii. Surveying fixed surface monuments, extensometers and inclinometers allow

determination of direction and rate of slope movement and depth and aerial extent of the failure plane;

iii. Extensometers provide an indication of displacement magnitude. iv. Manually-operated probe inclinometers are the most common means of long-term

monitoring of slopes. The location of the groundwater table is particularly important during stability and settlement analyses. High groundwater tables result in lower effective stress in the soil affecting both the shear strength characteristics or the soil and its consolidation behaviour under loading. It is important to identify the location of the groundwater table and determine the range in seasonal fluctuation.


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Figure 20: Major Slopes along Development Boundary. Source: Surbana


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6. Recommendations & Conclusion

6.1 Recommendation of additional soil investigation

The preliminary geotechnical report is based on limited borehole data provided by BEDB during the tender stage. It is recommended that additional soil investigation to be carried out to better define the soil condition of the proposed site. Additional exploration of the site would be required to obtain more detailed information concerning sub-surface conditions and to obtain samples for laboratory testing of classification and engineering properties in critical areas of concern such as

• Areas with unfavourable ground conditions

• Areas of major cut and fill

• Locations adjacent to existing major structures

There are no hard and fast rules for borehole spacing but generally 1 borehole per 5 hectare in a green field site and 250 m to 500m for the construction of roads and highways. Spacing can be increased or decreased depending on the ground conditions. From the preliminary site investigation, the soil strata across the site are highly heterogeneous. Hence it is recommended to carry out additional soil investigation at closer interval to obtain a more refined soil profile. The most commonly used field testing methods in sub-surface investigation are:

• Boreholes with standard penetration tests (SPT), and collection of disturbed and undisturbed soil samples for laboratory testing

• Cone penetration tests (CPT)

Figure 21 shows the proposed additional site investigation layout plan. 60 boreholes and 28 cone penetration tests are proposed across the Site. All the boreholes should be terminated within the original natural ground after confirmation of hard strata when 3 consecutive SPT N-values of 50 blows or more are obtained or when bedrock is encountered. The collected disturbed and undisturbed soils samples are used for laboratory tests. The types of laboratory tests commonly used to determine soil classification and engineering properties are listed below:

• Soil classification test o Particles Size Distribution o Atterberg limits o Moisture content o Unit weight o Specific gravity

• Engineering / Mechanical tests o One Dimensional Consolidation Test (Oedometer Test) o Unconfined Compression Test (UCT) o Unconsolidated Undrained Triaxial Test (UU), o Compaction test

The total stress strength parameter like undrained shear strength, Su is required for short term undrained stability analysis of embankment on cohesive soils and for foundation design

(e.g. footing) in cohesive soils. The effective strength parameters like c’ and φ’ are for long term stability analysis of foundation, embankment and slopes, particularly cut slopes.


Pg. 22 of 23

Consolidation parameters allow evaluation of deformation of the subsoil when there is a change of stress in the subsoil. The knowledge of groundwater level, groundwater pressure and potential flooding is important in soft clay as they will affect the effective stress of the subsoil and also the design. Water level observation in completed boreholes and existing wells (if any) should be taken daily during the ground investigation, particularly in the morning.

Figure 21: Proposed Additional Borehole and CPT Locations. Source: Surbana


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6.2 Conclusion

This preliminary geotechnical report is prepared based on the limited soil information provided during the tender stage, and is therefore could only provide some broad understanding of the soil conditions of the Site. For detailed analysis of long term settlement and slope stability, additional soil investigation has to be carried out as proposed above. A final geotechnical report will be prepared upon receiving the additional soil investigation report.

b4000 - preliminary geotechnical report (draft) 2010-11-1

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