Appendix F Geotechnical Investigation

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1 Appendix F Geotechnical Investigation

2 Cie Sucriere de Bel Ombre Ltee Residential Development at Bel Ombre Environmental Impact Assessment F1 Geotechnical Investigation This appendix includes the Factual Report on Trial Pits and Percolation Tests, undertaken by Water Research Co Ltd in October Issue 13 December 2012 J:\CURRENT PROJECT\2908 EIA BEL OMBRE PRECINCT\4.5 - REPORTS AND MEMOS\ENVIRONMENT\EIA (ISSUE, )\ EIA (ISSUE, ).DOCX Page F1

5 Contents 1 Introduction Introduction Scope of works 5 2 Desk study information Site location and topography Published geology 6 3 Description of field works Geotechnical investigation works Trial Pits In situ testing Percolation Test Laboratory testing Soil testing Moisture Content and Atterberg limits Particle size distribution Bulk and Dry Density 9 4 Results and ground conditions Identified soil profile Topsoil Weathered Basalt Groundwater Percolation Tests 11 5 Closing remarks 13 6 References 14 7 Figures 15 Appendices OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 3 of 19

6 Appendix A Appendix B Appendix C Location of Exploratory Holes Photographs of site works Exploratory log Appendix C-1 Trial pits Logs Appendix C-2 Photographs of Trial pits Appendix D In-Situ Test Results Appendix D-1 Percolation Test Results Appendix E Laboratory Schedule and Test Results OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 4 of 19

7 1 Introduction 1.1 Introduction In August 2012, Water Research Co Limited (Water Research) was instructed by Cie Sucrière de Bel Ombre to carry out logging of Trial Pits and perform Percolation Tests. The works are to support the evaluation of the subsurface ground on the proposed site and its suitability for drainfields and on-site disposal of wastewater. 1.2 Scope of works Water Research brief comprised the following items: Inspect, log and photograph Trial Pits, Carry out in situ percolation tests in Trial Pits, Factual reporting including logs, photographs and all relevant information obtained from the field works. This Factual Report presents the findings as per the brief described above. OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 5 of 19

8 2 Desk study information 2.1 Site location and topography The site under investigation is located at some 900m to the east of the village Bel Ombre, in the district of Savanne. The site is surrounded by sugarcane fields except to the south and west where it is bounded by the B9 Black River Savanne Coast Road and an IRS development, respectively (Figures 2.1 to 2.3). The site was covered with sugarcane at the time of the investigation. A natural water course runs from north to south (to the sea) on the east end of the site. 2.2 Published geology According to the Carte Geologique au 1:50,000 of Mauritius (Figure 2.4, Ref. 1), the site is located on weathered basalt and pyroclasts of the Intermediate volcanic series. The intermediate volcanic series are composed of basaltic flows overlying basalt agglomerate. The basalts are not very homogeneous in composition and are made up of fine grained olivine rocks, grey to black in colour. The very compact types are rather rare and are in general porous and often vesicular, the vesicles being sometimes infilled with a powdery bluish material. They are also slightly fissured and in some places the fissures and cavities are filled with light cream coloured clay (Ref. 2). Pyroclastic rocks are deposits formed by consolidation of fragments ejected during an eruption. The largest ejected masses of lava fall in and around the vent and become embedded in dust and ash. The smaller particles of ash and dust may be blown by wind and spread over large areas in layers; they may become hardened into rocks that are called tuffs. Tuffs include material less than 2mm in diameter (Ref. 2). According to the Soil Map of Mauritius (Figure 2.5, Ref. 3) the natural soil strata on the site area consists of very stony low humic latosols. Low Humic Latosols occur in the subhumid and lower rainfall zones from 1,000mm to 2,750mm annually, in areas within a distinct dry season. These are deep (over 100cm) to moderately deep soils (over 60cm) with good internal drainage. These soils have a weak to moderately strong structured A horizon varying from red to brown in colour over a red to reddish brown B horizon. Overall, the physical properties of these soils are good although in the drier areas they tend to be compact and somewhat sticky when wet (Ref. 2). OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 6 of 19

9 3 Description of field works 3.1 Geotechnical investigation works The site work was carried out between the 23 rd and 25 th August The scope and location of fieldworks was specified by the Client and was undertaken in accordance with the BS 5930 (1999) (Ref. 4). The field works included: Inspection and logging of excavated trial pits, In situ percolation tests The location of the Trial Pits is shown in Appendix A. The photographs of site works are presented in Appendix B. The following Sections present a general description of the works carried out. 3.2 Trial Pits The original scope of works comprised 6No. Trial Pits but at the end of the works a total of 9No. Trial Pits were excavated, all by the Contractor. TP 1 to TP 6 were in the original scope. Groundwater was encountered in TP 1 and TP 7 was excavated to replace it to be able to carry out percolation test. Groundwater was also encountered in TP 7. TP 8 and TP 9 were excavated to confirm groundwater levels. The depths of the trial pits varied between 2.0m and 3.4m below ground level (bgl) see site plans in Appendix A - an Engineer logged the Trial Pits from the surface. The logs and photos of the Trial Pits are presented in Appendices C-1 and C-2, respectively. A summary of the excavated trial pits is presented in Table 3.1. The Easting and Northings in Table 3.1 are from a hand held GPS. Table 3.1 Summary of Trial Pits TP No. Depth (m) E N Elevation (m amsl) Remarks TP Groundwater encountered TP TP TP TP TP TP To replace TP 1 TP To investigate water level OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 7 of 19

10 TP No. Depth (m) E N Elevation (m amsl) Remarks TP To investigate water level 3.3 In situ testing Percolation Test The percolation tests were implemented Trial Pits 2 to 6 with the purpose to determine the rate of infiltration of the ground. The specification of the Works requested the test to be performed on hand-excavated cubic pits of 300mm sides in accordance to BS 6297:2007 (Ref. 5). Following the excavation of the test pits at the bottom of the Trial Pits, water was added to the test pits and left to saturate overnight. Following the saturation period, the pits were quickly filled with water up to the rim of the pit and the reduction on water level with time recorded. The test is completed when the water column infiltrates to 25% of the initial water depth, or when 25% is not reached in 6hr, and the test is repeated three times. The percolation rate (Vp) is obtained by dividing the time in seconds for the water to seep away from 75% to 25% by 150mm (corresponding to the depth between 75% and 25% of the initial water depth). BS 6297 states that drainage field disposal should only be considered when percolation tests indicate average values of Vp between 15 and 100. BS 6297 also states that suitable soil conditions include well drained and well aerated subsoils, including sand, sandy and clay loams. Less suitable characteristics include fast and slow draining soils. Heavy clay soils might not provide sufficient dispersal characteristics to be suitable for drainage fields without a more extensive investigation (Ref. 5). 3.4 Laboratory testing The Client scheduled the geotechnical laboratory testing as per the schedule presented in Appendix E. The laboratory testing aimed at determining site parameters concerning the classification and compaction of the strata. The programme of laboratory testing was carried out in accordance with BS1377 (1990) (Ref. 6) at the Geotechnical Services laboratory. The test schedule included: Moisture Content Determination Atterberg limits Particle Size Distribution (wet sieving) Bulk and Dry Density OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 8 of 19

11 3.5 Soil testing Moisture Content and Atterberg limits 6No. Moisture content determinations and 6No. Atterberg Limits tests were carried out using BS 1377: Part 2 recommended procedures Particle size distribution The wet sieving method is to be used in accordance with BS 1377: Part 2. 6No. tests were carried out; the results of sieving analysis and the grain size distribution semi-log curves are presented in Appendix E Bulk and Dry Density 6No. determination of the bulk and dry densities were carried out in accordance with BS 1377: Part 2. OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 9 of 19

12 4 Results and ground conditions 4.1 Identified soil profile The depth and thickness of the various strata encountered on Trial Pits as interpreted in the exploratory logs are summarised in Table 4.1. Table 4.1 Depth intervals for various encountered strata TP No. Topsoil CWB to HWB & HWB HWB to MWB TP TP (HW) TP TP TP TP TP TP TP (HW) The following definitions were considered for weathered basalts: Residual Soil: No recognisable rock texture. Surface layer contains humus and plant roots. Completely Weathered (CW) basalt: Rock completely decomposed by weathering in place but texture still recognisable. Can be excavated by hand. Highly Weathered (HW) basalt: Rock so weakened by weathering that fairly large pieces can be broken and crumbled in the hands. Sometimes recovered as core in careful rotary drilling. Moderately Weathered (MW) basalt: Considerable weathered throughout. Possessing some strength large pieces cannot be broken by hand, reasonable core recovery. Often limonite stained. Difficult to rip. Slightly Weathered (SW) basalt: Distinctly weathered through much of the rock fabric with slight limonite staining. Strength approaches that of the fresh rock. Requires explosive for excavation. Highly permeable open joints. OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 10 of 19

13 The following sections summarise the test results for each of the encountered strata. 4.2 Topsoil Topsoil was encountered in all Trial Pits as from the surface and reaching up to a depth of 0.5m. The stratum was generally described as soft to firm orangish brown gravelly high plasticity clay with frequent roots. Average thickness of this layer is 0.4m. 4.3 Weathered Basalt Completely to Highly Weathered Basalt was encountered in all Trial Pits, except TPs 2 and 9, as from 0.3m depth and reaching up to a maximum depth of 2.3m. The stratum was generally described as very weak to weak orangish pinkish brown basalt. Average thickness of this layer is 1.3m. Highly Weathered Basalt was encountered in TPs 2 and 9 as from 0.3m depth and reaching up to the maximum excavated depth in TP 9. The stratum was generally described as very weak to weak pinkish brown with black mottles vesicular with frequent amygdales. Highly to Moderately Weathered Basalt was encountered in all Trial Pits as from 1.2m depth and reaching up to the maximum explored depth. The stratum was generally described as weak to moderately weak orangish pinkish brownish grey with black discoloration vesicular with frequent amygdales. 4.4 Groundwater Groundwater was encountered in the Trial Pits in proximity of the natural water course located on the east end of the site, as detailed in Table 4.2. Table 4.2 Measurement of Groundwater level TP No. Depth to groundwater (m) TP TP TP TP Percolation Tests 5No. percolation tests were carried out as per BS 6297 on the Highly to Moderately Weathered Basalt. The percolation rates, calculated using the procedure described on BS6297 (2007), are presented in Table 4.3 as well as in Appendix D. Time vs. water level data and graphs are also presented in Appendix D. OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 11 of 19

14 Table 4.3 Summary of Percolation Rates Infiltration TP No. Depth Duration of Test 1 Test 2 Test 3 Average Test No. (m) Tests (min) Vp (s/mm) Va 1 TP , 105 & TP & TP , 75 & TP , 38 & TP OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 12 of 19

15 5 Closing remarks The Report presents factual information from the inspection and logging of excavated trial pits and performing and processing of in-situ percolation tests. The Works were carried out between the 23 rd and 25 th August The identified soil sequence included topsoil overlying Completely to Highly and Highly to Moderately Weathered Basalts. Groundwater was encountered in the Trial Pits situated in the proximity of the natural water course. 5No. Percolation Tests carried out in TP 2 to TP 6. The results are presented in this Report. We trust that the information contained in this report is satisfactory. Should you have any questions, please do not hesitate to contact this office. OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 13 of 19

16 6 References 1. L. Giorgi and S Borchiellini (1999), Les aquifères de l Ile Maurice (Map). 2. Proag, V. (1995), The geology and water resources of Mauritius, analysis, Mahatma Gandhi Institute. 3. Directorate of overseas surveys (UK) (1962), Soil Map of Mauritius, Public Works and Survey Department, Port Louis Mauritius. 4. British Standards (1999), BS 5930, Code of practice for site investigations. 5. British Standards (2007), BS 6297, Code of practice for the design and installation of drainage fields for use in wastewater treatment. 6. British Standards (1990), BS 1377, Methods of tests for soils for Civil Engineering purposes. OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 14 of 19

17 7 Figures Site Figure 2.1 Geographical Site Location OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 15 of 19

18 North Site Figure 2.2 Site Location OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 16 of 19

19 Site Figure 2.3 Aerial View of the Site OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 17 of 19

20 Site Figure 2.3 Location of the site on geological map OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 18 of 19

21 North Site Figure 2.4 Location of the site on soil map OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Page 19 of 19

22 Appendices OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Appendix

23 Appendix A Location of Exploratory Holes OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Appendix

26 Appendix C Exploratory log OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Appendix

27 Appendix C-1 Trial pits Logs OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Appendix

37 Appendix D In-Situ Test Results OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Appendix

38 Appendix D-1 Percolation Test Results OPG CSB Report for Trial Pits & Percolation Tests at Bel Ombre Appendix

39 INFILTRATION TEST No. 1 (Trial Pit No. 6) PROJECT : Geotechnical Investigations at Bel Ombre Pit Dimension mm: 400x400x350 CLIENT : Bel Ombre Depth of Trial Pit (m) : 2.80 Location : Bel Ombre Test Depth (m) : 2.80 Date : Initial depth of water: 300mm Filled in by : SM Checked by : NC Test Stratum Description: Highly to Moderately Weathered Basalt Time Time Water Level ( mm ) (s) (h,min,s) Test 1 Test 2 Test m28s m27s h3s h18s Test Results Test No. V P (s/mm) Test a = 0.40m Test b b = 2.8 m Test c = 0.35m Average a V p : Percolation Rate c V a : Average Percolation Rate Notes: i. Tests performed according to BS 5930 and BS ii. Percolation Rate (Vp) is the average time in seconds required for the water to drop 1mm. iii. Saturation overnight was carried out on test pits.

40 Infiltration Test TP6 BEL OMBRE Water Le evel (mm) Test 1 Test 2 Test Time (s)

41 INFILTRATION TEST No. 2 (Trial Pit No. 4) PROJECT : Geotechnical Investigations at Bel Ombre Pit Dimension mm: 400x430x400 CLIENT : Bel Ombre Depth of Trial Pit (m) : 3.20 Location : Bel Ombre Test Depth (m) : 3.20 Date : Initial depth of water: 300mm Filled in by : GL Checked by : NC Test Stratum Description: Highly to Moderately Weathered Basalt. Time Time Water Level ( mm ) (s) (h,min,s) Test 1 Test 2 Test m20s Test Results Test No. Test 1 V P (s/mm) a = 0.40m Test b b = 3.20m Test 3 c = 0.40m Average a V p : Percolation Rate c V a : Average Percolation Rate Notes: i. Tests performed according to BS 5930 and BS ii. Percolation Rate (Vp) is the average time in seconds required for the water to drop 1mm. iii. Saturation overnight was carried out on test pits.

42 Infiltration Test TP4 BEL OMBRE Water Le evel (mm) Test 1 Test 2 Test Time (s)

43 INFILTRATION TEST No. 3 (Trial Pit No. 3) PROJECT : Geotechnical Investigations at Bel Ombre Pit Dimension mm: 300x400x300 CLIENT : Bel Ombre Depth of Trial Pit (m) : 3.40 Location : Bel Ombre Test Depth (m) : 3.40 Date : Initial depth of water: 300mm Filled in by : GL Checked by : NC Test Stratum Description: Highly to Moderately Weathered Basalt. Time Time Water Level ( mm ) (s) (h,min,s) Test 1 Test 2 Test Test Results Test No. V P (s/mm) Test a = 0.30m Test b b = 3.40m Test c = 0.30m Average a V p : Percolation Rate c V a : Average Percolation Rate Notes: i. Tests performed according to BS 5930 and BS ii. Percolation Rate (Vp) is the average time in seconds required for the water to drop 1mm. iii. Saturation overnight was carried out on test pits.

44 Infiltration Test TP3 BEL OMBRE Water Le evel (mm) Test 1 Test 2 Test Time (s)

45 INFILTRATION TEST No. 4 (Trial Pit No. 5) PROJECT : Geotechnical Investigations at Bel Ombre Pit Dimension mm: 300x300x300 CLIENT : Bel Ombre Depth of Trial Pit (m) : 3.40 Location : Bel Ombre Test Depth (m) : 3.40 Date : Initial depth of water: 300mm Filled in by : BR Checked by : NC Test Stratum Description: Highly to Moderately Weathered Basalt Time Time Water Level ( mm ) (s) (h,min,s) Test 1 Test 2 Test m40s m40s m Test Results Test No. V P (s/mm) Test a = 0.30m Test b b = 3.40m Test c = 0.30m Average a V p : Percolation Rate c V a : Average Percolation Rate Notes: i. Tests performed according to BS 5930 and BS ii. Percolation Rate (Vp) is the average time in seconds required for the water to drop 1mm. iii. Saturation overnight was carried out on test pits.

46 Infiltration Test TP5 BEL OMBRE Water Le evel (mm) Test 1 Test 2 Test Time (s)

47 INFILTRATION TEST No. 5 (Trial Pit No. 2) PROJECT : Geotechnical Investigations at Bel Ombre Pit Dimension mm: 300x300x300 CLIENT : Bel Ombre Depth of Trial Pit (m) : 1.70 Location : Bel Ombre Test Depth (m) : 1.70 Date : Initial depth of water: 300mm Filled in by : GL Checked by : NC Test Stratum Description: Highly to Moderately Weathered Basalt. Time Time Water Level ( mm ) (s) (h,min,s) Test 1 Test 2 Test m38s Test Results Test No. V P (s/mm) Test a = 0.30m Test 2 b b = 1.70m Test 3 c = 0.30m Average a V p : Percolation Rate V a : Average Percolation Rate c Notes: i. Tests performed according to BS 5930 and BS ii. Percolation Rate (Vp) is the average time in seconds required for the water to drop 1mm. iii. Saturation overnight was carried out on test pits.

48 Infiltration Test TP2 BEL OMBRE Water Le evel (mm) Test 1 Test 2 Test Time (s)

[GEOTECHNICAL SURVEY]

COMPAGNIE DE BEAU VALLON LIMITEE SUBDIVISION of LAND AT RICHE EN EAU, ST HUBERT [GEOTECHNICAL SURVEY] Rue De La Canelle Ebene City Ebene Tel/Fax No: (230) 465-8154 Email: kaselor@orange.mu October 2015