Upgrading and reconstruction of part of existing Le Coco Beach hotel

Transcription

1 Annex D Upgrading and reconstruction of part of existing Le Coco Beach hotel SUN RESORTS (MAURITIUS) LTD Site investigation report JUNE 2009

2 REPORT ON SUBSOIL INVESTIGATION AT COCO BEACH HOTEL BELLE MARE Prepared for Sun International Ltd Prepared by Water Research Co Limited Page 1 of 34

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4 Executive Summary Water Research was instructed by SIP Project Managers to carry out geotechnical ground investigation at the Coco Beach Hotel in accordance to specification provided by WSP South Africa. The investigation comprised the formation of 12No. Trial Pits and field and laboratory tests. The aim of the works was to determine subsurface ground and groundwater conditions for the design of the proposed upgrading of the Coco Beach Hotel. The upgrading works include demolishing the existing structures and redeveloping the site with single-storey and double-storey structures. Upgrading and construction of access roads are also included in the proposed redevelopment works. The site can be divided on areas of basalt flow outcrops and areas of superficial sand deposits. The profile on areas not covered by rock outcrops consist of: i) topsoil, ii) undisturbed loose to medium dense Coral Sand and iii) loose to medium dense sand or firm very sandy clay fills. The fill represents the materials used for levelling of the site for the existing development. Clayey deposits in the form of Residual Soil and CWB are encountered on a limited spatial distribution and with a limited thickness. Highly to Slightly weathered basalts underlie the above mentioned strata. Groundwater was encountered in TPs 7 and 12 at an approximate depth of 2m, equivalent to +0.0m elevation. The geotechnical properties for the sandy deposits were discussed. On the bases of the soil characteristics and vertical loads expected, conventional shallow foundations (pads, strip footings or rafts) may be adopted as foundation solution for the development. These foundations are to be located on basaltic rock or loose to medium dense sand layers. The definition of the foundation bearing capacity and dimensions must consider the allowable bearing capacity and the settlement under the given bearing pressure. The bearing capacity and settlement calculations carried out indicated that depending on the foundation dimensions the allowable foundation net bearing capacity is controlled by both bearing capacity considerations and the adopted service limit state of 25mm. A general net bearing capacity of 165kPa can be considered for design. The bearing capacities were computed for vertical loading only; eccentric, overturning or inclined loading should be taken into account for detailed design. Settlements were computed for isolated foundations. Page 3 of 34

5 The site is generally covered by loose Top Soil, which is expected to be removed and not used for engineering purposes. Most of the excavated material consist of fine to medium sand and as such will be suitable for cut and fill as General Granular Fill. The sand deposits are considered suitable as a capping layer for the hardstanding areas; although as uniformly graded they can be classified as difficult to compact. The CBR values obtained for Residual Soil and Completely Weathered Basalt (both materials encountered with limited extend on the site) are considered suitable for subgrade but unsuitable as a capping layer. Any excavation will be most likely in medium dense sand and basalt rock with various degrees of weathering, with water ingress as from 2.0m bgl. Excavation on sand will generally be nonproblematic with conventional excavating plant and without ripping. Short term stable slopes on sand can be achieved with 1v:2.5h slope ratio. Residuial soil to Moderately Weathered basalt can be excavated as per loose to medium dense sand but some ripping may be required. Small boulders may be disposed of by the excavating plant. Large boulders would typically require preblasting or use of percussion hammers or chisels to facilitate excavation. Excavation on bedrock will require drilling and blasting. A Design Sulphate Class of DS-1 and ACEC concrete class AC- 1 can be used for buried concrete structures. The surface of the investigated access road was encountered in good condition. The access road investigated is on an area of rock outcrop on which, according to the observations on site, the road asphalt was installed on top of a layer of concrete used to level the rock outcrop surface. Page 4 of 34

6 Contents Executive Summary 3 1 Introduction Introduction Scope of works and Report format 7 2 Desk study information Site location and topography Published geology 8 3 Description of field and laboratory works Geotechnical investigation works Trial Pits In-situ testing Percolation test in Trial Pits Field Density Dynamic Cone Penetrometer Laboratory testing 11 4 Results and ground conditions Identified soil profile Topsoil Fill Material Coral Sand Residual Soil and CW Basalt Highly to Slightly Weathered Basalt 18 Page 5 of 34

7 4.7 Road structure Groundwater 20 5 Geotechnical engineering assessment Identified soil profile Fill Material (Sand and Clay) Coral Sand Shallow foundations Bearing capacity Settlements Further considerations on foundations General Earthworks Buried concrete 27 6 References 28 7 Figures 29 Appendix A Location of Trial Pits 43 Appendix B Photographs of site works 44 Appendix C Trial Pit details 45 Appendix C-1 Trial Pits logs 46 Appendix C-2 Photographs of Trial Pits 47 Appendix D Laboratory & In situ Test Results 48 Appendix D-1 Field Density Test Results 49 Appendix D-2 DCPT Results 50 Appendix D-3 Percolation Test Results 51 Appendix D-4 Laboratory Test Results 59 Page 6 of 38

8 1 Introduction 1.1 Introduction On 31 st July 2008, Water Research Co Limited (Water Research) was instructed by SIP Project Managers to carry out geotechnical ground investigation at the Coco Beach Hotel, Belle Mare. The Engineer of the project is WSP South Africa and the Client is Sun Resorts Limited. The geotechnical ground investigation aimed to determine subsurface ground and groundwater conditions for the design of foundations and other engineering works for the proposed upgrading of the Coco Beach Hotel. We understand that the upgrading works include demolishing the existing structures and redeveloping the site with single-storey and double-storey structures, some founded on engineering fill above existing ground level. Upgrading and construction of access roads are also included in the proposed redevelopment works. 1.2 Scope of works and Report format Water Research s brief comprised the following items: Formation of 12No. Trial Pits Field tests Laboratory tests Report on findings and general geotechnical evaluation. The field work was carried out between the 4 th and 6 th August Laboratory testing was completed on the 26 th September This Report is presented in the following format: Desk study information: including geological maps and plans Factual information: comprising description of fieldwork; exploratory hole logs and laboratory test results Geotechnical assessment: comprising profile definition; discussion on geotechnical parameters for foundation design; preliminary recommendations for selection of foundation solutions, including bearing capacity and settlements; and earthworks. Page 7 of 38

9 2 Desk study information 2.1 Site location and topography Le Coco Beach Resort is located at Belle Mare, approximately 5km east of the village of Centre de Flacq (Figures 2.1 to 2.3). The hotel complex has an approximate plan area of 335,000m 2 and is bounded to the north and east by the sea, to the west by the B59 Belle Mare-Palmar-Trou d Eau Douce Road and to the south by the Club Valtur Le Flamboyant Hotel. Access to the site is through the B59 Road. The Coco Beach Resort includes hotel facilities and landscaped areas with grass and trees and numerous rock outcrops. The area of the hotel is characterised by abrupt variations on elevation resulting from the lava flow outcrops; the general elevation of the existing ground ranges between 0m and +10m above mean sea level (amsl) (Figure 2.2). 2.2 Published geology According to the Carte Geologique au 1:50 000, Schema hydrogeologique of Mauritius, the site is located on superficial alluvium and elluvium (Figure 2.5, Ref. 1). Although Mauritius is overwhelmingly volcanic in nature, some sedimentary formations can be observed along the coast. These are related to the formation of coral reefs that have given rise to sandy beaches and sand dunes along more than 20% of the coastline (Ref. 2). According to the Soil Map of Mauritius the natural strata on the site area consists of regosols (coral sand) that occurs on deep unconsolidated coral around the coast line (Figure 2.4, Ref. 3). The encountered Regosols are characterised by an A horizon of dark brown sand or loamy sand formed by a mixture of coral sand and organic matter over a light grey, very pale brown coral sand. The soils are normally highly permeable (Ref. 4). According to the hydrogeological survey the project site is under the Aquifer IV, Aquifer of Nouvelles Decouvertes - Plaine des Roches/Midlands - Trou d Eau Douce (Figure 2.6, Ref. 1). Page 8 of 38

10 3 Description of field and laboratory works 3.1 Geotechnical investigation works The ground Investigation contract was awarded to Water Research on 31 st July 2008 with site works carried out between 4 th and 6 th August The Coco Beach Resort was operational at the time of the site investigation. The scope and location of fieldworks, the in situ testing, sampling and laboratory testing were specified by the Engineer and undertaken in general accordance with BS 5930 (1999) (Ref. 5). The site and laboratory works included: Formation of 12No. Trial Pits to a maximum of 3m depth or top of bedrock and including sampling In situ testing including in situ density tests, percolation tests and Dynamic Cone Penetration tests. Soil sampling for geotechnical laboratory testing at regular intervals within all strata. Sampling included small disturbed samples and large bulk samples. Geotechnical laboratory testing including: classification testing (Moisture Content, Atterberg Limits, Particle Size Distribution, maximum and minimum density), Small Shear Box, California Bearing Ratio (CBR), Standard Compaction and chemical testing. The location of the Trial Pits and DCP tests is shown on Appendix A. Appendix B presents the photograph of site activities. 3.2 Trial Pits 12No. Trial Pits, designated TP1 to TP12, were excavated using the bucket of a JCB 3CX type mechanical backhoe excavator to a maximum depth of 2.80m bgl; no shoring was required. The exposed subsoil was photographed, sampled and logged by a Geotechnical Engineer according to BS5930:1999 (Ref. 5) immediately after completion of the excavation. Small bulk and large bulk disturbed samples were taken at regular intervals for geotechnical testing. The encountered strata are described on the Trial Pit logs shown in Appendix C. Table 3.1 presents details of the Trial Pits. Page 9 of 38

11 Table 3.1 Summary of Trial Pits TP No. Depth (m) Elevations (m, amsl) TP TP TP TP TP TP TP TP TP TP TP TP In-situ testing Percolation test in Trial Pits Percolation (falling head) tests were performed to determine the rate of percolation of the ground in accordance to BS6297 (Ref. 7). The percolation result can be used to determine the rate at which surface water can be disposed of in soakaways for the design of effluent disposal or surface water drainage systems. The tests were carried out on the 5 th August The falling head percolation tests were carried out on hand excavated rectangular pits of 0.40m by 0.40m sides and 0.30m depth. The tests were performed at depths varying between 0.45m to 0.60m at the bottom of purpose excavated shallow pits adjacent to Trial Pits No. 2, 6, 7 and 12. Previous to the test the bottom of the percolation pit was covered with a 50mm thick layer of uniformly graded gravel. Water was then added and the percolation test pit was kept saturating until the time of the percolation test. Saturation overnight was not carried out since the water seeped away immediately after pouring. In order to start the test the percolation pit was quickly filled with water, once the water reached 250mm height the source of water was closed and the reduction on water level with time recorded. The test specification requires measuring the time taken for the water to reduce from 250mm to 0mm. In general, the water filtered fast on an average time of 10min and maximum of 20min. 3No. consecutive tests were carried out at each location. Page 10 of 38

12 The details, processing of results and the time vs. water level curves are presented in Appendix D-3. The soil percolation rate was obtained using the procedure described on BS6297 (1983) (Ref 7) Field Density Field Density tests were carried out between 0.40 and 0.60m depths in Trial Pits No. 2, 6, 7, 11 and 12. The sand replacement method in accordance with the requirements of BS1377: Part 9 (1990) (Ref. 6) was used by the Ministry of Public Infrastructure (MPI) personnel. The method is suitable for granular soils and involves the use of a sand-pouring cylinder. Firstly, a small hole is dug about 100mm in diameter and not more than 150mm in depth and the soil removed weighted. The volume of the hole is then determined by pouring sand into it using a pouring cylinder. The sand container is weighted before and after this operation and the mass of sand filling the hole determined. Since the density of the sand is known, the volume of the hole can be determined and hence the bulk density of the in situ soil. The test results are presented in Appendix D Dynamic Cone Penetrometer TRRL type (Refs. 8 and 9) dynamic probing tests were carried along the main access road to the hotel at the locations shown in Appendix A. The MPI was contracted to carry out these tests. Dynamic probing involves driving a 20mm diameter solid cone (60 o angles) into the ground using repeated blows of a hammer with a mass of 8kg, falling a distance of 575mm. Typically, the rate of driving is between 15 to 30 blows per minute. As the cone is driven into the ground, the number of blows for each 100mm penetration is recorded. The penetration resistance provides a measure from which the California Bearing Ratio (CBR) can be calculated using the following formula: Log 10 (CBR) = log 10 (mm/blow) The details of the tests, the penetration vs blows and CBR results are presented in Appendix D-2. The existing asphalt and concrete road surfaces at the location of the tests were removed previous to the execution of the tests. 3.4 Laboratory testing The scope of laboratory testing aimed to determine project wide parameters concerning the strength, compressibility, compaction and chemical characteristics of the ground to enable the design of foundations. The laboratory testing was carried out in accordance with BS1377 (1990) (Ref. 6) at the University of Mauritius laboratories. The laboratory work comprised: 12No. moisture content and 2No. Atterberg limits 5No. Particle Density (Specific Gravity) by gas jar or Pyknometer 7No. Particle size (7No. wet sieving and 2No. Sedimentation) Page 11 of 38

13 7No. Dry density/moisture content compaction 4No. maximum and minimum dry density/moisture content 8No. California Bearing Ratio (CBR) 5No. small shear box 4No. Sulphate Content, 4No. Total Sulphate Content, 4No. Chloride Content, 4No. Carbonate Content and 4No. ph value. The complete set of laboratory results is given in Appendix D-4. Page 12 of 38

14 4 Results and ground conditions This Chapter summarises the in situ and laboratory test results. The geotechnical interpretation is presented in Chapter 5. The complete set of test results is presented on Appendix D Identified soil profile The depth and thickness of the various strata as interpreted in the exploratory holes are summarised in Table 4.1. Table 4.1 Depth intervals (in m) for encountered strata TP No. Topsoil Fill Material (Sand) Coral Sand Weathered Basalt TP TP TP TP TP (RS) TP Clay TP TP Clay TP (CWB) TP TP TP Clay BS5930 (1999) was adopted for the engineering classification of weathered basalt (Ref. 5). The following definitions were considered for weathered basalt: Residual Soil (RS): 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. Unsuitable for foundations of large structures. Page 13 of 38

15 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. Fairly good foundation material and may be suitable for pavement construction. 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. The following sections summarise the test results for each of the encountered strata. 4.2 Topsoil Topsoil was encountered in all Trial Pits except TP7 and TP12. Top Soil was encountered as loose dark brown clayey sand with many roots in typical thicknesses varying between 0.15m and 0.25m across the site. No testing was carried on Topsoil as it is unlikely to be used for engineering purposes. 4.3 Fill Material Fill Material is encountered as loose to medium dense light brown to dark cream gravely clayey medium to fine sand with roots and cobbles this is termed Sand fill. Sand fill thickness varies between 0.20m and 2.6m across the site but is typically 0.4m thick. Fill material is also found in TPs 6, 8 and 12 but as firm yellowish brown sandy high plasticity clay with cobbles and boulders this is termed Clay fill. Clay fill thickness varies between 0.15m to 1.67m bgl. Both fill materials represent the levelling of the natural ground (most likely lava flows) for the development of the existing hotel. 3No. moisture content determinations carried out in sand fill indicated values between 4% and 10%, with an average of 7% (Fig. 4.1). 2No. moisture content determinations on clay fill indicated values of 22% and 31%, with an average of 26% (Fig. 4.1). 1No. Atterberg Limit carried out in clay fill indicated Liquid Limit (LL) of 54% and Plastic Limit (PL) of 45%, for a Plasticity Index (PI) of 9%. 1No. particle density measurement carried out in clay Fill (TP 8 at 1.60m depth) gave a value of No. particle density measurements carried out in sand fill indicated values of 2.67 (TP 10 at 1.4m) and 2.70 (TP 11 at 0.6m) 1No. PSD test was carried out in clay fill indicating percentages as follows: 45% gravel, 33% sand and 22% fine particles (Fig. 4.2). 1No. PSD test was carried out in sand fill indicating the following composition: 45% gravel, 51% sand and 4% fine particles (Fig. 4.2). 1No. percolation test carried out at 0.5m depth on TP6 indicated an average Percolation Rate (V a ) of 0.07s/mm. Page 14 of 38

16 1No. Chemical test set carried out as per BS1377-Part 3 (1990) (Ref. 6) on fill clay material from TP 6 produced the results shown on Table 4.3. Table 4.3 Summary of physical/chemical test results Parameter Results ph 7.60 Water-soluble sulphates (as SO 3 ) 0.02% (0.07g/l) Total Sulphate Content (as SO 3 ) 0.29% Chloride Content as Cl % Carbonate Content (as CO 2 ) 18% 2No. Proctor (vibrating hammer) dry density/moisture content relationship tests carried out in sand fill indicated maximum dry unit weight of 15.82kN/m 3 and 16.2kN/m 3 with optimum moisture content values of 25% and 23% respectively. 1No. Proctor test on clay fill indicated maximum dry unit weight of 14.54kN/m 3 with optimum moisture content of 31%. 1No. Maximum and minimum dry densities test indicated a minimum dry density of 13.2kN/m 3 and a maximum dry density of 15.6kN/m 3 this result is considered representative for both type of fill encountered. 2No. Field Dry Density tests were carried out on sand fill giving density values of kn/m 3 (Relative Density (Dr) of 29%) and 13.81kN/m 3 (Dr = 29%) at 6% and 10% moisture contents, respectively. The results of 2No. unsoaked and 1No. soaked CBR test carried out on sand fill and 1No. unsoaked carried out in clay fill material are shown in Table 4.2. Appendix D-4 includes the detailed results. Table 4.2 CBR tests results on Fill material Dry unit weight kn/m 3 Relative Density (Dr) % CBR Value (%) (min) Un-soaked - TP 4 at 0.3m Sand Fill Un-soaked - TP 11 at 0.6m Sand Fill Soaked - TP 11 at 0.6m Sand Fill Un-soaked - TP 6 at 1.0m Clay Fill Page 15 of 38

17 2No. small direct shear test carried out in fill material (sand) resulted on angles of shearing resistance of 36 and 38.5, for relative densities of 11% (dry unit weight of kn/m 3 ) and 21% (13.64kN/m 3 ), respectively. 4.4 Coral Sand The term Coral Sand is used in this Report for undisturbed natural sand. The Coral sand was generally encountered as loose to medium dense whitish cream medium to fine sand of shells and corals. The identified thickness of Coral sand varied between 1.80m and 2.35m across the site, for an average thickness of 2.1m. 6No. moisture content determinations carried out indicated values between 4% and 15%, with an average of 7% (Fig. 4.1). 4No. PSD indicate a material with 3% to 23% gravel, 72% to 97% sand and 0% to 5% fine particles (Fig. 4.2). 3no. percolation tests carried out indicated average Percolation Rates, V a of 0.01s/mm and 0.05s/mm. The results are summarised in Table No. particle density measurement carried out in coral sand indicated a value of Table 4.5 Summary of Percolation Rates Lot No. Test Depth (m) Percolation Rate (s/mm) No. Chemical test suites were carried out as per BS1377-Part 3 (1990) (Ref. 6) on Coral sand from TP2 and TP7 given the result shown on Table 4.6. Table 4.6 Summary of physical/chemical test results Parameter TP 2 TP 7 ph Water-soluble sulphates (as SO 3 ) 0.00% (0.01g/l) 0.01% (0.04g/l) Total Sulphate Content (as SO 3 ) 0.52% 0.63% Chloride Content as Cl % 0.02% Carbonate Content (as CO 2 ) 38% 37% Page 16 of 38

18 3No. Maximum and Minimum dry densities tests indicated average maximum and minimum values of 15.55kN/m 3 and 12.03kN/m 3, respectively. 3No. Field Dry Density tests gave values of 13.57kN/m 3 (Dr=53%), 12.91kN/m 3 (Dr=36%) and 13.01kN/m 3 (Dr = 39%) at 9%, 11% and 20% moisture content, respectively. The results of 2No. un-soaked and 1No. soaked CBR test carried out in Coral sand are shown in Table 4.4. Table 4.4 CBR tests results on Coral Sand Dry unit weight kn/m 3 Relative Density (Dr) % CBR Value (%) (min) Un-soaked - TP 1 at 1.5m Coral Sand Un-soaked - TP 2 at 0.8m Coral Sand Soaked - TP 7 at 1.5m Coral Sand No. small direct shear tests carried out in Coral sand resulted on angles of shearing resistance of 35.5, 37 and 39.5 for Dr = 50% (dry unit weight 13.43kN/m 3 ), Dr = 49% (13.42kN/m 3 ) and Dr = 49% (13.39kN/m 3 ), respectively. 4.5 Residual Soil and CW Basalt Residual Soil (RS) and Completely Weathered Basalt (CWB) were encountered in only two Trial Pits as thin layers of high plasticity gravely clay overlying weathered basalts (bedrock). RS was encountered on TP5 as firm yellowish brown high plasticity gravely clay with thickness of 0.30m. CWB was encountered in TP9 firm yellow mottled dark brown high plasticity gravely clay with boulders and with a thickness of 0.7m. 1No. moisture content determination indicated a value of 13% (Fig. 4.1). 1No. Atterberg Limit showed LL of 34% and the sample was recorded as non-plastic. 1No. PSD carried out in CWB indicates a material with 4% cobbles, 53% gravel, 34% sand and 9% fine particles (Fig. 4.2). 1No Chemical tests suite was carried out as per BS1377-Part 3 (1990) (Ref. 6) on a sample from TP9. Table 4.7 shows the range of measured values of the parameters. Page 17 of 38

19 Table 4.7 Summary of physical/chemical test results Parameter Results ph 7.90 Water-soluble sulphates (as SO 3 ) 0.01% (0.03g/l) Total Sulphate Content (as SO 3 ) 0.46% Chloride Content as Cl % Carbonate Content (as CO 2 ) 33% 1No. Proctor Test carried out indicated Maximum Dry Unit weight of 16.67kN/m 3 with optimum moisture content of 21%. The result of 1No. un-soaked CBR carried out on CWB indicated a min CBR of 12.5%, at a moisture content of 24%. 4.6 Highly to Slightly Weathered Basalt Where identified, the Moderately and Slightly Weathered Basalts was encountered at depths varying between 0.8m and 2.0m bgl, to a proven maximum thickness of 0.3m. The depth of exploration was limited by the excavatability of the basaltic rock encountered. Highly to moderately weathered basalt was generally described as moderately weak to moderately strong dark grey with brown discolorations and vesicular. Moderately to Slightly Weathered basalt was generally described as moderately strong to strong dark grey with horizons of slightly yellowish brown high plasticity clay and vesicular. 4.7 Road structure The pavement support layers of the main access road were investigated using the TRRL type dynamic probing test. The number of blows for each 100mm penetration and the interpreted CBR are summarised in Table 4.8 and presented in Appendix D-2. Page 18 of 38

20 Table 4.8 Summary of DCPT results Location. ΣBlows Depth (mm) CBR Remarks Starting 50mm of pavement drilled with chisel and hammer mm drilled without reaching soil base. Test carried out outside pavement area mm drilled without reaching soil base. Test carried out outside pavement area. 4 Concrete surfacing found. No test carried out Starting 50mm drilled in asphalt with chisel and hammer Starting 50mm drilled in asphalt with chisel and hammer Page 19 of 38

21 4.8 Groundwater Groundwater was encountered in TP7 and TP12 at depth of 2.02m and 2.1m bgl, corresponding to elevations 0.2m and -0.1m, respectively. Page 20 of 38

22 5 Geotechnical engineering assessment Based on the ground conditions and characteristics of the proposed development it is envisaged that shallow foundations will be appropriate foundation solution for the expected structures. The following sections discuss geotechnical soil parameters for design and other foundations and earthworks topics. Some of the soil properties presented below are obtained from well established empirical correlations, which are implemented assuming a particular response/behaviour for the strata evaluated. Due judgement should be exercised when the indicated properties are used for design. 5.1 Identified soil profile The site can be divided on areas of basalt flow outcrops and areas of superficial sand deposits; Figure 5.1 shows identified areas of rock outcrops. In general the Trial Pits confirmed the strata sequence described in the published geology. In areas not covered by rock outcrops, the identified soil profile consists of: clayey sand as topsoil undisturbed loose to medium dense fine to medium Coral Sand. This layer is encountered overlying the bedrock mostly on the east side of the site. Loose to medium dense sandy or firm very sandy clay fills covering the rock outcrops on the west side of the development. These strata show occasional cobbles/boulders of weathered basalt. The fill represents the materials used for levelling of the site for the existing development. Weathered basalts. Clayey deposits in the form of Residual Soil and CWB are encountered on a limited spatial distribution and with a limited thickness. Highly to Slightly weathered basalts underlie the above mentioned strata. Bedrock was encountered on Trial Pits 4, 5, 6, 8, 9 and 11 to depth varying between 0.8m and 2.0m; these TPs covered the north, west and centre of the site. Bedrock was not encountered on TPs 1, 2, 3, 7, 10 and 12 located on the south and east of the site (TPs final depths varying between 2.5m and 2.8m). Groundwater was encountered in TPs 12 and 7 at an approximate depth of 2m, equivalent to +0.0m elevation. The encountered strata are described on the Trial Pit logs shown in Appendix C-1. The depth and thickness of the various strata as interpreted for design are shown in the geological cross sections of Figures 5.2 to 5.4. The soil strata that will control the shallow foundation design are the loose to medium dense sand layers; their geotechnical properties are discussed below. The properties of Weathered Basalt are not discussed in detail as its depth and strength indicate that it is unlikely to deteriorate the performance of the shallow foundations. Page 21 of 38

23 5.2 Fill Material (Sand and Clay) The following properties can be considered for design: In situ bulk density = 15.1kN/m 3 Both fill materials showed dominant gravel and sand fractions (78% for clay fill and 96% for sand fill). The clay component in the Clay fill is classified as high plasticity silt. Particle density for Clay fill of 2.72 and for Sand Fill 2.68, Average Percolation Rate (V a ) of 0.07s/mm. Average Field Dry Density of 13.82kN/m 3 (Dr = 29%) and maximum and minimum dry densities of 13.2kN/m 3 and 15.6kN/m 3, respectively. These data confirm the loose to medium dense state of the in situ strata. CBR values of at least 21% for sand fill. CBR of at least 2.9% for clay fill. Maximum Proctor dry densities with vibrating hammer in sand fill are 15.82kN/m 3 and 16.2kN/m 3. Maximum dry density for clay fill is 14.54kN/m 3 with optimum moisture content of 31%. An angle of shearing resistance of 32 can be used for design based on the loose to medium dense density of the strata and the results of direct shear tests. Stiffness for the Remediation fill can be computed following Stroud 1989 (Ref. 15), who indicates the following function: / E = N q q u where E / is the Young s modulus in MN/m 2, N is the SPT blow count, q is the foundation applied load and q u is the ultimate bearing stress. For q/q u ratio of 3; we have approximately E / = N. For loose to medium dense sand, and accounting for overburden correction, an N=10 can be considered. Thus E / for the fill can be estimated as 10MN/m 2 (i.e. 10MPa). 5.3 Coral Sand The following properties can be considered for design: In situ bulk density = 15.1kN/m 3 Coral sand contains 72% to 97% sand size materials with 0% to 5% fine particles Particle density of 2.71 Average Percolation Rates (V a ) between 0.01s/mm and 0.05s/mm. Field Dry Density varies between 12.91kN/m 3 (Dr=36%) and 13.57kN/m 3 (Dr=53%). Average maximum and minimum dry densities tests indicated values of 15.55kN/m 3 and 12.03kN/m 3, respectively. The results confirmed the loose to medium dense condition of the stratum. CBR values of at least 26.8% for sand at Dr = 56%. An angle of shearing resistance of 32 can be used for design based on the loose to medium dense density of the strata and the results of direct shear tests. Page 22 of 38

24 Stiffness of 10MN/m 2 (i.e. 10MPa) can be associated with the properties identified. 5.4 Shallow foundations On the bases of the soil characteristics and vertical loads expected, conventional shallow foundations (pads, strip footings or rafts) may be adopted as foundation solution for the development. Raft foundations may be necessary for large loads for which spreading of bearing pressure is required, for structures with settlement restrictions or to reduce the effect of lateral forces or overturning moments. Footings not located on areas of shallow basaltic rock can be founded within the loose medium dense sand layer. Soils of this description provide a good bearing layer for shallow lightly loaded foundations, both in terms of bearing capacity and settlement. The observed weathered basalt bedrock is expected to be a sound founding material or, where underlying the sand strata, providing a rigid boundary. Therefore, the soil profile for foundations on the sandy layers can be idealised in two zones depending on the depth to bedrock, including: i) north, west and centre zones consisting of sand overlying bedrock to a depth where bearing capacity is influenced (less than 2m depth), and ii) east and south zones consisting purely of sands or where the bedrock depth is such that rock does not influence bearing capacity calculations Bearing capacity Bearing capacity was calculated for square, rectangular and strip footings of various dimensions. Following the above discussed, the analysis were carried out for footings on infinite and finite sand strata. The lower bound bearing capacity value is for a strip footing on infinite homogenous sand layer as shape factors for square or circular footing and the proximity of rigid bedrock result in higher computed bearing capacities. The encountered strata are granular and thus drained bearing capacity applies. Table 5.1 shows estimates of net allowable bearing capacity for single strip footings and pads of various dimensions founded 1.0m and 0.6m bgl. Bearing capacities were calculated using the Brinch-Hansen coefficients, a factor of safety of 3.0 and the recommended angle of shearing resistance and ground water level. Finite sand layer analyses were carried out through bearing capacity factors representing the condition of limited-depth sand resting over material of infinite stiffness (Refs. 10 and 11). The bearing capacities are computed for vertical loading only; eccentric, overturning or inclined loading should be taken into account for detailed design. Page 23 of 38

25 Table 5.1 Net allowable bearing capacity for strip footings founded on Medium Dense Sand Footing dimensions (m) Bedrock Depth (m) Foundation depth (m) 1m Strip m Strip m Strip m Strip x x x x x x x x x x Net Bearing Capacity (kpa) The results indicate that the influence of the rigid bedrock in the bearing capacity calculated is negligible for bedrock depths larger than the width of the foundation strictly this depth is considered as B The results also show the large increase in bearing capacity when the bedrock is close to the foundation level Settlements Settlements for isolated individual strip and pad foundations (i.e. ignoring any additional settlement due to interactions with other adjacent foundations) are shown on Table 5.2 for a series of nominal stresses. The settlement of the cohesionless material was calculated using the SPT N based Burland and Burbidge (1985) method (Ref. 12). The value of SPT N for analysis was computed from the design angle of shearing resistance (32 o ). The equivalent uncorrected SPT N value used for analysis was 10. Table 5.2 Settlement for footings founded on sand Footing dimension Founding depth Depth to Bedrock Stress Settlement (m) (m) (m) (kpa) Immid. 30 years 1m Strip Infinite Infinite x Infinite Infinite x Infinite Infinite x Infinite Page 24 of 38

26 The settlements indicated as Immediate should take place as the load is applied. The 30 years settlement is related to creep on the sand. 5.5 Further considerations on foundations CIRIA 27 (Ref. 16) and Tomlinson (Ref. 13) recommend Terzaghi and Peck criterion that on sands the differential settlement is unlikely to exceed 75% of the maximum movement, and since most ordinary structures can withstand 20mm of settlement between adjacent columns a limiting maximum total settlement of 25mm was postulated. Raft foundation can usually tolerate somewhat greater total settlements and the limiting maximum settlement was increased to 50mm. Skempton and MacDonald concluded that for a limiting angle of distortion of 1 in 500 the limiting maximum differential settlement is about 25mm, the limiting total settlement is 40mm for isolated foundations, and 40 to 65mm for raft foundations. Studies have shown that buildings on sands rarely settle by more than 50mm and in the majority of cases the settlement is of the order of 25mm or less. Differential settlements between independent foundations are of greater significant to the stability of the superstructure than the magnitude of the total settlement. In the encountered conditions differential settlement can arise from variations on the thickness of overburden. The definition of the foundation bearing capacity and dimensions need to be based on checking the allowable bearing capacity and the settlement under the given bearing pressure. The bearing capacity and settlement calculations shown above indicate that depending on the foundation dimensions the allowable net bearing capacity is controlled by both bearing capacity considerations or the adopted service limit state of 25mm. A general net bearing capacity of 165kPa can be considered for the site. The following issues should be considered during foundation design and construction: During construction an inspection at founding level is recommended ensuring suitable quality and consistency. Hard and soft pockets such as pockets of clay and boulders might need to be removed and backfilled in with suitable fill. Overexcavation and re-levelling using compacted fill may reduce the settlement and increase bearing capacity. Fill proposed to bear footings should be compacted. The detailed load conditions, including lateral or overturning forces acting on the foundations, should be taken into consideration for the detailed design of the proposed foundations. The detailed foundation design should take into account any potential differential settlement, including the footing group effects. Page 25 of 38

27 5.6 General Earthworks Re-levelling, comprising cut and fill activities, for hardstanding areas, elevated founding level or access roads are likely to be carried out as part of the project. The site is generally covered by soft/loose Top Soil, which is expected to be removed and not used for engineering purposes. Because the encountered soils are relatively pervious even when compacted, they are not affected significantly by their water content during the compaction process. Consequently, as observed in the laboratory results, the peaked curved relationship between dry density and water content (Proctor curve) that is characteristic of cohesive soils is ill defined for the encountered sands. For these soils, the normally used compaction criterion is Relative Density (Dr). As the encountered sand strata are uniformly graded they can be classified as difficult to compact. Most of the excavated material consist of fine to medium sand and as such will be suitable for cut and fill as General Granular Fill - according to the British Specification for Highways Works. The geotechnical properties of the identified sand deposits (i.e. loose to medium dense sand) are considered suitable as a capping layer for the hardstanding areas. Capping with a laboratory CBR of at least 15% should provide an adequate platform for construction of the sub-base when compacted to the appropriate thickness. For reference, the Highways Agency in England (Ref. 14) indicates a simple means of assessing the equilibrium in-service (i.e. long term) CBR of the subgrades. The CBR values for Sand (poorly graded), sand (well graded) and Sandy Gravel (well graded) are 20, 40 and 60, respectively. The CBR values obtained for Residual Soil and Completely Weathered Basalt (both materials encountered with limited extend on the site) are considered suitable for subgrade but unsuitable as a capping layer. Any excavation will most likely be in medium dense sand and basalt rock with various degrees of weathering, with water ingress as from 2.0m bgl. Excavation on medium dense sand will generally be non-problematic with conventional excavating plant and without ripping. The choice of plant for bulk excavation is largely determined by the quantity and by the length of haul to the disposal point. Short term stable slopes on sand can be achieved with 1v:2.5h slope ratio. Highly to Moderately Weathered basalt can be excavated as per loose to medium dense sand but some ripping may be required for layers showing lower degree of weathering. Small boulders may well be disposed of by the excavating plant. Large boulders of moderately to slightly weathered basalt may not be rippable and other means of excavation or blasting will be required. These larger boulders would typically require pre-blasting or use of percussion hammers or chisels to facilitate excavation. Excavation on bedrock will require drilling and blasting. When foundation trenches are excavated, the surface of the sound rock may be found to be highly irregular, thus necessitating deep excavation in pockets in a narrow trench to reach a suitable stratum. Variations on the bedrock surface can be smoothed by excavating the rock with final levelling applied with concrete blinding. Page 26 of 38

28 The surface of the investigated access road was encountered in good condition. Note that the access road is on an area of rock outcrop on which, according to the observations on site, the road asphalt may have been installed on top of a layer of concrete used most likely to level the rock outcrop surface. This condition can be associated with DCPT tests No. 2, 3 and 4 for which test was not carried on the access road due to the hard surfaces encountered. High CBR values were determined on all the other DCPT tests. 5.7 Buried concrete An aggressive Chemical Environment for Concrete (ACEC) site classification has been carried out based on sulphate content and ph tests on soil samples and following the UK Building Research Establishment (BRE) Special Digest SD1, Concrete in Aggressive Ground (2005) (Ref. 17). Based on the test results available, a Design Sulphate Class of DS-1 and ACEC concrete class AC-1 can be used for buried concrete structures. Page 27 of 38

29 6 References 1. L. Giorgi and S Borchiellini (1999),Ile Maurice Carte Geologique et Hydrogeologique (1999) 2. Prem Saddul (2002), Mauritius, A geomorphological analysis, Mahatma Gandhi Institute. pp Directorate of overseas surveys (UK) (1962), Soil Map of Mauritius, Public Works and Survey Department, Port Louis Mauritius. 4. Proag, V. (1995), The geology and water resources of Mauritius, analysis, Mahatma Gandhi Institute. 5. British Standards (1999), BS 5930, Code of practice for site investigations. 6. British Standards (1990), BS 1377, Methods of tests for soils for Civil Engineering purposes. 7. British Standards (1983), BS6297, Code of practice for design and installation of small sewage treatment works and cesspools. 8. Transport and Road Research Laboratory (1986), Operation instructions for the TRRL dynamic cone penetrometer, TRRL Information Note. UK. 9. Frank Graham Geotechnical Engineers (1988), The use of continuous dynamic probing in ground investigation. 10. Meyerhof, G. G. (1974), Ultimate bearing capacity of footings on Sand Layer overlying clay. 11. Winterkorn, H. F. and Fang H. Y. (1995), Foundation Engineering Handbook. 12. Burland, JB and Burbidge, MC (1985), Settlement of foundations on sand and gravel. Proc Instn Civ Engrs, part Tomlinson (2001), Foundation Design and Construction, Seventh Edition, Pearson Education Limited. 14. Highways Agency (1994), Design Manual for Roads and Bridges, HD25/94 Part Stroud, M A (1989), The Standard Penetration Test its application and interpretation, Penetration Testing in the UK, Thomas Telford 16. Padfield, C. J. and Sharrock, M. J. (1983), Settlement of structures on clay soils, CIRIA Special Publication BRE (2005), Concrete in aggressive ground, Special Digest 1 Part C Assessing the aggressive chemical environment. Page 28 of 38

30 7 Figures Site Figure 2.1 General location of the site Page 29 of 38

31 Site Figure 2.2 Site Location Page 30 of 38

32 Site Figure 2.3 Aerial photo of site location Page 31 of 38

33 Site Figure 2.4 Location of the site on soils map Page 32 of 38

34 Site Figure 2.5 Location of the site on geological map Page 33 of 38

35 Site Figure 2.6 Location of the site on the Aquifer map Page 34 of 38

36 Depth (m) Moisture content (%) Beach sand Fill material CWB Coco Beach Hotel at Belle Mare Beach Sand, CWB and fill material Moisture Content Date: Sep 2008 Job: OPG 8148 SUN Figure: 4.1 Page 35 of 38

37 Depth (m) Particles passing sizes (%) Silt size Sand size Gravel size Cobbles Clay Size Coco Beach Hotel at Belle Mare Beach Sand, Fill Material and CWB Particle size distributions Date: Sep 2008 Job: OPG 8148 SUN Figure: 4.2 Page 36 of 38

38 Relative density (%) Shear Angle, φ ( ) Fill Material Coral Sand Coco Beach Hotel at Belle Mare Fill Material and Coral Sand Relative density v/s Shear Angle φ Date: Sep 2008 Job: OPG 8148 SUN Figure: 4.3 Page 37 of 38

39 Figure 5.1 Location of basalt flow outcrops and sand deposits Page 38 of 38

40

41

42

43 Appendix A Location of Trial Pits Appendix

44

45 Appendix B Photographs of site works Appendix

46 Appendix C Trial Pit details Appendix

47 Appendix C-1 Trial Pits logs Appendix

48 Appendix C-2 Photographs of Trial Pits Appendix

49 Appendix D Laboratory & In situ Test Results Appendix

50 Appendix D-1 Field Density Test Results Appendix

51 Appendix D-2 DCPT Results Appendix

52 Appendix D-3 Percolation Test Results INFILTRATION TEST No. 1 (Trial Pit No. 2) PROJECT : Coco Beach Hotel Pit Dimension mm: 400x400x300 CLIENT : Sun International Ltd Depth of Trial Pit (m) : 2.6 Location : Belle Mare Test Depth (m) : 0.45 Date : 5/8/2008 Initial depth of water: 250mm Filled in by : NC Checked by : E.S Test Stratum Description: Loose whitish cream fine CORAL SAND. Time Water Level ( mm ) (min) Test 1 Test 2 Test Test Results Test No. V P (s/mm) Test a = 0.4m Test b b = 0.45 m Test c = 0.3m Average 0.01 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. Appendix

53 Water Level (mm) Time (s) Appendix

54 INFILTRATION TEST No. 2 (Trial Pit No. 6) PROJECT : Coco Beach Hotel Pit Dimension mm: 400x400x300 CLIENT : Sun International Ltd Depth of Trial Pit (m) : 1.5 Location : Belle Mare Test Depth (m) : 0.5 Date : 5/8/2008 Initial depth of water: 250mm Filled in by : NC Checked by : E.S Test Stratum Description: Very loose light grey with black mottles fine to medium gravelly SAND. FILL MATERIAL. Time Water Level ( mm ) (min) Test 1 Test 2 Test Test Results Test No. V P (s/mm) Test a = 0.4m Test b b = 0.5 m Test c = 0.3m Average 0.07 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. Appendix

55 300 Water Level (mm) Time (s) Appendix

56 INFILTRATION TEST No. 3 (Trial Pit No. 7) PROJECT : Coco Beach Hotel Pit Dimension mm: 400x400x300 CLIENT : Sun International Ltd Depth of Trial Pit (m) : 2.3 Location : Belle Mare Test Depth (m) : 0.6 Date : 5/8/2008 Initial depth of water: 240mm and 300mm Filled in by : NC Checked by : E.S Test Stratum Description: Loose to medium dense cream with occasional black horizons fine to medium CORAL SAND of corals and shells. Time Water Level ( mm ) (min) Test 1 Test 2 Test Test Results Test No. V P (s/mm) Test a = 0.4m Test b b = 0.6 m Test c = 0.3m Average 0.01 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. Appendix

57 Water Level (mm) Time (s) Appendix

58 INFILTRATION TEST No. 4 (Trial Pit No. 12) PROJECT : Coco Beach Hotel Pit Dimension mm: 400x400x300 CLIENT : Sun International Ltd Depth of Trial Pit (m) : 2.4 Location : Belle Mare Test Depth (m) : 0.5 Date : 5/8/2008 Initial depth of water: 190mm and 270mm Filled in by : NC Checked by : E.S Test Stratum Description: Loose to medium dense whitish cream with occasional black horizons medium CORAL SAND of fragments of sand and coral. Time (min) Test 1 Test 2 Test Test Results Test No. V P (s/mm) Test a = 0.4m Test b b = 0.5 m Test c = 0.3m Average 0.05 a V p : Percolation Rate c V a : Average Percolation Rate Water Level ( mm ) 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. Appendix