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Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

Site Investigation Report: Sports Development, Marsh Rd, Rhyl

RJ Colin York (5839499)

1


Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

Site Investigation Report: Sports Development, Marsh Rd, Rhyl Introduction The development consists of a swimming pool (approx. dimensions 20m x 10m x 1.5m) and accompanying entrance/reception, and two multi-storey sections (up to 3 stories). For further detail of the arrangement see “Design 4 – General Arrangement” (p15). The site is located near Rhyl, North Wales; the access road to the site is called Marsh Road. The site’s grid reference is SJ 002 802. The site is currently open and on level ground which was previously used as a refuse tip. An ex-tip manager who worked on the refuse tip states that tipping began around 1970, below ground level the layer of refuse is 12 to 15 feet thick and the layer is roughly 25 years old. Refuse includes domestic waste and demolition rubble. Prior to the use of the land as a refuse tip, the ground was marshy. There are no services located on the site, including water, electricity and gas.

Objectives The objectives of the site investigation and site report are to: • Interpret groundwater information and determine water table level/s (p2-3). • Describe the soil’s geological type (p3-4). • Describe the soil’s physical properties (p4-5, p9). • Present and interpret subsurface profiles showing soil formations (p10-11). • Comment on soil tests and recommended values (p4-5). • Recommend/Propose geotechnical design of temporary work and permanent works commenting on their risks and uncertainty (p5-8). • Construct a conclusion to all points listed above (p8).

Groundwater Investigation Reviewing the detailed soil sections (p9) it can be seen that water levels appear down to a depth of 10m. To determine the water table level/s, I shall analyse both the water level (morning and evening) and the seepage rates for each borehole, separately. Borehole Number 3 Water levels are given for the top 5m only, appearing in the made ground and soft clay layer below. Since the clay will have a low permeability there is a good chance that a water table lies above this. Seepage at 1.5m is slow, meaning the water table must be below that, however at 3.5m seepage is fast, therefore a water table must lie at a depth between 1.5m and 3.5m. A water level of depth 1.7m was recorded; for now I will assume this to be the level of the first water table. The rate of seepage below the soft clay (6.0m) layer falls to medium suggesting another water table exists below the clay layer. A fast seepage rate is recorded at a depth of 10.4m. A second water table may lie between depths of 6.0m and 10.4m. Note: All further depths are given relative to the ordinance level of borehole 3 at depth of 0m. This means depth readings will correspond directly to the depths given on the soil sections (p9) but not directly to the depths given on the boring records

Borehole Number 4 Water levels are recorded in the top 5m depth, above and in the soft clay layer as above and also at a depth of 9.8m within a second layer of clay. Seepage rates are slow at a depth of 1.6m ∴ first water table lies below this. Seepage rates below soft clay layer is medium, given this and the fact there was a water level found at 9.8m I will assume there is a water table in the region above the second clay layer.

RJ Colin York (5839499)

2


Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

A seepage rating is taken at the level of the band of sand, given that it is fast it must lie below/within a water table. Borehole Number 5 Water levels are again found within the made ground above the soft clay layer. The lowest depth reading being 1.6m, a very similar value to that obtained from borehole 3, giving further proof of a water table at such a level. A water reading is also recorded at a depth of 5.8m which does not conform to the pattern of readings obtained in previous boreholes. Seepage rates suggest there is a water table below 1.1m (i.e. the first water table at 1.6m), probably below 4.4m and above 11.7m (water table at 5.8m?). Borehole Number 6 All water levels are recorded in the made ground, the highest water reading at 1.9m depth. There is a medium seepage rate at 1.8m, matching with the first water table. At depth 4.3m, just below the soft clay layer, seepage rate is slow ∴water table exists below 4.4m. At 6.5m seepage rate is fast, correlating to the water level found at 5.8m in borehole 5 this suggests a water table lies between 4.4m and 6.5m and possibly at 5.8m. Summary Combining data from all four boreholes one could assume there are three water tables appearing as follows: 1. Within the made ground and above the soft clay layer at a depth of approximately 1.6m. 2. Within the compact sand layer at a depth of approximately 5.8m. 3. Above or within the second clay layer (borehole 4 and 5) at a depth of approximately 10m. Due to the sites proximity to the sea, it is possible that the water tables are affected by the tide. One could assume that water table 2 and 3 are the same water table at different levels due to a different tide level. The difference in depth between water table 2 and 3 of approximately 4m, this relates closely to the tidal range in the area of about 4m (tidal range at Conwy, a near-by town). The water table would be free to vary within the layer of permeable sand. This final assessment identifies the water tables as follows: i. Water table 1 as a perched water table with the soft clay layer acting as an aquiclude. ii. Water tables 2 and 3 as the same water table with varying depth due to tidal motion. Water table ii lies above (caps) the aquifer.

Ground Investigation Geological History Solid and drift maps (p12-14) give general information about the soil of the site. Underlying rock (solid) is ‘lower mottled sandstone’ and transported rock debris overlying the solid bedrock (drift) is ‘marine or estuarine alluvium’ – not surprising as the site is located next to the River Clwyd. In fact, it can be seen from the drift/solid maps that the site sits upon an old section of the river channel. Using this information it is possible to characterise the soils found by the site investigation by the manor in which they were formed or deposited. Referring back to the drift map, areas surrounding Rhyl and the River Clwyd have a drift of Boulder Clay, formed during a period of glaciation. Areas around Rhyl and the River Clwyd have a drift of marine and estuarine alluvial deposits. Taking this into account and the fact that there are nearby deposits of old river gravel and glacial sand and gravel, I believe the strata to have formed as follows: 1. Stratum 10 – boulder clay formed during a period of glaciation. Formed underneath a glacier.

RJ Colin York (5839499)

3


Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

2. Strata 9, 8, 7 – glacial silt, glacial sand and gravel. Probably deposited as ice sheets melted/retreated. Note the presence of coal fragments, probably eroded and transported from the strata south of Rhuddlan. 3. Strata 5,6 and 4 – contain clays, sands and ‘older river gravel’, laid down postglaciation, either deposited by a river or lake. Possibly head, undifferentiated as shown on the drift map stratum 4 as storm gravel beach deposits. 4. Strata 3 – marine and estuarine deposits, proven by the presence of shell fragments in stark contrast to strata below. 5. Strata 2,1 – man-made strata of clay – acting as a permeable lining to the refuse tip – and refuse above. Capping materials of topsoil, stone and concrete apparent. Note that borehole data gives no evidence of a submerged forest even though the drift map shows it appearing at the surface closer than a mile away from the site. Geological Descriptions and Physical Properties of Strata Table 1 - Soil Types and Physical Properties (determined by soil tests) Number Description Moisture Undrained Standard Content Shear Penetration Strength Resistance m Cu N-value (%) kN/m2 1 MADE GROUND 2 - 55 2 Soft 2-7 CLAY(contaminated) 3 Compact SAND with 9 – 26 Shell Fragments 4 SAND and GRAVEL 5 Firm/Stiff CLAY with 12.1 – 20.4 64.6 – 37 Occasional Stones 128.0 6 Band of SAND 7 Fine-Medium SAND 9 - 59 and GRAVEL with Coal Traces 8 Dense SAND with 18 - 34 Occasional Gravel and Clay Lenses 9 SILT with lenses and 28.5 40.6 zones of silty clay 10 Firm/Stiff/Very Stiff 19.6 – 26.3 46.9 – 15 CLAY with 171.0 occasional stones and lenses of silt and sand, laminated in parts 11 Compact brown 22 sandy SILT 12 Dense brown SILT 59 and SAND

Density D kg/m3 2103 2285 2040 2201

-

Moisture Content Typical of moisture content values for saturated soils are between 2% and 5%. Soils in the strata 9 and 10 are ‘fully-saturated’ whilst soil in stratum 5 is semi-saturated. As the three strata are below the water table, one would expect a high degree of saturation. Stratum 5 may have a lower degree of saturation due to the draining effect of the band of sand which intersects it.

RJ Colin York (5839499)

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Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

Undrained Shear Strength Table 2 - Undrained Strength Classification Stiffness State Hard Very Stiff Stiff Firm Soft Very Soft

Undrained Strength (kN/m2) >300 150-300 75-150 40-75 20-40 <20

All samples of clays tested were firm or stronger. The strength of stratum 5’s samples average as stiff. Stratum 10’s samples have strength values evenly distributed across the classes; firm, stiff and very stiff. Standard Penetration Resistance Table 3 - Density Index of Sands N-value Very Loose Loose Compact (Medium Dense) Dense Very Dense

Classification 0-4 4-10 10-30 30-50 >50

Referring to Table 1 sands fall within a range of densities: stratum 3 – mainly compact, stratum 7 - compact/dense, stratum 8, compact and stratum 12, very dense. Table 4 - Density Index for Clays N-value Very Soft Soft Firm (Medium) Stiff Very Stiff Hard

Classification 0 2 4 8 16 >32

Referring to Table 1 clays fall within a range of densities: stratum 2 – soft/stiff, stratum 5 hard, stratum 10 – very stiff and stratum 11 – very stiff/hard. Values may have been affected by stones in the soil. Colliding with a stone during a test will give an unrealistically high N-value. The above results should be read with caution. Soil Profile See p9, p10 and p11 for Detailed Soil Sections, Subsurface Profile and 3D Subsurface Profile respectively. Contamination Sulphate values were recorded from a range of strata and depths (see p24). The maximum value of 0.16% can be regarded as small. Sulphate becomes a danger when levels in the soil increase above 0.5% (Pile Design and Construction Practice by Tomlinson).

RJ Colin York (5839499)

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Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

Geotechnical Design The following information acts only as a recommendation and guide to the final foundation design. Described below is one solution to the design problem, for alternative solutions see “Alternative Design Solutions” on p8. Deign Solution Parameters • Swimming pool is built below ground level. • Swimming pool water can be emptied for cleaning. • Characteristic loads of say, Dead Load – 5000kN and Imposed Load – 4000kN (4000kN includes weight of swimming pool). • Columns at say, 6x6m grid (as shown on drawing p15). Temporary Works inc. Ground Water Control 1. Drainage wells to lower water table. 2. Excavation for pool construction. Since the pool is to be constructed below ground level, an excavation is required. If the excavation were to take place without lowering the water table the following problems would be encountered: • The base of the excavation would flood. • The sides of the excavation would fail by slipping. • Excavated materials would be heavy (compared to unsaturated, ‘dry’ material). To prevent the above problems the water table must be lowered, this can be done by installing wells. Wells should be installed at a level that will lower the water table (from approx 1.6m) to a depth below the excavation, say 3m depth and across the whole area of the construction site. The upper water table present at this level must be drained to the lowest point possible, approximately 3.5m (just above soft clay stratum). A well density will need to be determined to enable the water table to remain at a suitable depth across the site. Once the water table is lower the excavation for the pool can be made. The excavation area should be larger that the pool area giving sufficient room to construct the pool structure. The void between the pool structure and the excavation can be backfilled with a compressed material. Once the pool structure has been installed and the void backfilled the wells can be removed allowing the water table to rise to its original level. Permanent Works 1. Compression Piles 2. Anchor Piles 3. Swimming Pool Structure Compression Piles-Type The construction of piles depends on many factors including: • Structural Strength (Loading) • Arrangement • Cost • Soil Structure • Environmental Aspects (e.g. noise during construction) Bored piles, specifically Continuous Flight Auger piles satisfy most factors as long as due care is taken. The use of CFA piles on the Rhyl site will allow: • Good structural strength • Flexible arrangement (no need to take into account heave cause by displacement piles) RJ Colin York (5839499)

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Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

• Cheap cost. CFA Piles are very quick to install in comparison to other pile types. • CFA boring will drill through all types of soil found on the Rhyl site. • Boring is a relatively quiet procedure compared to driven pile techniques. As the site’s water table will intersect the piles depth it will be necessary to use Tremmy filling. This will give the pile a good quality of concrete regardless of its location, above or under the water table. If the fill is found to be too loose to be self-supporting then casing will be required to prevent materials falling into the auger flight – this phenomenon is called flighting. Flighting may also occur in stratum 3, as sand is a granular non-cohesive material. Flighting will be more likely in looser sand strata, special note should be made of the loose sand stratum found in borehole 3. To prevent flighting of this stratum it may be necessary to install casing to this level. Compression Piles-Arrangement Each pile will take a fraction of the overall structural load, including factored dead and variable loads. Arrangement of piles should reflect the load distribution of the structure across the site. To aid the distribution and to prevent differential settlement ground beams should be constructed, connecting pile groups. Piles transferring the load of the swimming pool should have compressive and tensile load carrying capacity; these piles are discussed in the following section “Anchor Piles”. As a rough guide 15x300mm diameter piles would be necessary to carry the unfactored loads of 5000kN + 4000kN (dead + variable). As the swimming pool applies a greater force/m2 on the soil compared to the surrounding structure a greater density of piles will be required to support the pool structure. Compression Piles-Load Transfer The depth of the pile will determine its bearing capacity (along with other factors including its skin area). To determine the design depth of the pile it is necessary to analyse the bearing capacity of the stratum it will pass trough and rest upon. Good skin friction will be generated from strata with high cohesion - firm clay strata 5 and10. Good base load will be given by soils with a low compressibility and high values of shear strength – dense/compact sand and gravel strata 3, 4,7, 8. As a rough guide a depth of 8m would allow all piles to rest on the compact sand stratum 3. As the pile will not pass through a cohesive layer, skin friction may be low therefore more load may be transferred through base load by widening the base’s effective area through under-reaming. Anchor Piles Piles supporting the swimming pool will have two loading modes. 1. As compressive members carrying the load of the swimming pool. 2. As tensile members carrying soil overburden whilst the swimming pool is empty (of water). To enable this, an anchor pile is used. An anchor pile will transfer the compressive loads of the pool to the soil strata in an identical manner to the compressive piles described above, however, to transfer tension the pile must either use a negative skin friction or be tied by a disk anchor or physical connection to the bedrock. On this site there are two options: • Bore deeper piles into cohesive stratum, which may mean boring to depths of over 15m. • Use the weight of the upper granular strata to anchor the pile by means of a disk anchor. Factors of structural satisfaction, cost and risk should be balanced to give a final solution.

RJ Colin York (5839499)

7


Site Investigation Report: Sports Development, Marsh Rd, Rhyl

05/12/2007

Swimming Pool Structure If the swimming pool is to be constructed below the ground level (approx. 2.5m) it will require a retaining wall to both prevent the soil from falling into the pool and to contain the pool water. To enable the side walls to resist these forces a large reinforced beam is cast at the top edge of the pool – in effect acting as a retaining wall. After construction the swimming pool structure will exist below the first water table. At the design stage flow-nets surrounding the swimming pool ‘box’ should be drawn and pressures on the base of the swimming pool structure analysed.

Conclusion Summary of Risks Risks discussed in context, above: 1. Flighting of looser non-cohesive soils. 2. High water table effecting the formation of the piles. 3. Complex design of swimming pool foundation. 4. Overburden loads and excess pore water pressures on base of swimming pool. General risks: 5. Bad pile construction causing: a. Massive eccentric loading. b. Off-vertical piles c. Poor quality concreting d. Poor insertion of rebar cage leading to exposed bent reinforcement. 6. General H&S construction risks. Specific risks not discussed in “Geotechnical Design”: 7. Base load transferred to the local submerged forest. The weak layer will provide little bearing capacity and will give massive settlement. 8. SO 3 in soil damaging concrete piles. 9. Saturated materials in excavation causing failure of side walls and/or flooding of base.

Alternative Design Solutions 1. Construct swimming pool above ground level relieving the need for anchor piles. Pro: no anchor piles, Con: Inefficient use of space. 2. Soil overburden load on structure caused by the emptying of the swimming pool transferred through foundation structure to outer piles transferred to soil as moments, i.e. structure acting as rigid member. Pro: no anchor piles, Con: expensive solution. 3. Partially flexible raft foundation rather than piled. Pro: cheaper than piled foundation, Con: great deal of settlement, could result in failure of structure. 4. Partially flexible raft foundation laid upon stratum 3. Pro: would not deflect as much as alternative solution 3, Con: very expensive to remove and dispose of large amounts spoil.

References 1. 2. 3. 4. 5.

Cover Image - Aerial Photograph, Rhyl by Microsoft Virtual Earth Table 2, Table 3 – Craig’s Soil Mechanics by R.F. Craig Table 4 - The Foundation Engineering Handbook by Manjriker Gunaratne Sulphate levels - Pile Design and Construction Practice by Tomlinson Geological Drift Map, Sheet 95

RJ Colin York (5839499)

8


Detailed Soil Sections Borehole

Depth (m)

3

0 m

m Cu

Cu

6

5

N m

N 11

S

Borehole

Borehole

Cu

7

Cu

VS

N

0

18 17 M

S

14

m

N

M

15

7 41

7

13

7

4

2

55 2 F

53 7

2 M

M 22

5

S 14

20

16

5

11 M

20

14 F

9

12

12 12 11

11 26

16

23

26

14 20.4

78.3

10

10 F 24 40 17.9

64.6

12.1

128.0

12.4

105.2

F 37

32

F 22

M

18

28.5

9

34

59

12

40.6

15

23.3

24.2

50.1

19.6

63.9

21.6

54.7

15

68.2

21.9

152.0

21.6

157.2

20.8

70.0

20.6

74.8 26.3

46.9

21.9

64.6

19.8

171.0

21.4

51.9

15

20

20

22 M 59

Key SAND

SILT

CLAY

GRAVEL

MADE GROUND

SAND & GRAVEL

SAND & SILT

WATER LEVEL & SEEPAGE Recorded Water Level

LOOSE/ SOFT

COMPACT/ FIRM

DENSE/ STIFF

STONE/ GRAVEL

VS Very Slow Seepage COAL

SHELL

S

Slow Seepage

M

Medium Seepage

F

Fast Seepage

Depth (m)

4

Borehole


Depth (m)

0

Borehole

4

Borehole

3

Borehole

Borehole

5

6

1. Made Ground

Borehole

3

0

1. Made Ground

2. Soft Clay

2. Soft Clay 5.0

5.0

3. Compact Sand with Shell Fragements

3. Compact Sand with Shell Fragements

4. Compact Sand & Gravel 10.0

5. Firm Clay with Stones

10.0

5. Stiff Clay with Stones

6. Band of Sand

8. Dense Sand

5. Stiff Clay with Stones

7. Compact Sand & Gravel with Traces of Coal

8. Dense Sand 9. Silt

9. Silt 15.0

15.0

10. Firm Clay with Ocassional Stones

20.0

10. Firm Clay with Ocassional Stones

11. Firm Silt

NOT TO SCALE, For Key see “Detailed Soil Section”

11. Firm Silt 12. Sand & Silt

20.0

Depth (m)

Subsurface Profile


3D Subsurface Profile NOTES Exaggerated vertical scale x and y displacements are proportional Some strata may be simplified See â&#x20AC;&#x153;Detailed Soil Sections for keyâ&#x20AC;?

Borehole

6

Borehole

5 Borehole

3

Borehole

4

Borehole

6

Borehole

5 Borehole

3 Borehole

6

Borehole

5 Borehole

3

Borehole

4


SI - Sports Development, Rhyl  

site investigation report

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