OR/14/052 Results

Boon, D, Kirkham, M, and Scheib, A. 2014. Physical properties of till deposits from Anglesey, north west Wales. British Geological Survey Internal Report, OR/14/052.

The results of the laboratory analyses and in-situ penetrometer tests are summarised in the following section, with further details in the Appendix 2.

Particle size distribution

No. 16 particle size distribution (PSD) tests were carried out on samples collected from coastal exposures. Sample location and weights are provided in Table 1. The particle size distribution results are summarised in Table 4, grading curves are given in Figure 3 and the spatial distribution of data is summarised in Figure 4.

The till deposits on Anglesey are predominantly well-graded FINE matrix dominated soils but 4 out of the 18 tills contained less than 35% fines and are therefore COARSE soils. Variability in the proportion of the fine fraction will influence the geotechnical and hydrological properties of the till. For example, the shape of the grading curve for the upper brown till and grey till from Cemlyn Bay is almost identical however the red till contains 10% more fines making the red till a ‘coarse’ soil and the grey till a ‘fine’ soil in terms of classification for engineering purposes.

The tills from nearby Hen Borth have a similar shaped grading curve to those at Cemlyn Bay but the red till is similar in composition to the grey till. On the south coast the tills at Sea Zoo and Penrhos have similar shaped grading curves but the Penrhos tills have a slightly higher proportion of fines. The red and grey tills from Beaumaris in the east of the island are distinctly sandy and the upper red till is particularly enriched in fine to medium-grained sand.

The till at Porth Nobla is by far the coarsest, contains 19 to 25% fines, 25–33% sand, 45–47% gravel and 3% cobbles and is classified as a ‘COARSE’ soil for engineering purposes. The wide grading of the tills means their particles can become extremely tightly packed achieving high relative densities when compacted under an ice sheet. The effects of weathering include reduction in bulk density and an increase in natural moisture content and compressibility.

The ‘sand’ body found between the grey and red tills in a drumlin at Cemlyn Bay is composed of around 17% clay, 26% silt, and 50% sand (silty SAND) with a small amount (5%) of medium to coarse gravel (Figure 3, Figure 4, and Table 3) and is a FINE soil (after Anon, 1995).

Figure 3    Particle size distribution curves.

Figure 4    Particle size distribution of Anglesey tills.

Bulk density and natural moisture content

The results are summarised in Table 4, and show that bulk density of red till range from between 2.02 to 2.26 Mg/m³. Values of dry density for red till are on average 8% lower than the bulk density. Natural Moisture Content ranges between 7.3 and 9.1% for red till and 13.0% for grey till, though the latter is a single test result and so may not be representative. Bulk density values will also of course vary depending on their degree of saturation. The bulk density of the hydrofracture sand is fairly uniform and ranges from 1.92 to 2.09 Mg/m³ with a NMC of 15.3%.

Table 4    Results of particle size analysis

Site name	Sample ID	Particle Size % passing
		Clay	Silt	Sand	Gravel	Cobbles	% Fine fraction (clay + silt)
Hen Borth	HB1 1/2	18.9	17.5	28.4	32.6	2.6	36.4
	HB1 2/2	16.7	15.7	26.0	34.9	6.7	32.4
	HB2 1/2	15.1	16.1	24.4	36.7	7.7	32.1
	HB2 2/2	18.2	18.7	29.0	34.2	0	36.9
Beaumaris	B2 1/2	8.5	10.6	44.8	36.1	0	19.1
	B2 2/2	18.3	23.0	23.2	35.5	0	41.3
	B1 1/2	19.6	22.8	38.8	18.8	0	42.4
	B2 2/2	19.0	21.6	36.2	23.2	0	40.6
Sea Zoo	SZ (mudstone)	19.0	18.0	45.8	17.2	0	37.0
	SZ 1/2	10.9	16.2	26.1	20.6	26.2	27.1
	SZ 2/2	14.1	21.5	32.4	32.0	0	35.6
Porth Nobla	PN 1/2	11.1	13.5	24.7	47.1	3.5	24.6
	PN 2/2	8.1	10.8	33.2	45.1	2.9	18.9
Cemlyn Bay	Grey till	15.0	15.7	25.4	41.9	4.1	30.7
	Red till	23.3	17.0	22.6	31.8	5.3	40.3
	Sand dyke	17.4	26.3	50.8	5.6	0	43.7
Penrhos	PR1 1/2	18.8	24.1	29.9	27.2	0	42.9
	PR1 2/2	24.2	31.3	15.8	28.8	0	55.5
	PR2 1/2	17.2	21.0	22.4	39.4	0	38.2
	PR2 2/2	18.3	23.0	23.2	35.5	0	41.3

Table 5    Results of soil density and moisture content tests

Sample ID	Geology Unit	Sample Type	Bulk Density, ρbulk Mg/m3	Dry Density, ρd Mg/m3	NMC, %
Cemlyn Bay A	Brown/Red Till	Ring	-		8.5
Cemlyn Bay B	Grey Till	Ring	-		13.0
Cemlyn Bay 7	Weakly cemented Sand	Ring	2.09
Cemlyn Bay 8	Weakly cemented Sand	Ring	2.02
Cemlyn Bay 9	Weakly cemented Sand	Ring	1.99
Cemlyn Bay (SB 50 kPa)	Weakly cemented Sand	Tin	2.00		15.3
Cemlyn Bay (SB 100 kPa)	Weakly cemented Sand	Tin	2.05		15.3
Cemlyn Bay (SB 200 kPa)	Weakly cemented Sand	Tin	1.92		15.3
Beaumaris 1	Red Till	Ring	2.04
Beaumaris 2	Red Till	Ring	2.43*	2.22*	8.8
Beaumaris 3	Red Till	Ring	2.26	2.10	7.3
Sea Zoo 1	Red Till	Ring	2.16	1.98	8.3
Sea Zoo 2	Red Till	Ring	2.21	2.01	9.1
Sea Zoo 4	Red Mudstone	Ring	2.09
Sea Zoo 5	Red Till	Ring	2.04
Sea Zoo 6	Red Till	Ring	2.02

* gravel protruding density ring — density overestimated. SB = Shear Box sample initial ‘pre-test’ values.

Shear strength of cemented sand (Cemlyn Bay)

Peak values of Phi’ and apparent cohesion, c’, determined by direct shear tests, are summarised in Table 6, and give an effective angle of internal friction of 34 degrees. This is fairly typical of a fine sand deposit, however the sand does possess a small amount of apparent cohesion (13 kPa).

Table 6    Summary of effective shear stress strength parameters for cemented and remoulded sand from drumlin at Cemlyn Bay, Anglesey (‘effective’ stresses = 50, 100, 200 KPa)

	Undisturbed sand	Remoulded, recompacted sand
Apparent cohesion, c’ (kPa)	13.3	0
Angle of internal friction, φ’	34.3	34.3

Figure 5    Plot of shear stress/normal stress for cemented and remoulded sand.

Panda penetrometer results

The Panda variable energy dynamic penetrometer (Panda) test results are summarised in Table 7. Full details are included in Appendix.

Table 7    Summary of ‘Panda’ penetrometer results

Site	Profile name	Unit	Cone resistance range (MPa)
Cemlyn Bay	P001	Sand lens — NB Invalid test due to brittle soil fracture	0.5–0.7
	P002	Sand lens	2.0–6.0
	P003	Sand lens	2.0–5.0
	P004	Sand lens	2.0–5.0
	P005	Grey till	5.0–10.0
	P006	Sand lens grey till at base	2.0–6.0 10.0
	P007	Grey till	10.0
Beaumaris	P008	Grey till	5.0–7.0
	P009	Grey till	3.0–6.0
	P010	Red till	6.0–10.0
	P011	Red till	4.0–10.0
	P012	Red till	3.0–6.0
Port Nobla	P014	Red till	7.0–10.0
	P015	Red till	2.0*–10.0