Engineering geology of British rocks and soils Mudstones of the ...

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4.4 Stress vs strain graph for Mercia Mudstone Weathering Zones 1, 3 and 4 (drained cell pressure 68.95 KN/m 2 ) showing change from brittle to plastic behaviour as weathering progresses (after Chandler, 1969) 21 4.5 Classification of mudrocks suggested by Grainger, 1985 22 6.1 Outcrop of the Mercia Mudstone Group showing areas used for the statistical analysis of the geotechnical database 29 6.2 Table structure for the geotechnical database 30 6.3 Standard box and whisker plot 32 6.4 Structure of extended box plots for ‘normal’ distribution of data values 33 7.1 Particle size grading envelopes (all data) 35 7.2 Particle size grading curves (Area 1, subdivided by formation) 36 7.3 Particle size grading curves (Area 2, subdivided by weathering zone, WG) 36 7.4 Activity plot, %
1 Introduction 1.1 BACKGROUND TO THE REPORT The Mercia Mudstone Group is a sequence of predominantly mudrock strata which underlies much of central and southern England and on which many urban areas and their attendant infrastructure are built. It is the mudstone that is most commonly encountered in construction and it is the mudstone with which this report is primarily concerned. In order to assist readability this report uses the widely used, though stratigraphically inaccurate, term ‘Mercia Mudstone’ rather than the strictly correct but repetitious term ‘mudstone of the Mercia Mudstone Group’ or ‘Mercia Mudstone Group mudstone’. Although the Mercia Mudstone appears to cause few serious geotechnical problems compared with other, higher plasticity, clays (Jones, 2000) it is significant to the construction industry because it is frequently encountered in civil engineering activities involving foundations, excavations and earthworks. Its nature is such that its properties may vary between a soil and a rock depending on its detailed lithology and its state of weathering. As a result of this, in some cases, weaker material may be found below stronger rather than the more normal weathering progression where the weakest material occurs at the surface and becomes fresher and stronger with depth. Consequently sampling and testing is difficult because the material may not be suited to either soil or rock techniques. There is a significant problem with regard to measuring particle size grading and plasticity due to the difficulties with disaggregating the component particles. Some parts of the Mercia Mudstone sequence may be subject to shrinking and swelling with changes in moisture content to a sufficient degree that structural damage to buildings or disruption in some types of construction work is caused. The Mercia Mudstone Group contains sandstone beds and evaporite minerals, mainly halite (sodium chloride) and gypsum (calcium sulphate, 2H 2O). These lithologies can cause significant problems to construction but they have not been addressed by this study on the basis that it is the mudstones that represent by far the greater part of the 1 sequence. Other reports may address these materials in the future. Halite is very soluble in water and is never encountered at the natural ground surface in this country having been removed by groundwater at some depth. Similarly gypsum, though less soluble, is frequently absent in the near surface zone. Sandstone represents a small proportion of the sequence in most areas and is rarely a problem to construction. This report on the mudstone of the Mercia Mudstone Group is the second of a series on the rocks and soils of Britain which aims to satisfy a need of geologists and engineers for reference works describing the engineering behaviour of important geological formations. 1.2 METHODOLOGY The properties and behaviour of geological materials are controlled by their texture, structure and mineral composition. These factors are a reflection of their depositional environment, diagenesis and subsequent tectonic history that also have a major influence on the current engineering behaviour of the strata as a whole. Also, the near-surface zone has been influenced by Earth surface processes acting in the more recent past. In many instances the behaviour of the material cannot be predicted unless the recent geological history of the site is understood. The Mercia Mudstone study comprised several interdependent parts. An extensive literature search was carried out at the start of the study to collect and review previous work thus guiding the activities of the study. At the same time an extensive geotechnical database was assembled from high quality site investigation reports which was then analysed to establish the typical range and values of the most commonly determined geotechnical parameters and to look for regional variation in geotechnical properties. When the scope of the database was clear, a sampling and testing programme was carried out to investigate in more detail some of the geotechnical properties and behaviour not satisfactorily covered in the database.
Page 1: Engineering geology of British rock
Page 4 and 5: BRITISH GEOLOGICAL SURVEY The full
Page 6 and 7: Summary This report is the second o
Page 9: Contents Summary iv Preface v 1 Int
Page 13 and 14: JURASSIC 205 5Ma Rhaetian TRIASSIC
Page 15 and 16: 5 Wessex Basin Somerset/Avon Worces
Page 17 and 18: Mendips and Charnwood Forest, the M
Page 19 and 20: minor amounts of disseminated, fine
Page 21 and 22: 11 South Devon Dolomite Calcite Cal
Page 23 and 24: 13 Illite South Devon Chlorite Illi
Page 25 and 26: BOLLIN MUDSTONE FORMATION The lower
Page 27 and 28: The chlorite content decreases slig
Page 29 and 30: 4 Geotechnical literature review 4.
Page 31 and 32: ( 1-3 )lb/sq.in. VOLUMETRIC STRAIN
Page 33 and 34: Table 4.3 Typical values for the en
Page 35 and 36: material to fully plastic for Zone
Page 37 and 38: 5 Drilling and sampling 5.1 INTRODU
Page 39 and 40: 6 Database design and method of ana
Page 41 and 42: 6.5 DATA ANALYSIS The data should b
Page 43 and 44: 5-9 10-24 25-99 100-499 >500 Number
Page 45 and 46: dry or wet sieving and the fine fra
Page 47 and 48: Plasticity index (%) 50 45 40 35 30
Page 49 and 50: Plasticity index (%) Plastic limit
Page 51 and 52: Plasticity index (%) Plasticity ind
Page 53 and 54: 40 30 20 10 Low Interm. High V.High
Page 55 and 56: tion agents may be unsuited to alka
Page 57 and 58: Depth (m) 0 5 10 15 20 25 30 35 40
Page 59 and 60: Depth (m) 0 5 10 15 20 25 30 s' (kP
Page 61 and 62:
the ratio c u / N with decreasing p
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Table 7.5 Typical permeabilities of
Page 65 and 66:
in situ condition as possible, wher
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8 In situ testing 8.1 INTRODUCTION
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the UK) by the Building Research Es
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9 Conclusions 9.1 GEOLOGY The Merci
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differences in the reported values
Page 75 and 76:
References AMBROSE, K. 1989. Geolog
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Coastal and Engineering Geology Gro
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WILLIAMS, A A B, and DONALDSON, G W
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INDEX TESTS — Natural moisture co
Page 83 and 84:
Natural moisture content (NMC, w %)
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Bulk density, BD (Mg/m 3 ) — AREA
Page 87 and 88:
INDEX TESTS — Dry density Dry den
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Liquid limit (LL) — AREA 4 No. St
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Liquid limit (%) 100 80 60 40 20 0
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Plastic limit, PL (%) — AREA 7 St
Page 95 and 96:
Plasticity index, PI (%) — AREA 2
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Plasticity index, PI (%) — AREA 9
Page 99 and 100:
Liquidity index (LI) — AREA 7 No.
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Total sulphates, TS (%) — AREA 8
Page 103 and 104:
Aqueous sulphates, AS (%) — AREA
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pH — AREA 8 Stratig. &/or weath.
Page 107 and 108:
Point load (axial), PLA (MPa) — A
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STRENGTH TESTS — Uniaxial compres
Page 111 and 112:
Undrained cohesion, CU (kPa) — AR
Page 113 and 114:
Compaction, OMC (%) — AREA 9 No.
Page 115 and 116:
Standard penetration test, SPT (N)
Page 117:
BGS Research Report RR/01/02 Engine
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