Environmental/engineering geology of alluvial settingsalliance.la.asu.edu/rockart/scans/AlluvialSettings.pdf · G.A. Kiersch/Engineering Geology 45 (1996) 325-346 327 quently stimulated

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ELSEVIER Engineering Geology 45 (1996) 325-346

ENGiNEERiNG GEQLQGY

Environmental/engineering geology of alluvial settings

George A. Kiersch Professor Emeritus Cornell University and Consultant, 4750 N. Camino Luz, Tucson, AZ 85718, USA

Received 31 May 1995; accepted 17 November 1995

Abstract

Alluvial deposits were recognized as important to mankind's welfare 2500 years ago when collector galleries (kanats) were constructed in Persia beneath alluvial slopes. Furthermore, the 4000-year history of observations along the rivers of Mesopotamia, Egypt, and China provided an early understanding of sediment loads, silting, and backfilling of river channels, elements basic to selecting a site for engineered works or locating aggregate sources.

Harold Fisk's work (Fisk, 1944, 1947) in the Lower Mississippi Valley developed three-dimensional concepts relative to deposition within large stream valleys and the identification of distinctive features concerned with the origin and active processes. Utilizing Fisk's methods and techniques led to recognition of the sequential depositional history that serves to delimit and map distinctive alluvial units. This systematic unravelling of the younger alluvial history is frequently the all important factor in engineering geology practice. Moreover, Fisk's contributions on the evolution of a major river system provide guidelines to characterize a candidate site, or design and operate an engineered works. Such principles are briefly reviewed in a series of selected geological settings related to major works, such as: (1) The intermontane alluvial valleys of Cenozoic age have a complex sequence of sediments and infiUing that is

closely related to its structural history which enlarged the basins and accelerated erosion of surrounding terrain. Such deposits may possess substantial groundwater and other resources.

(2) Construction of dams and hydrological projects within a deeply-filled alluvial valley is economical today, because of the greatly improved understanding of alluviation, characteristics of the deposits, and stratigraphic sequences common to the environs of a narrow valley.

(3) An inadequate understanding of the origin, distribution, and character of alluvial features can result in geological errors of judgment. Boulders in a gravelly unit maybe interpreted as "bedrock" and the design foundation-level for works.

(4) The intermixing of fluvioglacial deposits with Holocene sediments throughout 25 km of an ancient river valley resulted in physical changes that impacted tunnel construction beneath the channel.

(5) Morainal openwork gravels utilized as a reservoir bank may fail with time due to liquefaction and piping action. (6) Fine-grained alluvial sediments deposited within an ancient abandoned river channel can be a deceptive and

dangerous foundation material for major works. (7) Multiple alluvial units can be critical to understanding the depositional history and recency of tectonism.

0013-7952/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S0013-7952 (96)00020-8

326 G.A. Kiersch/Engineering Geology 45 (1996) 325-346

1. Introduction

Alluvial deposits were recognized as important to mankind's welfare in ancestral times when over 2500 years ago collector galleries - kanats (or ganats) - were constructed in Persia to capture the ground water beneath alluvial slopes for villages and irrigation of fields (Fig. 1). The techniques of construction spread quickly eastward to Afghanistan and China and westward to Egypt where one large system irrigated 4700 km z of land west of the Nile circa 500 B.C. (Tolman, 1937). Many kanats are in operation today, and perhaps the most famous system is in the Turpan oasis of China. It has thrived for 2000 years where extremes exist; one-half inch (12 mm) of rainfall per year and temperatures of 40-49°C. This underground distribution system also reduces evaporative losses from the network of galleries and kanats-canals. Today the city of Turpan and surrounding agricul- tural district are supplied water by an underground system that distributes flow from alluvial deposits along the front of the Tien Shan Mountains. Kanats and canals built centuries ago are still in use and the Chinese government built ten additional aqueducts in 1949 (kanats, etc.) to ensure additional supplies (Wren, 1983), and a reduct- ion of evapo-transpiration losses by the "underground"/covered canal distribution.

Over 4000 years ago, observations of inhabitants along the Tigris and Euphrates rivers of Mesopotamia, the Nile of Egypt and Yellow River of China were providing the earliest ideas on

ALLUVIAL DEPOSIT- AFTER: WATER $ATURAVED FORBES, 1965

.....~ U SHAFT- CONSTRUCTION GAK AND COLLECTION 1988

Fig. 1. A hillside section of alluvial deposits illustrating an ancient water-collecting system - the Kanat (from Kiersch, 1991, p. 3).

sediment loads, silting, terraces, and floodplain deposition, and the backfilling of river channels, all basic processes in the evolution of a fluvial system (Forbes, 1963, pp. 16-19). Thus a database of observations and historical data collected over time on the broad geo-related principles of a fluvial river system and processes of alluviation were available for further development by Fisk in the 1940s. Moreover, three-dimensional concepts of the processes active within stream valleys were set forth by Fisk (1944, 1947), as described by others in this volume and the distinctive features or deposits associated with a setting or environs were identified.

This characterization led to recognition of the sequential history of alluvial deposits; Fisk's methods and techniques delimited and mapped these young distinctive alluvial units - which have become indispensable to modern engineering geology practice. For example, today alluvial deposits may be the source of data critical to the design of an engineered works such as the composition and permeability of a potential aquifer, the diagenetic deterioration and/or weathering effects valuable in defining the sequential history, and the use of individual alluvial units to age-date or assess the recency of tectonism other than the A and B soil horizons in older alluvium. A much greater pro- portion of the modern engineering geologist's attention and efforts are concerned with alluvial deposits prior to 1960s. This is due not only to Fisk's contributions that unraveled the history of a river system but the sequence of young sediments deposited in modest-sized stream valleys as cited herein. For example, much knowledge was learned during the nuclear power plant building-boom of the 1960s to 1980s, concerning the importance of ancient soil profiles and individual alluvial units, particularly relative to age-dating the recency of tectonism or confirming the nonhazardous seismic conditions for design of the works. This was equally true for the foreign sites whether in Europe, Asia, Central or South America.

Fisk's principles and contributions on the evolution of a major river system set forth guidelines for characterizing a candidate site when designing an engineered works. Fisk's investigation of the Lower Mississippi Valley (Fisk, 1944, 1947) subse-

G.A. Kiersch/Engineering Geology 45 (1996) 325-346 327

quently stimulated an investigation of the modest-to small-scale intermontane alluvial valleys of the western US (Figs. 2 and 3) and foreign regions. These studies confirmed the importance of on-going, stage-faulting along margins of the valley walls (Anderson et al., 1988) and the occurrence of large, commercial-grade evaporite deposits (Peirce, 1974) within the Unit B beds (Figs. 3 and 4). The following review of selected case histories demonstrates the importance of the basic field and physical criteria of alluvial deposits recognized by Fisk and broadened by many of the investigator cited in this volume.

1.1. Al luvial deposits - criteria

Alluvial deposits are characterized by distinctive physical and field criteria as reviewed in this volume: occur in fluvial areas, stream channels, and floodplains; transport agents of running water (fluvial and floodplains) or meltwater (fluvioglacial); grains of mainly sand and gravel sizes, some

fines to boulders; lithology varies and reflects source(s); inherent primary structures may include scour-fill, graded- and cross-bedding; matrix usu- ally unconsolidated with fines removed; soil profile absent, except A and B horizons in some older alluvium; bedrock contact is generally abrupt with scour-channels frequent (after Hatheway and Leighton, 1979, p. 186).

1.2. Na tura l aggregate - sources

Three of the four common occurrences of suitable natural aggregate sources (after Rhoades, 1950) within the environs of an intermontane alluvial basin or a river valley channel terrace, and floodplain deposits were recognized by Fisk (1944).

1.2.1. River channel deposits The sands and gravels are generally charac-

terized by rounded particles and some uniformity throughout the various sizes. Large differences in

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Fig. 4. Typical Intermontane Basin structure and sequence of deposits after Anderson et al., 1988; a refinement of earlier interpretation of the 1950s (see Fig. 3).

gradation can occur between deposits because river transport tends to winnow and down-size or cause mechanical attrition. The soft materials chemically react and decompose the susceptible materials.

1.2.2. River terraces deposits Generally possess the characteristics of river

channel materials. However, because of accessibil- ity, terrace deposits may be more economical to exploit than channel sources. But, terrace particles are frequently cemented or coated by oxides, sili- ceous, or evaporite minerals due to ground water action or postdepositional weathering that render the alluvial material unsuitable for concrete aggregate.

1.2.3. Floodplains deposits Commonly are a variable heterogeneous mass

that is deficient in sizes larger than sand and thus of limited use for concrete aggregate. Sometimes, the floodplain materials occur overlying an earlier stage of acceptable sands and gravels deposited by the earlier meandering river; often these older deposits have the characteristics of a river channel.

1.2.4. Alluvial fan and cone deposits Normally are rudely layered in zones ranging

from very fine to very coarse sub-rounded particles. Although some deposits may include materials suitable for aggregate use, they will likely require

beneficiation and blending with an outside source to overcome deficient size grades.

2. I n t e r m o n t a n e al luvial val leys - bas in and range province

The increasing demands for groundwater supplies in many regions of the western states have caused the engineer/geologist/economist team to adopt a conservation concept for basin-resources and management (Fig. 2). This method provides both an improved supply and a more efficient use of the combined surface and underground water budget within any geological province. Basically, this approach requires a thorough pre-planning physical and quantitative database on the geological conditions throughout the basin. Ultimately, both the underground and surface reservoirs are probably operated as an integral system - whether for storage, supply, replenishment, or the disposal of polluted waters.

2.1. Groundwater resources

The generalized subsurface geology within three contrasting alluvial basins of the western states is shown on the three-dimensional diagram in Fig. 3. The three major alluvial/sedimentary units A, B, and C are delineated along with their age range, groundwater levels, and structural history based on the database and a model of 1950s that confirmed the following.

(1) The basin fill is not all alluviation derived from the nearby mountain ranges. Infilling by flow-through streams contributed fine- to coarse- grained sediments which were frequently dammed by volcanic outpourings, causing deposition of clayey sediments and evaporites.

(2) The dominant basin sediment is not a coarse gravel and sand alluvium, but rather fine-grained sediments.

(3) The actual groundwater reservoir potential is only part (one- third) of what had been anticipated earlier.

(4) The critical alluvial features of the basin are composed of units A-C.

Unit C: oldest, largely coarse alluvial debris


from adjacent mountains; materials well-cemented and of low to moderate permeability).

Unit B: majority of basinfill is fine-grained silt and clay of very low permeability, the deposits of ancient drainages and intermittent lakes with frequently gravel lenses and intercalated lavas. Sites for underground dams may exist as in upper-block in Fig. 3 where shallow C unit enclosed by B.

Unit A: youngest, near-surface sand and gravel alluvium deposited by drainages, and/or slope alluviation by fans and cones.

A refinement of the 1950s studies on basin structure and sequence of deposits had led to a 1980-1990s model of the basins, as interpreted by Anderson et al. (1988) and shown on Fig. 4. Rocks of Cenozoic age fill the basins and consist of unconsolidated to consolidated units and interbedded volcanics. The older sediments were deposited during and following the basin and range faulting that enlarged the individual basins and accelerated erosion of nearby mountain slopes. The oldest basins - early to middle Tertiary age - were backfilled many thousand feet and the fine-grained units are often interbedded with mid-Tertiary volcanic rocks. Alluvial fans at the base of mountains coalesced to form bajadas while playa-type deposition occurred in the lowest part of basin. Such an environment produced two facies of younger rocks: fine-grained, well-sorted deposits throughout the central sector, where evaporites are common (Peirce, 1974); and coarse-grained, poorly-sorted materials near the mountain fronts ranging from silt and clay to cobbles and boulders.

Groundwater management requires an in-depth, quantitative understanding of the subsurface and particularly the physical properties of individual alluvial units. However, a more economical water storage is attained when the resources of a ground water basin are utilized in conjunction with surface reservoirs. This practice imposes new engineering needs and requires additional geological guidance relative to: recharge, water quality, salt balance, sewage and industrial wastes, and in some coastal areas, seawater intrusion.

Such areal and site-related inter-dependence is also an important phenomenon of man-induced

ground subsidence, both shallow and deep, to improve the properties of a foundation or the characteristics of an excavation for engineered works.

2.2. Underground storage dams - small valleys

Another type of groundwater works that requires a detailed geological knowledge of a basin and its alluvial units is the siting and construction of an underground retention reservoir. Long, narrow alluvial-valley aquifer-stream systems occur in many parts of the western states. These aquifers are in hydraulic connection with associated streams, and invariably display a rapid inter- action of the surface and ground water. The aquifer system reflects a complex depositional environment with permeable hydrogeological units and are among the most intensively used by mankind (Rosenshein, 1988, p. 174).

The geological setting of a possible underground retention dam is illustrated in Fig. 5 within a small, alluvial-filled valley of an intermontane basin. The channel is carved in essentially impermeable bedrock; the overlying alluvium consists of open-void gravel (principal aquifer) with a thin unit of fines and coalescing lenses of clay (Unit B material of Fig. 3). This sequence is capped by coarse Holocene alluvium (Unit A). Recharge is near the mountain front.

A heavy draw-down by well "B" located down- gradient is rapidly depleting the ground water of the up-valley sector. In this setting it is not uncom- mon for wells such as "A" to go dry.

To insure against a complete draw-down in the upper-valley sector, an underground water-retention dam offers a possible solution (Kiersch, 1954, 1964). A grout barrier would be placed from holes on a grid pattern, using clay and a clay-cement slurry to seal the open-void gravel in the lower/central sector of the channel 1. This creates a small underground dam founded on bedrock

1Mixes vary on any site according to the alluvial composition. Technique is satisfactory if 90% of alluvium is above 0.40 mm in size (medium sand). Size range of 10% fines determines ulti- mate permeability of grout; only one-half (5% of gross) can be less than 0.25 mm (fine sand).

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Fig. 5. Three-dimensional diagram of modest-sized alluvial-flied basin; clay lenses within main fill overlie open-void gravels (principal aquifer). Setting may be suitable for creating underground water reservoir. Longitudinal subsurface section 1 shows conditions between wells "A" and "B" of natural gradient and water level. Section 2 is along same line after grout barrier constructed, showing underground reservoir for well "A". Cross-section 3 is along the line of grout barrier across valley (see 2) and shows placement keyed to the inter-bedded clay lenses within alluvium (from Kiersch, 1954, 1964).

9-12 m (30-40 ft) high (can be higher). The design allows for movement of water to the downstream area by overtopping the "cap", after the upstream reservoir is filled. Potential groundwater losses to downstream land owners are thus avoided.

Clay lenses would overlap one another at different elevations and grossly constitute a Unit B of low permeability. The clay bodies conve- niently serve as both a "cap" on the grout barrier and as a keystone to stabilize the groundwater dam.

Other methods to construct a barrier might be feasible depending on the geological conditions of

a site. For example, a trench (limit about 21 m (70 ft)) excavated to bedrock and backfilled with a clay slurry or clay core - or sheet piling across a narrow construction may be feasible. Some legal aspects are apparent, but a partial barrier as suggested would not violate the groundwater codes of many states• Such "reservoirs" serve as ideal storage basins for local needs, that otherwise may require an expensive surface storage structure. Furthermore, they also eliminate evapo-transpiration losses, provide an improved moisture content of the overlying soil for cultivation, and there are essentially no surface land costs for the site.

332 G.A. Kiersch/Engmeering Geology 45 (1996) 325 346

2.3. Dam foundations - deep inner-filled valleys

An "underground" dam foundation within a deep inner-filled alluvial valley is anchored to clay lenses within the channel fill. The improved understanding of the fluvial system and deposition of alluvial sediments within a restricted river channel by Fisk has made possible critical advances in the engineering geology practices for unconsolidated foundations. Among them is the consolidation grouting of deep alluvial foundations for major dam structures (Haffen and Karpinski, 1963).

Heretofore many excellent hydrological sites have been unused because it was uneconomical to excavate and backfill the tens to hundreds of feet of ancient alluvium occurring beneath the stream channels. Success by the French at sites on the Rhine and Rhone rivers in the 1950s (Haffen and Karpinski, 1963) opened up new vistas for dam construction and such sites now receive consider- ation around the world (Cambefort, 1963).

The 120-m-high (394 ft) earth-fill dam at the Serre Poncon narrows of the Durance River valley in French/kips rests on mainly 110 m (360 ft) of alluvial deposits as shown in Figs. 6 and 7. The early Pleistocene, glacial-scoured, U-shaped valley was later eroded and a deep inner-canyon created, which is backfilled with alluvial materials over 110 m (360 ft) in depth (Fig. 7). The sands and gravels were deposited as sheets and lenses following the configuration of the ancient stream channel, which formed permeable zones and groundwater paths within the alluvium (Haffen and Karpinski, 1963, p. 36-39).

A large-scale consolidation grout curtain (placed in test-blocks) was constructed in the alluvial gravels and sand deposits in 1957; a photo (Fig. 8) of an exploration shaft (post-grout) demonstrates the heterogeneous nature of alluvium ranging from sand to boulders. The grout-slurry was a mix of finely crushed cement slag with a 2-3% setting agent, caustic soda (Haffen and Karpinski, 1963).

2.4. Underflow of engineered works

When engineered works such as dams, embankments and levees are located in fluvial contexts, underflow may occur and particularly when the

alluvial feature (s) are unrecognized or even mistakenly interpreted as "bedrock" by the engineer- geologist team. Golder Dam, planned as the nucleus for a new commercial and residential com- munity in southern Arizona in 1960, experienced just such an error in geological judgment. The site, 29 km (18 miles) north of Tucson is within the valley of Canada del Oro, where a substantial thickness of alluvial sands and gravels with boulders overlies a thick accumulation of grus, the old weathered granitic detritus transported from the slopes of nearby Catalina Mountains (Fig. 9). Throughout the planning and construction of dam, the importance of a thick sand and gravel formation, with its highly permeable interbeds and lenses, was not fully recognized, even though the gravels transmitted a major portion of Canyon del Oro's underground flow. The first reservoir impound- ment in July 1964 experienced leakage that was observed 6 days later at the downstream toe of the dam. This increased by March 1965 to 0.31 m3/s (5000 gpm) (partially-filled reservoir) and to 0.69 m3/s (11 000 gpm) by April 1966 when the reservoir was nearly filled.

A systematic geological investigation was under- taken in 1965-1967 (Kiersch, 1968) that deter- mined the causes of leakage. Most importantly, the embankment dam itself was inadequately built and the impervious core cut-off stopped 1.5-12 m (5-40 ft) above the bedrock by design. Unfortunately, the dam had been founded on permeable sands, gravels, and boulders that were mistaken for "bedrock" by the inexperienced project geologist (Fig. 9). The main leakage zone extended for 305 m (1000 ft) throughout the channel sector, with additional bank storage losses in the sand and gravel formation (unit) along the western margin of the reservoir (Fig. 9).

Litigation by the owner established that the constructor, engineer-design firm, and the consult- ing geologist were responsible for committing errors of technical judgment. They incorrectly interpreted the geological facts and core-boring data that led up to the faulty design and underseep- age. The owner, through litigation, was awarded a sizable financial settlement, which was sufficient to repair the faulty dam through gravity grouting


Fig. 6. Serre Poncon damsite, Durance River valley, France, during exploration of the channel and alluvial flU over 110 m deep. The dam alignment and height within folded bedrock of glacial-scoured valley are shown, with distribution channel fill (from Haffen and Karpinski, 1963, Photo 1).

of the construction joint/opening formed at the top of the puddled core and pressure grouting of the underlying sands and gravels (Figs. 9 and 10) combined with the construction of a downstream berm and dewatering well system (Kiersch and James, 1991, p. 531-534).

Forecasting the depth to bedrock and establish-

ing the elevation of a suitable foundation or cut- off design for engineered works in alluvial environs requires an understanding of the geological history and depositional events, as reflected in the characteristics of the alluvial units. Equally important however is the mature geological ability to analyze core-boring data and/or geophysical profiles.

334 G.A. Kiersch/Engineering Geology 45 (1996)325-346

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® A,..i.. ® ® Fig. 7. Geological cross-section of Serre Poncon inner valley - looking downstream from a point upstream of main axis (Fig. 6). The relationship between the steep-walled inner limestone canyon, the backfilled channel with alluvium, and the rubble deposit along the right bank is shown, as are the locations of principal shaft and cored-borings of the exploration stage. Elevations are in meters above sea level (from Haffen and Karpinski, 1963, Fig. 6).

3. Fiuvioglacial deposits

3.1. Hudson River channel

The ancestral Hudson River drainage area was impacted by four stages of Pleistocene glaciation. Cretaceous lacustrine clays overlie the weathered bedrock within the pre-Tertiary channel of the Hudson drainage at the Narrows, New York (Fig. 11) and intermixed fluvioglacial deposits occur in River channel. The youngest Wisconsin drift is related to the terminal moraine of Hill Harbor Drift deposited across central Long Island, the Narrows of Hudson River and southward across Staten Island (Fig. 12). This till of clayey and sandy gravels frequently conceals the deposits of earlier advances. For example, most of southern Manhattan is covered by drifts up to depths of 30 m (100 ft) that conceal many buried channels, ancient as well as recent, situated below sea level (Baskerville, 1982, p. 104). Scouring action contributed to the intermixing of the earlier glacial deposits such as occurred in the Hudson River valley (Fig. 13A,B). On Long Island the Harbor Hill moraine (Fig. 12) is a preferred location for construction of cemeteries, parks, and even local sources of perched water (Fickies, 1995). Furthermore, the topographically-high moraine has provided the ideal location for the military

forts, Hamilton and Wadsworth, to guard New York harbor (Fig. 12) for over 200 years. During at least one of the three inter-glacial stages, soft clays of ancient Lake Hudson were deposited within the sequence of glacial sands (Fig. 11). Holocene organic clayey silts and reworked drift materials of the modern Hudson River channel overlie the youngest (Wisconsin) glacial sand unit (Berkey et al., 1933, p. 56-66). The design and construction of the Verranzo-Narrows bridge were not adversely impacted by the sequence of glacio- sediments at the site, as the structure is anchored to the high-quality foundation bedrock of schist and gneiss at depth (Fig. 11).

The site of the Pennsylvania Railroad tunnel beneath the Hudson River, Weechawken, New Jersey to lower Manhattan, New York (Fig. 13B) encountered a very different sequence of glacio- sediments within the ancestral Hudson drainage. Re-worked or modified glacial drift materials of silt, clay, and muds near the modern channel floor overlie earlier glacial deposits, that occur as lenses of gravel, clays and silts as shown in Fig. 13B. These two Hudson River valley sites some 14 km (9 miles) apart demonstrate the type of depositional changes and the reworking and mixing of glacial materials that can occur with younger Holocene sediments of a modern river valley. Such modifications and re-working of glacial deposits


t

Fig. 8. Grouted alluvial fill of channel, Serre Poncon Dam - as exposed in wall of exploration shaft. Note size of boulders and sand with gravel matrix grouted (from Haffen and Karpinski, 1963, Photo 2).

by stream flow are even more pronounced at the George Washington Bridge site 11 km (7 miles) farther upstream from the Pennsylvania Railroad tunnel (Fig. 13A).

3.2. Reservoir bank failure

A reservoir located in morainal gravels failed when the glacial outwash was eroded at Cedar Ridge Gorge, Washington. The gorge is underlain by lacustrine sediments and flanked by morainal gravels distributed by the ancestral Puget Sound Glacier (Fig. 14). A masonry dam built in the

gorge in 1914 impounded a water supply for Seattle. The geological evaluation of the glacial deposits and reservoir setting was largely neglected by the builder. Fine-grained lacustrine clay, silt, and sand deposits overlie the bedrock and are capped by coarse, openwork gravels as shown in Fig. 14. When partially filled, the reservoir waters are in direct contact with the porous gravels and inter-bedded lacustrine sediments of the glacial valley margins. The openwork gravels are now known to be modified in some areas with a coating or veneer of boulder-clay.

Within 4 years after the reservoir began opera-

336 G.A. Kierseh/Engineering Geology 45 (1996) 325-346

GOLDER DAM GEOLOGIC CROSS SECTION ALONG AXIS

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tion, the openwork gravels were impacted at Boxley Creek 1830 m (6000 ft) upstream and eventually liquefied due to piping action fed by reservoir waters. An oversteepening of the phreatic line around the washout area was attributed to the bouldery-clay veneer of gravels (Kiersch and James, 1991, p. 524-526). The bank failure con- sisted of between 610000 and 1 530000 m 3 (800 000 and 2 million cubic yards) of detritus washed out within 2 h and perhaps as little as 20 min; peak discharges were between 915 and 6000 m3/s (3 000 and 20 000 ft/s). Today any reservoir involved with permeable alluvial-glacial deposits should install piezometer arrays to monitor the phreatic line near likely seepage outlets and the first filling controlled in increments with analysis by flow nets (Cedergren, 1989).

4. Piedmont coastal plain

4.1. Ancestral Potomac River sediments

The fine-grained alluvial sediments of an abandoned channel of an ancient Potomac River channel demonstrate a common foundation problem inherent to unconsolidated materials throughout the Piedmont Coastal Plain region (Fig. 2) at the site for the Washington Monument (Fig. 15). The initial cornerstone was laid in 1848 and construction progressed to a height of only 47 m (154 ft) when stopped by the Civil War. When construction resumed in 1880, engineers quickly realized the original foundation design would not support the obelisk monument and the foundation was re-designed. Settlement and tilting had occurred due to the soft underlying foundation of unconsolidated sand and clay, and sand with gravel and clay that overlie the bedrock (Fig. 15). Apparently unrecognized until the 1930s, the site was within

G.A. Kierseh/Engineering Geology 45 (1996) 325-346 339

EXPLANATION ~ ~l!7 | /'~)l

~il Ice Movement .71179 i ~\ {gt " ' - ' - , , ~i!;.

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MAP NEW YORK CITY AND Base Mop, C.P. Berkey, 1933 SURROUNDINGS- affected C.A, Boskerville, 19B~

Wisooniin elaoiation with Terminal Moraine Long G.o,~eA.~,e,so~, I s land- Staten Island. ,9~,,

Fig. 12. Map of New York City-Long Island and surroundings affected by the four stages of Wisconsin ice movement/glaciation. Locations of three geological cross-sections across Hudson River Valley (Fig. 11 and Fig. 13) are shown (modified after Berkey et al., 1933, and Baskerville, 1982).

an ancient abandoned channel of the Potomac River (Reed and Obermeier, 1982) as shown on the areal setting cross-section (Fig. 15). Unfor- tunately, geological guidance on the significance

of the physical characteristics inherent to alluvial deposits of ancient channels was unavailable when the site was chosen and the monument constructed. Fisk (1947) in his paper on fine-grained

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342 G.A. Kiersch/Engineering Geology 45 (1996) 325 346

WASHINGTON MONUMENT

O R I G I N A L

C O N C R E T E F O U N D A T I O N S U N D E R P I N N I N G \ N

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S T R E N G T H E N E D BY U N D E R P I N N I N G

A F T E R :

D A R T O N , 1 9 5 0

G I L L E T T E , 1 9 3 3 G . A . K I E R S C H

R E E D & O B E R M E I E R , 1 9 8 2 1 9 8 9

Fig. 15. Alluviation within ancestral channel of Potomac River, Washington Monument, DC (from Kiersch, 1991, p. 20).

sediments described in detail the stratigraphy and characteristics of abandoned channel deposits.

5. Multiplie alluvial units

5.1. Age-dating tectonism

Multiple alluvial units at a youthful site can be a major aid to unravel the sequence of depositional history as well as establishing recency of tectonism. Several faults traverse the foundation rock of the Cedar Springs earthfill dam on the Mojave River in southern California near Cajon Pass. The most recent age of fault movement is associated with two distinctive sequential units of Holocene alluvium within the Mojave River valley (Keaton, 1991, p. 462-464). The composition, weathering

effects, and/or occurrence of materials suitable for radiocarbon dating when present, are useful aids in establishing the date of last movement.

The structural and stratigraphic relationships of the Holocene alluvium and the underlying sand- stone and granite bedrock at fault No. 2, are shown on Fig. 16. An older, coarser gravelly unit with sand lenses rests on the bedrock and has been offset vertically up to 1.5 m (4.9 ft). A younger, finer-grained alluvial unit overlies to ground level and is not offset by fault No. 2 (Fig. 16, inset). Dating the time of last movement is an important part of assessing the history of faulting and site feasibility. For example, if movement occurred at least once within the past 35 000 years or recurred within the past 500 000 years, the fault is consid- ered active.

NW 9 7 2 " - -


SE

971.

970"

~969 ' o

_l t~

i CIo~ '

i

967] I

FINER ALLUVIUM

toot)

966 ~ FAULT No. 2 /

DISTANCE (.METERS)

Fig. 16. Cedar Springs damsite, southern California, shows structural and stratigraphic relationships of alluvial units and faulting. Sequential history and recency of tectonism are related to distinctive units of alluvium (see inset) (from Keaton, 1991, p. 463, and Stankov, 1992).

5.2. Seismic hazard- works design

The Cedar Springs project utilized distinctive alluvial units to assist with dating the recency of fault movement within the proposed dam foundation; the San Andreas fault zone is within 8 kin. In this geological setting, procedures of design and construction were implemented to accommodate the major seismic hazard in a responsible manner (Sherard et al., 1974). The dam was re-designed more conservatively by reducing its height from 92-66 m (302-216 ft), shortening its length from 1000 to 660 m (3281-2164 ft), widening the dam crest 40-100%, and shifting the dam axis so the entire clay core is founded on granite north of fault No. 1 (Fig. 17).

6. Summary

This paper has focused on a few selected alluvial valley settings where the environs and character of deposits can be important relative to the location, design, or operation of engineered works. An incorrect evaluation of the geological history, depositional environments, or aUuviation sequences and stratigraphy relative to the region- al-areal tectonism can contribute to major errors of judgment by the geological practitioner and even the failure of an engineered works. Some of the modem utilizations of alluvial valley settings cited are:

(1) Intermontane alluvial valleys can be commercial sources of ground water, evaporites, salt,

344 G.A. Kiersch/Engineering Geology 45 (1996) 325 346

Qh ! :S&NDS]ONE (PLb.I~,IUCENEi ;a~' GRANITIC ~OC~ [ME$OZC:,~I

Fig. 17. Cedar Springs damsite, southern California - geological map with location of faults Nos. 1 and 2 and distribution of alluvial units and bedrock (from Keaton, 1991, p. 462).

clays, and natural aggregate. However, many valleys possess very limited resources and any such forecast should be based on an in-depth recon- struction of the depositional and structural history. For example, the most suitable aggregate deposits are probably in the stream channel, while the river terrace, floodplain, and alluvial fan occurrences are less desirable sources. The actual groundwater reservoir potential is only a fraction of what had been anticipated by early 1950s studies. Commercial evaporites, salt, and/or clay deposits may occur in the basin sediments - due to its sequential history.

(2) Dam foundations within a deep inner-filled alluvial valley can be anchored to clay lenses or interbeds within the channel fill. Consolidation grouting of deep alluvial deposits is possible due to an understanding of the characteristics of alluvial sediments; today construction of formerly unused hydrological sites is economical for major dam projects.

(3) Underflow of dams, embankments, and levees may occur when the origin and distribution of alluvial features are unrecognized; for example when the principal sand and gravel unit contains frequent boulders that are mistakenly interpreted from borings as "bedrock" and selected as the design foundation.

(4) Fluvioglacial deposits of the lower Hudson River channel at three sites within a distance of 25 km demonstrate the varied physical occurrences of reworked glacial and Holocene channel sediments. Such changes seriously impacted the construction of a railroad tunnel beneath the channel.

(5) Morainal openwork gravels overlying lacustrine sediments as a reservoir bank failed when liquefied due to piping actions within 4 years. When a reservoir involves permeable alluvial-glacial deposits, piezometer arrays should be installed to monitor the phreatic line near likely seepage outlets and its first-filling performed in monitored increments.


(6) Ances t ra l f ine-grained a l luvia l sediments wi th in an a b a n d o n e d channel o f the P o t o m a c River is a c o m m o n set t ing tha t m a y go unrecognized when si t ing works t h r o u g h o u t the P i e d m o n t / C o a s t a l P la in region. F i sk in his 1947 p a p e r on f ine-grained sediments recognized the i m por t ance o f a b a n d o n e d channel c layey depos i t s and their significance to the s i t ing-design o f engineered works (F i sk , 1947).

(7) Mul t ip le a l luvia l uni ts c a p p e d by Ho locene a l luv ium can be cri t ical to unde r s t and ing the depo- s i t ional h is tory and es tabl ishing the recency to tec tonism, and especial ly when the o ther age- da t ing techniques are no t appl icable .

Acknowledgment

R o b e r t Fickies , engineer ing geologis t wi th the New York Geo log ica l Survey, k ind ly suppl ied the a u t h o r wi th pub l i shed d a t a on the d i s t r ibu t ion a n d p roper t i e s o f the y o u n g glacial depos i t s t h r o u g h o u t the New York Ci ty region. The text has benefi ted f rom the review and commen t s o f the vo lume edi tors , R o g e r T. Saucier , L a w s o n M. Smith, and W h i t n e y J. A u t i n o f the U S A r m y Engineer Wa te rways Expe r imen t Sta t ion. K i m Duffek o f K a n o a I l lus t ra t ions , Tucson, Ar i zona , p r e p a r e d / modi f ied some o f the line drawings and Jane H o f f m a n n o f Tucson r ep roduced the text.

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