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Engineering soil mapping from airphotos
Photogrammetria
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
E N G I N E E R I N G SOIL M A P P I N G F R O M A I R P H O T O S A. H O L D E N
Ministry of Roads and Road Traffic, Salisbury (Rhodesia)
(Received February 9, 1968)
SUMMARY
Southern Africa, rapidly developing but short of highly trained technicians particularly in the civil engineering field, has of recent years turned more and more to airphoto interpretation as the tool for terrain evaluation, enabling large areas to be identified, classified and evaluated with the barest minimum of field work. The preparation of detailed engineering soil maps, showing drainage pattern, soil type and classification and construction material availability, has become routine in Rhodesia; particularly on bridge, airfield, road and dam construction projects. With the necessity of producing trained staff in the shortest possible time, methods of data storage have become vital. In Rhodesia an index system, built up from experience, has been prepared and is available to the interpreter. By referring to a simple form of card index system on which are recorded similar geological, topographical, climatical and drainage patterns, he can obtain the necessary information to enable him to undertake airphoto evaluation of an area even though he may never have had previous experience with the type of country or geology in which his assigment falls.
INTRODUCTION
The methods described in this paper are those in use at the Ministry of Roads Airphoto Interpretation Section, Salisbury. By these methods it has been found possible to evaluate large areas of Rhodesia and to store vital information for reference for future projects. Considerable success has been achieved in the field of location of materials suitable for road and dam construction, particularly in the identification of laterites, hitherto a difficult material to locate.
Photogramrnetria, 23 (1968) 185-199
186
A. HOLDEN
GENERAL PROCEDURE
Any terrain evaluation from airphoto projects depends on the availability of modern high quality airphotos. Rhodesia, although at the time well served by airphoto coverage, introduced in 1963 a 5-year 1 : 25,000 blanket photography plan to cover some 30,000 sq. miles, involving up to 10,000 photographs. It is planned that on completion of this project a further 5-year plan will be introduced to ensure that the blanket coverage is repeated at 5-year intervals. This service supplying as it does modern photographs of every area, enables the airphoto interpreter to undertake his terrain evaluation with speed and accuracy. In the Ministry of Roads for any particular project, the aerial photographs to cover the required area (in the case of a road project, this is usually 3-5 miles either side of the possible route), are made up in the form of an index mosaic. Usually 18 X 18 inch photographs are used to a scale of approximately 1 : 12,500 (blanket photography enlarged), and care is taken to ensure that all photographs are tone matched to facilitate materials identification. This is particularly important for laterite. This index mosaic is given an initial stereo inspection to establish geology, topography, soil boundaries and drainage. Great attention is paid to the drainage pattern due to its vital guide to the soil conditions. These elements are delineated on the mosaic and detailed interpretation is then undertaken to establish the possible route. This detailed interpretation ascertains the best possible combination of elements to produce the most economical route, i.e., the route must (a) traverse, where possible, the soil types with best engineering properties, thus reducing the amount of imported construction material; (b) be aligned to take advantage of natural drainage; (c) be a route requiring the minimum of earth works; and (d) be a route as near as possible to suitable materials for base and sub-base. Having decided on the possible route and using the index reference cards applicable to the particular area, the next step undertaken is that of the engineering soil mapping and classification of the soil types, as well as the location of the different types of constructional materials by interpretation. Once the detailed interpretation of the area is complete, a field visit is made to confirm the actual soil boundaries, to sample and test the soil types, and in the case of deposits of constructional materials, to determine the type, quality and quantity. Uncontrolled mosaics are now made from the index mosaics and overlays are prepared showing drainage pattern, vlei areas, perched water table areas, soil boundaries, obstructions and other relevant details to assist the construction teams in due course.
Photogrammetria, 23 (1968) 185-199
ENGINEERING SOIL MAPPING FROM AIRPHOTOg
187
DETAILS OF PROCEDURE
Identification of laterite areas Laterite formation Laterite, a hydrated ferric oxide, with alumina, silica and calcium carbonate, is usually found near vlei areas and/or in areas where natural drainage is impeded. Dissolved salts, from basic igneous rocks, containing large contents of ferromagnesian materials, are carried relatively short distances into the granite clan areas and trapped in such areas by impeded drainage. Subsequent high ground evaporation results in a mineral deposit being built up in the sand forming a ferruginous calcereous concretion. In Rhodesia all base layer, low plasticity, laterites are normally found near geological boundaries between granite and rocks of the gabbro, dolerite, basalt group, rich in magnesia lime and iron minerals. Laterites are rarely thick; 2-3 ft. is an average thickness as, being impervious, their very formation seems to put an end to the drainage which is essential to their occurrence. Laterite identification Areas where both the granite and gabbro clans are evident are carefully selected. The index mosaic is examined and all zones of influence stretching from the gabbro clan into the granite are marked. Such zones are identified by photo tone, a distinct intermediate tone pattern being observed. Fig.1 illustrates zones of dolerite influence into the granite. Near vlei areas, areas of impeded drainage with sparse vegetation conditions denoting soil infertility, flat areas abounding in termite hills where little defined drainage is evident, are all marked on the index mosaic. Particular note is made of any natural barriers such as rock bars and termite hill barriers that may cause a perched water table, and areas immediately on the high side of such barriers are recorded as likely laterite areas. Sharp moisture seepage demarcation lines are particularly noted (see Fig.l). After airphoto interpretation has been completed in the office, a field visit is arranged and a number of the marked areas are investigated. It is rare to find no trace of laterite in areas so identified, but sometimes areas are small and unworkable. Often the site visit enables the interpreter to correct his interpretation, but it has been found that generally there is a set pattern for any particular area. Unfortunately, laterite deposits are rarely completely uniform due to their method of formation, and usually there is considerable variation in over-burden thickness. As a rule over-burden is heavier towards the high side of the area, with laterite on the surface near the vlei edge.
Photogrammetria, 23 (1968) 185-199
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A. HOLDEN
TABLE I AIRPHOTO ANALYSIS CARD
Physical and airphoto features FRONT:
(1) Geology: Zambesi Valley. Alluvial fine sands and silts of Triassic origin. (2) Altitude: 1,200-1,500 ft. (3) Rainfall: 24--28 inches. (4) Topographic form: Flat country between Zambesi River and escarpment.
(5) Drainage form: dendritic, (6) Erosion form: U (7) Vegetation: Mopani and Jesse bush with thorn scrub. Occasional baobab. (8) Land use: Nil, possibly sugar where irrigation possible. Extensive game reserve (hunting and viewing). (9) Surface material (tone): Light for Mopani (vlei grass--little to no bush), dark for Jesse bush (heavy concentration). Some contact areas easily seen. Mopani requires 18 inches cover; Jesse requires 6-8 inches cover; contact requires 12 inches cover.
BACK:
(10) Surface material (type and distribution): Fine grained sands and silts varying from highly impervious (Mopani) to well drained (Jesse) (in great depth).
(11) Subsoil (type and distribution): As above (10). Plasticity increasing with depth.
(12) Construction materials: Concrete sands and aggregates: all rivers, small and large contain some form of river sand; larger rivers, particularly those rising in escarpment, contain ballast; check phototone for mottled effect on sand areas. Gravel: less plastic fine sands from rises in Jesse areas are suitable for sub-base when stabilized with 3% lime; work to approx. 6 ft. deep; base gravels are scarce; screened river ballast can be investigated (needs stabilizing with cement however); ballast easily seen on photography as old river meanders. Generally base gravels must be imported from the escarpment.
Gravels are obtainable from the foot of the escarpment. Look for lightly treed mounds, easily seen on airphotos. Rock for crushing: only available in the form of limestone deposits. Look for small kopjes with very light photo-tone in the escarpment itself.
(13) Observations: Construction difficulties will be experienced over plastic sand silts. Route road to miss all Mopanis if possible (avoid light photo-tone). Heaving of the subgrade is a common phenomenon in the Mopani areas due to sodic nature of the soils themselves.
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ENGINEERING SOIL MAPPING FROM AIRPHOTOS
TABLE II AIRPHOTO ANALYSIS CARD
Physical and airphoto features FRONT:
(1) Geology: Great dyke, serpentines with pyroxenite pillows, boulders and ridges with silt clays on high ground; black cotton clay in vleis. (2) Altitude: 3.000 3,500 ft. (3) Rainfall: 20-24 inches. (4) Topographic form: undulating.
(5) Drainage form: Saucer vleis to parallel between ridges. (6) Erosion form: saucer, (7) Vegetation: Sparse bush with small trees. (8) Land use: pastoral. (9) Surface material (tone): Medium to dark on ridges; light in vleis due to heavy grass growth on black cotton clay.
BACK:
(10) Surface material (type and distribution):
(13) Observations:
Sand silts with some plastic gravels. (11) Subsoil (type and distribution): Sandclay with solid pyroxenite at no great depth.
Granite sandveld on either side of the dyke yields quartz gravels; no laterite apparent even though expected No base gravel in dyke. Plastic gravels only suitable for fill or sub-base.
(12) Construction materials: plastic gravels.
Identification of other road building materials A n index system is available to assists the i n t e r p r e t e r in materials location and for the p r e p a r a t i o n of the engineering soil m a p ( T a b l e I - I I l are samples of index cards). With a k n o w l e d g e of the t y p e of geology, the average annual rainfall, and the altitude range to be expected, the i n t e r p r e t e r can select the c a r d or cards to suit his p a r t i c u l a r circumstance. This m e a n s that although he m a y personally never have h a d experience in a p a r t i c u l a r type of a r e a he can l e a r n from the experience of p e o p l e w o r k i n g similar areas before him. H e will k n o w where not to l o o k for c o n s t r u c t i o n a l m a t e r i a l s and t h e r e b y save himself countless hours, if not days, in wasted effort. A c o m p a r i s o n between two d y k e areas (Table II, I I I ) clearly shows that the a p p a r e n t difference in altitude a n d rainfall has led to the f o r m a t i o n of laterites a n d ferricretes on either side of the d y k e where the rainfall is between 32 and 36 inches/year. W h e r e the rainfall is m u c h lower no laterites or ferricretes are found. (The G r e a t D y k e in R h o d e s i a extends for s o m e 300 o d d miles and as a result traverses various climatic conditions over its length, although geological for-
Photogrammetria, 23 (1968) 185-199
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A. HOLDEN
TABLE 1II A I R P H O T O A N A L Y S I S CARD
Physical and airphoto [eatures FRON~I :
(1) Geology: Great dyke, mainly serpentine with large pyroxenite intrusions. (2) Altitude: 4,500-5,000 ft. (3) Rainfall: 32-36 inches. Mainly heavy in December to May each year. (4) Topographic form: Very hilly country, generally rough with large surface boulders.
(5) Drainage form: Deep eroded gullys, parallel running, from crest of dyke to sandveld on both sides. (6) Erosion form: sharp "V". (7) Vegetation: Very sparse on serpentine but heavily wooded on pyroxenite (msasa trees). (8) Land use: Mining chrome and asbestos; some cattle grazing. (9) Surface material (tone): Medium to dark.
BACK:
(10) Surface material (type and distribution): Fractured serpentine and pyroxenite-very stony ground with boulders from _ 11~-10 inches; heavy mineralised sandclay in between boulder beds.
(l l) Subsoil (type and distribution): Solid rock throughout--either pyroxenite or serpentine.
(12) Construction materials: Laterites: where the dyke butts onto granitic formations, long beds of ferricrete are found, i.e., both sides of the dyke--these ferricretes are referred to as vlei ironstones on some geological maps. (plasticity index: _ 4-20)
Stone: for surfacing and concrete in the form of crushed serpentine obtainable from some mines--especially where asbestos is mined. (13) Observations: Rough cross-section as sketched below; no gravel in dyke but gravels as per sketch.
BLAC K
VLEI ~xERRICR ETE7
BLACK
VLEI
mations are similar throughout). Generally other road building materials such as quartz reefs, arkose reefs and similar can be identified without difficulty from the airphotos. Sharp backbone type ridges are characteristic of a quartz reef, generally with a light phototone around the circumference. Arkose kopjes produce a very light phototone particularly around their base. In gneiss areas excellent road building materials are available and these can be located by phototone, topography and erosion form. For instance, where a "V" shape erosion form is apparent gravels can be expected (otherwise the drainage is "U" shaped), and gravels are usually located on rises with noticeably light phototone.
Photogrammetria, 23 (1968) 185-199
ENGINEERING SOIL MAPPING FROM AIRPHOTOS
191
Engineering soil maps A irphoto interpretation Using the index cards which also show topography, drainage form, erosion pattern, vegetation, land use and other general observations which may be of assistance in interpreting from the airphotos of any particular area, the interpreter can prepare his engineering soil map. The concept of a climatically influenced engineering pattern of behaviour of certain rocks and their derived soils means that although the geology of two areas may be similar, given different climatic conditions the engineering evaluation of these areas can vary greatly. This and other factors are also recorded on the index cards. Report forms are used in the preparation of the index cards (Table IV). These report forms are submitted by all airphoto and field officers at regular intervals. Table I - I I I are three index cards from completely different types of geological formations found in Rhodesia, i.e., Table I is from an area within the Zambesi
TABLE I V PHYSICAL AND AIRPHOTO INTERPRETATION FEATURES REPORT FORM
(1) (2) (3) (4) (5) (6) (7)
(8) (9) (10) (11) (12)
Land use Surface material (tone) Surface material (type and distribution) Subsoil (type and distribution) Construction materials (type and distribution) (13) Observations
Geology Altitude Rainfall Topographic form Drainage form Erosion form Vegetation
KEY
Topographic ]orm: (l) Flat (2) Gentle slopes (3) Steep slopes (4) Escarpment (5) "Rounded" hills (6) "Domed" hills (7) Ridge (8) "Castle" (9) "Step" or terrace (10) Flood plain Vegetation1: (1) Grass (2) Scrub (3) Bush (4) Forest
Drainage form: (1) Dendritic (2) Angular (3) Parallel (4) Radial (5) "Tongued" (6) Vlei (7) Pan (8) "Vanishing"
Land use: (1) Cultivated (2) Pasture (3) Forestry
Erosion form: (1) "V" shape (2) "U" shape (3) "Saucer" shape (4) Sheet
Photo tone (grey): (1) Light (2) Medium (3) Dark (4) Mottled
1 Qualified by density and uniformity
Photogrammetria, 23 (1968) 185-199
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A. HOLDEN
Valley typical of a large area in the northern part of Rhodesia. Tables II and II1 are two typical dyke formation index cards but from different altitude and rainfall areas.
Soil sampling ]or classification Once the detailed airphoto interpretation is complete a site visit is made for the purposes of soil sampling for classification purposes. All soils sampled during the field visit are taken at points indicated from the airphoto study. In areas of uniform soil type and rainfall, samples are usually taken with regard to topography, that is low points, slopes and ridges are sampled seperately. In gneiss areas of similar rainfall, a remarkably uniform plasticity pattern has been observed from non-plastic on the ridges to plasticities of approximately 14-16 on the slopes to approximately 30 in the vleis. In schist areas studies have indicated that, from an engineering point of view, soils exhibit remarkably consistent strength properties and although plasticity and fineness may vary quite considerably the strength figures remain of the same order. Generally, therefore, in such areas sampling is also dictated by topography. All soils sampled are subjected initially to normal plasticity and grading tests. Soils with the same geological origin under the same climatic conditions, and of the same colour, mica and organic content are then grouped. Groups arc formed so that the variations within any one group are of the order of: plasticity index up to 4; C.I. two symbols1; Minus 200 B.S.S. sieve up to 10%. Some examples of possible groups are: (1) 0-4/A-B/10-20 (2) 9-13/B-C/25-35 (3) 20-24/A-B/35-45 (4) 0-4/E-F/5-15 (5) 9-13/D-E/15-25 (6) 20-24/E-F/9-19 These groups are then subjected to strength tests, and the road cover design requirement is calculated taking into consideration traffic intensity and weight and general rainfall conditions. C.I. is coarseness index and is the per cent retained on the No.7 B.S. sieve. It is expressed thus: C.I. Value Symbol 0-10 A 11-20
B
21-30 31-40 41-50 51 and greater
C D E F
Photogrammetria, 23 (1968) 185-199
Fig.1 (pp.193-194). Zones of dolorite influence into the granite.
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197
ENGINEERING SOIL MAPPING FROM A IRPtIOTOS
The final soil number is then prepared for use on the soil map. It has been found essential that this classification number conveys as much information as possible regarding the soil itself and to this end the final number is formed as geological origin/average plasticity index: average minus 200 B.S.S. sieve/cover requirement in inches (non-plastic is recorded as )0). The geological origin is symrequirement in inches non-plastic is recorder as ()0). The geological origin is symbolised (Table V). Examples of possible final soil numbers are: (1) GR/0215/6 in. (Granite; P.I. of 2; minus 200 15%; 6 inches cover required). (2) GN/1130/8 in. (Gneiss; P.I. of 11; minus 200 30%; 8 inches cover required). For coarse grained materials (C.I. of C - F inclusive) the suffix C, D, E or F is added, as appropriate, as follows: (1) GR/0215C/6 in. (Granite; P.I. of 2; minus 200 15%; 21-30% on 7 B.S.S. sieve; 6 inches cover required). (2) G N / l l 3 0 D / 8 in. (Gneiss; P.I. of 11; minus 200 30%; 31-40% on 7 B.S.S. sieve 8 inches cover required). These final soil numbers are now added to the soil map as are the proposed and approved constructional material areas. Where only one soil is evident in the profile, one soil number appears in the engineering soil map. Where a top soil over-lies this is indicated thus: 9 in. GR/1543/8 in. (9 in. topsoil) GR/3859/16 in. (sub-soil) A typical engineering soil map is shown in Fig.2.
Fig.2. Engineering soil map. Soil mapping and drainage pattern. Region: Sinoia. Road: Tsatsi-Mazoe (control nr. L 50) Section: index mosaic R. 27, Ch. 220-530. Scale 1 : 10,000 nominal. ~ ROC K.
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Photogrammetria, 23 (1968) 185-199
198
A. HOLDEN
TABLE V SYMBOLS OF GEOLOGICAL ORIGIN
Material
Symbol
Material
Symbol
Alluvium Amphibolite Andesite Aplite Arkose Banded ironstone Basalt Breccia Calcite Calcrete Charnockite Chert or Chalcedony Coal Conglomerate Diorite Dolerite Dolomite Epidorite Felsite Ferricrete Fluorspar Gabbro Gneiss Granite Greywacke
AI Am An Ap Ar Bi Bk Br Ca Ct Cn
Grit Kalahari sand Laterite Limestone Marble Mica Mudstone Norite Pegmatite Phylite Pisolite Porphyry 1 Pumice Pyroxenite Quartz Quartzite Rholite Sandstone Schist 2 Serpentine Shale Silcrete Slate Syenite Talc
Gt Ks L Ls Ma Mc Mu No Pe PI Pi Po Pu Py Q Qu Rh Sa Sc Se Sh St SI Sy Ta
Ch Co Cg Di Do Dm Ep FI Fc Fs Ga Gn Gr Gw
1 Porphyry is prefixed by its geological origin, i.e., granite porphyry = GrPo. 2 When origin is doubtful, schists are symbolised Sc, otherwise prefixes are added thus: chlorite schist = Cs; greenstone schist = Gs; hornblende schist = Hs; micaceous schist = Ms; talc schist = Ts.
OBSERVATIONS A detailed index system is being built up and eventually sufficient inform a t i o n will be recorded of details of soil types, sources of constructional material etc., to form the complete basis of interpretation for any n e w project in any area in the country. I n addition to the index cards, airphotos showing typical geological formations with i n f o r m a t i o n m a r k e d with regard to the significance of particular photo tones a n d vegetation characteristics are on display in the airphoto section to assist the interpreter. I n respect of vegetation characteristics details are also recorded of the particular type of tree or shrub generally associated with a certain type of gravel, i.e., in the low veldt area of Rhodesia where rainfall is low ( 1 0 - 1 5 inches) laterite deposits are covered with a c o n c e n t r a t i o n of the " R o o i Bos" or " M b o n d i "
Photogrammetria, 23 (1968) 185-199
ENGINEERING SOIL MAPPING FROM AIRPHOTOS
199
shrub (Combretum apiculatum) similarly a concentration of "Mahobohobo" or "Mazhanje" (Euapaca kirkiana) in a granitic sandveld area usually denotes the presence of quartz float. The advent of colour photography should make the preparation of the engineering soil map and the location of the gravels, particularly laterites, a far simpler exercise. Meanwhile photo tone (or "grey" tone) and vegetation and land form are the basic factors used in most identification operations from the air photos.
ACKNOWLEDGEMENTS This paper is published with the permission of the Commissioner of Roads and Road Traffic, Salisbury, Rhodesia. The author wishes to acknowledge the assistance given to him by members of his staff and the Regional Road Engineer's staff, Sinoia Region.
Photogrammetria, 23 (1968) 185-199
Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
E N G I N E E R I N G SOIL M A P P I N G F R O M A I R P H O T O S A. H O L D E N
Ministry of Roads and Road Traffic, Salisbury (Rhodesia)
(Received February 9, 1968)
SUMMARY
Southern Africa, rapidly developing but short of highly trained technicians particularly in the civil engineering field, has of recent years turned more and more to airphoto interpretation as the tool for terrain evaluation, enabling large areas to be identified, classified and evaluated with the barest minimum of field work. The preparation of detailed engineering soil maps, showing drainage pattern, soil type and classification and construction material availability, has become routine in Rhodesia; particularly on bridge, airfield, road and dam construction projects. With the necessity of producing trained staff in the shortest possible time, methods of data storage have become vital. In Rhodesia an index system, built up from experience, has been prepared and is available to the interpreter. By referring to a simple form of card index system on which are recorded similar geological, topographical, climatical and drainage patterns, he can obtain the necessary information to enable him to undertake airphoto evaluation of an area even though he may never have had previous experience with the type of country or geology in which his assigment falls.
INTRODUCTION
The methods described in this paper are those in use at the Ministry of Roads Airphoto Interpretation Section, Salisbury. By these methods it has been found possible to evaluate large areas of Rhodesia and to store vital information for reference for future projects. Considerable success has been achieved in the field of location of materials suitable for road and dam construction, particularly in the identification of laterites, hitherto a difficult material to locate.
Photogramrnetria, 23 (1968) 185-199
186
A. HOLDEN
GENERAL PROCEDURE
Any terrain evaluation from airphoto projects depends on the availability of modern high quality airphotos. Rhodesia, although at the time well served by airphoto coverage, introduced in 1963 a 5-year 1 : 25,000 blanket photography plan to cover some 30,000 sq. miles, involving up to 10,000 photographs. It is planned that on completion of this project a further 5-year plan will be introduced to ensure that the blanket coverage is repeated at 5-year intervals. This service supplying as it does modern photographs of every area, enables the airphoto interpreter to undertake his terrain evaluation with speed and accuracy. In the Ministry of Roads for any particular project, the aerial photographs to cover the required area (in the case of a road project, this is usually 3-5 miles either side of the possible route), are made up in the form of an index mosaic. Usually 18 X 18 inch photographs are used to a scale of approximately 1 : 12,500 (blanket photography enlarged), and care is taken to ensure that all photographs are tone matched to facilitate materials identification. This is particularly important for laterite. This index mosaic is given an initial stereo inspection to establish geology, topography, soil boundaries and drainage. Great attention is paid to the drainage pattern due to its vital guide to the soil conditions. These elements are delineated on the mosaic and detailed interpretation is then undertaken to establish the possible route. This detailed interpretation ascertains the best possible combination of elements to produce the most economical route, i.e., the route must (a) traverse, where possible, the soil types with best engineering properties, thus reducing the amount of imported construction material; (b) be aligned to take advantage of natural drainage; (c) be a route requiring the minimum of earth works; and (d) be a route as near as possible to suitable materials for base and sub-base. Having decided on the possible route and using the index reference cards applicable to the particular area, the next step undertaken is that of the engineering soil mapping and classification of the soil types, as well as the location of the different types of constructional materials by interpretation. Once the detailed interpretation of the area is complete, a field visit is made to confirm the actual soil boundaries, to sample and test the soil types, and in the case of deposits of constructional materials, to determine the type, quality and quantity. Uncontrolled mosaics are now made from the index mosaics and overlays are prepared showing drainage pattern, vlei areas, perched water table areas, soil boundaries, obstructions and other relevant details to assist the construction teams in due course.
Photogrammetria, 23 (1968) 185-199
ENGINEERING SOIL MAPPING FROM AIRPHOTOg
187
DETAILS OF PROCEDURE
Identification of laterite areas Laterite formation Laterite, a hydrated ferric oxide, with alumina, silica and calcium carbonate, is usually found near vlei areas and/or in areas where natural drainage is impeded. Dissolved salts, from basic igneous rocks, containing large contents of ferromagnesian materials, are carried relatively short distances into the granite clan areas and trapped in such areas by impeded drainage. Subsequent high ground evaporation results in a mineral deposit being built up in the sand forming a ferruginous calcereous concretion. In Rhodesia all base layer, low plasticity, laterites are normally found near geological boundaries between granite and rocks of the gabbro, dolerite, basalt group, rich in magnesia lime and iron minerals. Laterites are rarely thick; 2-3 ft. is an average thickness as, being impervious, their very formation seems to put an end to the drainage which is essential to their occurrence. Laterite identification Areas where both the granite and gabbro clans are evident are carefully selected. The index mosaic is examined and all zones of influence stretching from the gabbro clan into the granite are marked. Such zones are identified by photo tone, a distinct intermediate tone pattern being observed. Fig.1 illustrates zones of dolerite influence into the granite. Near vlei areas, areas of impeded drainage with sparse vegetation conditions denoting soil infertility, flat areas abounding in termite hills where little defined drainage is evident, are all marked on the index mosaic. Particular note is made of any natural barriers such as rock bars and termite hill barriers that may cause a perched water table, and areas immediately on the high side of such barriers are recorded as likely laterite areas. Sharp moisture seepage demarcation lines are particularly noted (see Fig.l). After airphoto interpretation has been completed in the office, a field visit is arranged and a number of the marked areas are investigated. It is rare to find no trace of laterite in areas so identified, but sometimes areas are small and unworkable. Often the site visit enables the interpreter to correct his interpretation, but it has been found that generally there is a set pattern for any particular area. Unfortunately, laterite deposits are rarely completely uniform due to their method of formation, and usually there is considerable variation in over-burden thickness. As a rule over-burden is heavier towards the high side of the area, with laterite on the surface near the vlei edge.
Photogrammetria, 23 (1968) 185-199
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A. HOLDEN
TABLE I AIRPHOTO ANALYSIS CARD
Physical and airphoto features FRONT:
(1) Geology: Zambesi Valley. Alluvial fine sands and silts of Triassic origin. (2) Altitude: 1,200-1,500 ft. (3) Rainfall: 24--28 inches. (4) Topographic form: Flat country between Zambesi River and escarpment.
(5) Drainage form: dendritic, (6) Erosion form: U (7) Vegetation: Mopani and Jesse bush with thorn scrub. Occasional baobab. (8) Land use: Nil, possibly sugar where irrigation possible. Extensive game reserve (hunting and viewing). (9) Surface material (tone): Light for Mopani (vlei grass--little to no bush), dark for Jesse bush (heavy concentration). Some contact areas easily seen. Mopani requires 18 inches cover; Jesse requires 6-8 inches cover; contact requires 12 inches cover.
BACK:
(10) Surface material (type and distribution): Fine grained sands and silts varying from highly impervious (Mopani) to well drained (Jesse) (in great depth).
(11) Subsoil (type and distribution): As above (10). Plasticity increasing with depth.
(12) Construction materials: Concrete sands and aggregates: all rivers, small and large contain some form of river sand; larger rivers, particularly those rising in escarpment, contain ballast; check phototone for mottled effect on sand areas. Gravel: less plastic fine sands from rises in Jesse areas are suitable for sub-base when stabilized with 3% lime; work to approx. 6 ft. deep; base gravels are scarce; screened river ballast can be investigated (needs stabilizing with cement however); ballast easily seen on photography as old river meanders. Generally base gravels must be imported from the escarpment.
Gravels are obtainable from the foot of the escarpment. Look for lightly treed mounds, easily seen on airphotos. Rock for crushing: only available in the form of limestone deposits. Look for small kopjes with very light photo-tone in the escarpment itself.
(13) Observations: Construction difficulties will be experienced over plastic sand silts. Route road to miss all Mopanis if possible (avoid light photo-tone). Heaving of the subgrade is a common phenomenon in the Mopani areas due to sodic nature of the soils themselves.
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ENGINEERING SOIL MAPPING FROM AIRPHOTOS
TABLE II AIRPHOTO ANALYSIS CARD
Physical and airphoto features FRONT:
(1) Geology: Great dyke, serpentines with pyroxenite pillows, boulders and ridges with silt clays on high ground; black cotton clay in vleis. (2) Altitude: 3.000 3,500 ft. (3) Rainfall: 20-24 inches. (4) Topographic form: undulating.
(5) Drainage form: Saucer vleis to parallel between ridges. (6) Erosion form: saucer, (7) Vegetation: Sparse bush with small trees. (8) Land use: pastoral. (9) Surface material (tone): Medium to dark on ridges; light in vleis due to heavy grass growth on black cotton clay.
BACK:
(10) Surface material (type and distribution):
(13) Observations:
Sand silts with some plastic gravels. (11) Subsoil (type and distribution): Sandclay with solid pyroxenite at no great depth.
Granite sandveld on either side of the dyke yields quartz gravels; no laterite apparent even though expected No base gravel in dyke. Plastic gravels only suitable for fill or sub-base.
(12) Construction materials: plastic gravels.
Identification of other road building materials A n index system is available to assists the i n t e r p r e t e r in materials location and for the p r e p a r a t i o n of the engineering soil m a p ( T a b l e I - I I l are samples of index cards). With a k n o w l e d g e of the t y p e of geology, the average annual rainfall, and the altitude range to be expected, the i n t e r p r e t e r can select the c a r d or cards to suit his p a r t i c u l a r circumstance. This m e a n s that although he m a y personally never have h a d experience in a p a r t i c u l a r type of a r e a he can l e a r n from the experience of p e o p l e w o r k i n g similar areas before him. H e will k n o w where not to l o o k for c o n s t r u c t i o n a l m a t e r i a l s and t h e r e b y save himself countless hours, if not days, in wasted effort. A c o m p a r i s o n between two d y k e areas (Table II, I I I ) clearly shows that the a p p a r e n t difference in altitude a n d rainfall has led to the f o r m a t i o n of laterites a n d ferricretes on either side of the d y k e where the rainfall is between 32 and 36 inches/year. W h e r e the rainfall is m u c h lower no laterites or ferricretes are found. (The G r e a t D y k e in R h o d e s i a extends for s o m e 300 o d d miles and as a result traverses various climatic conditions over its length, although geological for-
Photogrammetria, 23 (1968) 185-199
190
A. HOLDEN
TABLE 1II A I R P H O T O A N A L Y S I S CARD
Physical and airphoto [eatures FRON~I :
(1) Geology: Great dyke, mainly serpentine with large pyroxenite intrusions. (2) Altitude: 4,500-5,000 ft. (3) Rainfall: 32-36 inches. Mainly heavy in December to May each year. (4) Topographic form: Very hilly country, generally rough with large surface boulders.
(5) Drainage form: Deep eroded gullys, parallel running, from crest of dyke to sandveld on both sides. (6) Erosion form: sharp "V". (7) Vegetation: Very sparse on serpentine but heavily wooded on pyroxenite (msasa trees). (8) Land use: Mining chrome and asbestos; some cattle grazing. (9) Surface material (tone): Medium to dark.
BACK:
(10) Surface material (type and distribution): Fractured serpentine and pyroxenite-very stony ground with boulders from _ 11~-10 inches; heavy mineralised sandclay in between boulder beds.
(l l) Subsoil (type and distribution): Solid rock throughout--either pyroxenite or serpentine.
(12) Construction materials: Laterites: where the dyke butts onto granitic formations, long beds of ferricrete are found, i.e., both sides of the dyke--these ferricretes are referred to as vlei ironstones on some geological maps. (plasticity index: _ 4-20)
Stone: for surfacing and concrete in the form of crushed serpentine obtainable from some mines--especially where asbestos is mined. (13) Observations: Rough cross-section as sketched below; no gravel in dyke but gravels as per sketch.
BLAC K
VLEI ~xERRICR ETE7
BLACK
VLEI
mations are similar throughout). Generally other road building materials such as quartz reefs, arkose reefs and similar can be identified without difficulty from the airphotos. Sharp backbone type ridges are characteristic of a quartz reef, generally with a light phototone around the circumference. Arkose kopjes produce a very light phototone particularly around their base. In gneiss areas excellent road building materials are available and these can be located by phototone, topography and erosion form. For instance, where a "V" shape erosion form is apparent gravels can be expected (otherwise the drainage is "U" shaped), and gravels are usually located on rises with noticeably light phototone.
Photogrammetria, 23 (1968) 185-199
ENGINEERING SOIL MAPPING FROM AIRPHOTOS
191
Engineering soil maps A irphoto interpretation Using the index cards which also show topography, drainage form, erosion pattern, vegetation, land use and other general observations which may be of assistance in interpreting from the airphotos of any particular area, the interpreter can prepare his engineering soil map. The concept of a climatically influenced engineering pattern of behaviour of certain rocks and their derived soils means that although the geology of two areas may be similar, given different climatic conditions the engineering evaluation of these areas can vary greatly. This and other factors are also recorded on the index cards. Report forms are used in the preparation of the index cards (Table IV). These report forms are submitted by all airphoto and field officers at regular intervals. Table I - I I I are three index cards from completely different types of geological formations found in Rhodesia, i.e., Table I is from an area within the Zambesi
TABLE I V PHYSICAL AND AIRPHOTO INTERPRETATION FEATURES REPORT FORM
(1) (2) (3) (4) (5) (6) (7)
(8) (9) (10) (11) (12)
Land use Surface material (tone) Surface material (type and distribution) Subsoil (type and distribution) Construction materials (type and distribution) (13) Observations
Geology Altitude Rainfall Topographic form Drainage form Erosion form Vegetation
KEY
Topographic ]orm: (l) Flat (2) Gentle slopes (3) Steep slopes (4) Escarpment (5) "Rounded" hills (6) "Domed" hills (7) Ridge (8) "Castle" (9) "Step" or terrace (10) Flood plain Vegetation1: (1) Grass (2) Scrub (3) Bush (4) Forest
Drainage form: (1) Dendritic (2) Angular (3) Parallel (4) Radial (5) "Tongued" (6) Vlei (7) Pan (8) "Vanishing"
Land use: (1) Cultivated (2) Pasture (3) Forestry
Erosion form: (1) "V" shape (2) "U" shape (3) "Saucer" shape (4) Sheet
Photo tone (grey): (1) Light (2) Medium (3) Dark (4) Mottled
1 Qualified by density and uniformity
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A. HOLDEN
Valley typical of a large area in the northern part of Rhodesia. Tables II and II1 are two typical dyke formation index cards but from different altitude and rainfall areas.
Soil sampling ]or classification Once the detailed airphoto interpretation is complete a site visit is made for the purposes of soil sampling for classification purposes. All soils sampled during the field visit are taken at points indicated from the airphoto study. In areas of uniform soil type and rainfall, samples are usually taken with regard to topography, that is low points, slopes and ridges are sampled seperately. In gneiss areas of similar rainfall, a remarkably uniform plasticity pattern has been observed from non-plastic on the ridges to plasticities of approximately 14-16 on the slopes to approximately 30 in the vleis. In schist areas studies have indicated that, from an engineering point of view, soils exhibit remarkably consistent strength properties and although plasticity and fineness may vary quite considerably the strength figures remain of the same order. Generally, therefore, in such areas sampling is also dictated by topography. All soils sampled are subjected initially to normal plasticity and grading tests. Soils with the same geological origin under the same climatic conditions, and of the same colour, mica and organic content are then grouped. Groups arc formed so that the variations within any one group are of the order of: plasticity index up to 4; C.I. two symbols1; Minus 200 B.S.S. sieve up to 10%. Some examples of possible groups are: (1) 0-4/A-B/10-20 (2) 9-13/B-C/25-35 (3) 20-24/A-B/35-45 (4) 0-4/E-F/5-15 (5) 9-13/D-E/15-25 (6) 20-24/E-F/9-19 These groups are then subjected to strength tests, and the road cover design requirement is calculated taking into consideration traffic intensity and weight and general rainfall conditions. C.I. is coarseness index and is the per cent retained on the No.7 B.S. sieve. It is expressed thus: C.I. Value Symbol 0-10 A 11-20
B
21-30 31-40 41-50 51 and greater
C D E F
Photogrammetria, 23 (1968) 185-199
Fig.1 (pp.193-194). Zones of dolorite influence into the granite.
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197
ENGINEERING SOIL MAPPING FROM A IRPtIOTOS
The final soil number is then prepared for use on the soil map. It has been found essential that this classification number conveys as much information as possible regarding the soil itself and to this end the final number is formed as geological origin/average plasticity index: average minus 200 B.S.S. sieve/cover requirement in inches (non-plastic is recorded as )0). The geological origin is symrequirement in inches non-plastic is recorder as ()0). The geological origin is symbolised (Table V). Examples of possible final soil numbers are: (1) GR/0215/6 in. (Granite; P.I. of 2; minus 200 15%; 6 inches cover required). (2) GN/1130/8 in. (Gneiss; P.I. of 11; minus 200 30%; 8 inches cover required). For coarse grained materials (C.I. of C - F inclusive) the suffix C, D, E or F is added, as appropriate, as follows: (1) GR/0215C/6 in. (Granite; P.I. of 2; minus 200 15%; 21-30% on 7 B.S.S. sieve; 6 inches cover required). (2) G N / l l 3 0 D / 8 in. (Gneiss; P.I. of 11; minus 200 30%; 31-40% on 7 B.S.S. sieve 8 inches cover required). These final soil numbers are now added to the soil map as are the proposed and approved constructional material areas. Where only one soil is evident in the profile, one soil number appears in the engineering soil map. Where a top soil over-lies this is indicated thus: 9 in. GR/1543/8 in. (9 in. topsoil) GR/3859/16 in. (sub-soil) A typical engineering soil map is shown in Fig.2.
Fig.2. Engineering soil map. Soil mapping and drainage pattern. Region: Sinoia. Road: Tsatsi-Mazoe (control nr. L 50) Section: index mosaic R. 27, Ch. 220-530. Scale 1 : 10,000 nominal. ~ ROC K.
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----~____
Photogrammetria, 23 (1968) 185-199
198
A. HOLDEN
TABLE V SYMBOLS OF GEOLOGICAL ORIGIN
Material
Symbol
Material
Symbol
Alluvium Amphibolite Andesite Aplite Arkose Banded ironstone Basalt Breccia Calcite Calcrete Charnockite Chert or Chalcedony Coal Conglomerate Diorite Dolerite Dolomite Epidorite Felsite Ferricrete Fluorspar Gabbro Gneiss Granite Greywacke
AI Am An Ap Ar Bi Bk Br Ca Ct Cn
Grit Kalahari sand Laterite Limestone Marble Mica Mudstone Norite Pegmatite Phylite Pisolite Porphyry 1 Pumice Pyroxenite Quartz Quartzite Rholite Sandstone Schist 2 Serpentine Shale Silcrete Slate Syenite Talc
Gt Ks L Ls Ma Mc Mu No Pe PI Pi Po Pu Py Q Qu Rh Sa Sc Se Sh St SI Sy Ta
Ch Co Cg Di Do Dm Ep FI Fc Fs Ga Gn Gr Gw
1 Porphyry is prefixed by its geological origin, i.e., granite porphyry = GrPo. 2 When origin is doubtful, schists are symbolised Sc, otherwise prefixes are added thus: chlorite schist = Cs; greenstone schist = Gs; hornblende schist = Hs; micaceous schist = Ms; talc schist = Ts.
OBSERVATIONS A detailed index system is being built up and eventually sufficient inform a t i o n will be recorded of details of soil types, sources of constructional material etc., to form the complete basis of interpretation for any n e w project in any area in the country. I n addition to the index cards, airphotos showing typical geological formations with i n f o r m a t i o n m a r k e d with regard to the significance of particular photo tones a n d vegetation characteristics are on display in the airphoto section to assist the interpreter. I n respect of vegetation characteristics details are also recorded of the particular type of tree or shrub generally associated with a certain type of gravel, i.e., in the low veldt area of Rhodesia where rainfall is low ( 1 0 - 1 5 inches) laterite deposits are covered with a c o n c e n t r a t i o n of the " R o o i Bos" or " M b o n d i "
Photogrammetria, 23 (1968) 185-199
ENGINEERING SOIL MAPPING FROM AIRPHOTOS
199
shrub (Combretum apiculatum) similarly a concentration of "Mahobohobo" or "Mazhanje" (Euapaca kirkiana) in a granitic sandveld area usually denotes the presence of quartz float. The advent of colour photography should make the preparation of the engineering soil map and the location of the gravels, particularly laterites, a far simpler exercise. Meanwhile photo tone (or "grey" tone) and vegetation and land form are the basic factors used in most identification operations from the air photos.
ACKNOWLEDGEMENTS This paper is published with the permission of the Commissioner of Roads and Road Traffic, Salisbury, Rhodesia. The author wishes to acknowledge the assistance given to him by members of his staff and the Regional Road Engineer's staff, Sinoia Region.
Photogrammetria, 23 (1968) 185-199