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Ships and tunnels: particular loads
Ships and Tunnels:
Particular
Loads
S. B. de Vries
RCsum&L’auteur discute les impacts de lrois phtnomtmes sur conception de tunnels immersb: (I) la prbence de bateaux en train couler ou sous l’eau; (2) une ancre en tombbe; (3) une ancre draggage. On parle des effets de ces facteurs et des mkthodes conception pour les rkduire.
Abstract-The author deals with the impact of three types of phenomena on the design and immersed tunnels: (I) the occurrence of sinking or sunken ships; (2) a falling anchor; (3) a dragging anchor. The article discusses ways of calculating the effects of these phenomena, and design factors that should be considered to minimize these effects.
la de de de
Introduction
T
he appearance and design of structures in infrastructural transport connections is mainly a matter of optimizing technique and cost. Broadly speaking, such connections include crossings of water and roads. In principle, crossings for traffic connections can be executed as level intersections aboveground or underground. The possibility of a level intersection exists only for similar flows of transport, e.g. road/road or waterway/waterway. When road and waterway intersect, a through connection can be realized only by constructing crossings at different levels. Here, bridge and tunnel come under consideration (the ferry is not considered in this paper). The final choice of the crossing structure will be determined by the local boundary conditions. In a flat country, the structure appears as a bump or dent in the road. The designer always endeavors to keep this effect as small and smooth as possible. Height differences always will be minimized, from the point of view of cost as well as aesthetics. Herein lies an important choice for, with a structure such as a bridge or fly-over, the headroom for navigation or road traffic has been determined; with a tunnel, the bottom profile is permanently fixed. For both road and water crossings, there is always the danger that high loads will collide with the structure, be it fly-over or bridge. This factor always must be taken into account. In principle, this concern also may be valid for tunnels; only the “methodof attack” is different. This danger is dealt with in relation to tunnels. Present address: S. B. de Vries, Head of Building and Civil Structures Department, Rotterdam Public Works, Stadstimmerhuis, Marconiplein, Galvanistraat 15, Postbus 6633,3002 AP Rotterdam, The Netherlands.
Tunncll,ng
and Underground
Printed in Great Britain
SPace Technology.
Figure
1. Longitudinal
section
of a drilled
tunnel.
Backfill --
Figure
2. Longitudinal
section
of a submerged
Owing to the construction method, drilled tunnels pass beneath waterways at a great depth (Fig. 1). The soil cover over the tunnel is 1- 1.5 times the tunnel diameter; in practice, the soil cover amounts to 10 m or more. On the outside of such a tunnel, there can be hardly any load other than that of soil and water. A submerged tunnel “lies high”, usually having a thinner soil cover or no soil cover at all (Fig. 2). Loads on the outside of the tunnel, in addition to those of soil and water, may be of as for example, a specific origin, sinking or sunken ship, falling anchor, or dragging anchor. These three factors, which may have to be considered in the tunnel design, are discussed below.
Sunken Ship-General Concept The tunnel structure acts as a local part of the river or canal bottom, resulting in an increased coefficient of subgrade reaction. Simple calculations
Vol. 3. No. 4, pp. 369-373, 1988.
tunnel.
Figure 3. Protective sheetpile wall.
concrete
slab and
can prove that no special measures are necessary to counteract this effectcertainly, at least, not in practice, where several meters of soil cover the fairway part of the tunnel. A special case is the metro-tunnel in the Coolhaven in Rotterdam. The submerged tunnel in this harbor has a pile foundation and nearly no soil cover (see Fig. 3). The navigation situation at the entrance of the Delfshavense Schie causes an increased risk of collision by maneuvering ships.
0886.7798038 t3.00 + .Oll Pergamon
Press
plr
369
Table 1.
Relationship between the carrying capacity o/ a ship and the anchor weight.
Class number
Carrying capacity class (in tons)
Approx. anchor weight (in kg)
1 2 3 4 5 6
50-199 200-449 750-1149 1150-1549 1550-2549 2550-4999 50OO
350 650 850 1000 1650 2000
The sheetpiles of the b u i l d i n g trench have been burned off under water at the level of the river bottom, and the lower parts of the sheetpiles have not been removed. O n the top of the sheetpiles and the roof of the tunnel, a layer of protective concrete (prefabricated elements) has been applied. The resulting structure has sufficient resistance capacity against sinking or sunken ships and anchor loads. According to their function, ships should float and not sink, so that under normal circumstances the risk of a ship sinking u p o n the tunnel is very small. A falling or dragging anchor is a more probable hazard.
Falling Anchor General Aspects In the event that a tunnel without a covering of soil is struck directly by a falling anchor, the following factors are important: • The weight of the anchor. • The speed at which the tunnel is struck by the anchor. • The resilience of the concrete construction. The weight and speed of the anchor and the "response" of the structure determine the impact on the structure. Weight o/ the anchor. T h e necessary weight of the anchor depends on the dimensions of the ship. For i n l a n d ships, the relationship between the carrying capacity of the ship and the anchor weight is indicated in Table 1. Designers of immersed tunnels need to consider the anticipated size of vessels expected in the waterway in order to determine the calculated m a x i m u m anchor weight. For example, the m a x i m u m anchor weight for a 500,000ton d.w. ocean-going vessel is 29 tons.
The speed o] an anchor Jalling into the water. T o determine this speed, tests were performed with an anchor weighing 5215 kg in 13.60-m-deep water. The distance from the hawse hole to the water level was 8 m. As shown in Figure 4, the speed at which the anchor fell was reduced within approx. 1.00 m to a constant speed of approx. 7.0 m/s. This
370
figure can serve as the base for the calculations.
The resilience structure. When
o/
the
I
2
3
m/see 5
4
6
7
8
9
concrete
considering the resilience of the concrete structure against the anchor impact, a distinction must be made between local response and overall response. The difference between these responses is shown graphically in Fig. 5.
Response Structure The following four regions can be distinguished as a result of an impact load on a concrete structure in relation to the local response (see Fig. 6): (1) A crater region. (2) A crushed aggregate region. (3) A cracking region. (4) A scabbing region. Crater region. This region can be considered the "shooting point" of the impact. The crater depth can be determined by empirical formulas. In the case of a falling anchor, it appears that the crater depth will only be a few millimeters because of the relatively low striking speed of the anchor. Crushed aggregate region. The crushed aggregate region can be considered a transitional region in which the concrete reacts both elastically and plastically. In this region, the concrete disintegrates as a result of the pressure wave, while the reflected tensile waves given rise to further disintegration. A quantitative indication of this p h e n o m e n o n can be obtained by comparing the m a x i m u m compression stresses with the dynamic compressive strength of the concrete, which depends on the speed of loading. This figure increases as the load speed increases. However, the considered rate of fall for a falling anchor is so low that the increase in the compressive strength is only a p p r o x i m a t e l y 2 0 % . Because the compression stresses caused by this type of impact are of the same order of magnitude as the stresses at which crushing occurs, crushing of the concrete must not be ruled out. Cracking region. Cracking in this region is attributable to the transmission of elastic stress waves. Cracking occurs
TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
)
3 2 I
O, -I (m) -2 -3 -4 -5 -6 -7 -8 -9 -10 -II -12 -13 Figure 4. Speed of an anchor as it/alls through the water.
i
b
t Local responr, e
~
Localreil~onle
Figure 5. Difference between local response and overall response to a /ailing anchor.
impact
~ b b i n g
Figure 6. Different regions caused by local response. Volume 3, Number 4, 1988
Bendlnq moment due to normal Load Bending moment, including falling anchor i
c.b.b.a.
Primary rodioL crocks b.c. Secondary ring crocks
Additional, r e i n f o r c e m e n t m l O %
Figure 7. Characteristic cracking, local response, to a falling anchor.
= EJL :/ Figure 9. Additional bending m o m e n t resulting from a [ailing anchor.
i /
/
/
/
/ N
\
\
\
\
1
R
o
c 15m
k
~
-,-
l
o
t
h
30m
~u~
15m
~1
\ b
Figure 10. Protective construction with rock fill.
AsphaltMastic
~.
0.5m
3m
0"2m I
Figure 8. Spalling occurring as a result of a falling anchor. Rock Fill 10 - 80 kg
when the resulting stress exceeds the dynamic tensile strength. Around the point of impact, longitudinal (stresstensile) and transversal (shearing tensile) waves are created (see Fig. 7). The main tensile stresses in the first phase of impact can be estimated mathematically. In the case of a falling anchor, it appears that cracking is very local. Scabbing region. Scabbing occurs when the tensile stress resulting from the initial pressure wave and the reflected tensile wave on the inside of the tunnel roof exceed the dynamic tensile strength of the concrete. As a result, a horizontal crack may occur at some distance from the bottom. O w i n g to the spherical character of the waves, the spall will take the form of a spherical segment (see Fig. 8). In general, the extant reinforcement in the concrete structure will be sufficient to resist damage from cracking and spallings. For the crater and the crushed region, a 100- to 150-ram-thick layer of protective concrete on the roof
Volume 3, Number 4, 1988
I~
~
~|~
20m
-r
~- 1 u~-
30m
20m
--- I
Figure 11. Protective construction with asphalt mastic.
ROCK FILL
control wires 8.75 m
-_ ~ 3.6m..L
~-
-
~'1'1'1 ~S
10 m
"y"
-
; I
d _ 3.~d~ 7
.
.
.
10 m
.
:
i
R_R_R_R_RK_RF_R_IRL_RL_~CC O N S ~
i
I'!'PI' s; 4 o
3 .2
3o
A
B
1
z. 1.
Figure 12. Survey o[ test location.
TUNNELLING
AND UNDERGROUND
SPACE TECHNOLOGY
371
of the tunnel will offer sufficient protection. With regard to the overall response, the load on the structure resulting from a falling anchor increases the forces in the structure. The dynamic effect of the load must be included in determining the quantity of the reinforcement required. The dynamic load factor for the overall response is determined by means of a one-mass spring system with linear spring characteristic. In this case, the designer may want to consider permitting a lower safety coefficient when the required quantity of reinforcement is calculated. In practice, additional reinforcement will be about 10% of the reinforcement due to normal load (see Fig. 9).
GR IPPtI~(~ FORC[
KN6Q0
500
~00
3OO
200
1GO
0 I I
o
CONSTRUCTION
I
i
] 20
Dragging A n c h o r
I i
' ! 15
i
10
I 5
--
, dist~e
in m
Figure 13. Test results from a rock fill layer and a combination rock-sand fill.
General Aspects Theoretically, this effect can be calculated. Apart from n a u t i c a l considerations of the anchor load, it is important that an anchor dragging on the bottom cannot catch behind an obstacle in the bottom, because this will lead to fracture of the anchor chain. A good solution to this problem is to guide a dragging anchor around a structure that lies in or just below the bottom level of the river. Corners of the tunnel roof can be leveled at the topside, and additional special protective construction can be applied (see Figs 10 and 11). In principle, these measures can be similar to those used for sunken structures for pipeline crossings in river bottoms. T h e vulnerability may be even greater in the latter kind of structure, where it is the pipeline, and not the anchor chain, that is fractured, causing a calamity to occur. A test scale of approximately 1:1 will be reported. A requirement imposed on a protective structure is that a dug-in anchor must break out before passing the tunnel or underwater pipe lying in the river bed, and then dig in again after passing it. A n u m b e r of materials may be used as a protective structure: rock fill, asphalt mastic, synthetic material, concrete, or a combination of these materials. After comparing these materials based on the m a n n e r of application, the necessary structure height and cost were taken into consideration, and tests were performed for the following materials: (1) Rock fill, embedded in sand or otherwise/ (2) A combination of asphalt mastic and penetrated rock fill. Rock-]ill protective structure. The rock fill structure consisted of 10-60 kg of rock fill; the bottom of the structure was covered by polypropylene cloth. In
372
~
RIRrl
K~ 5Oo m
m
0 t I i
(ONS'II~U( 1 ION
t
i
i
~
!
i
I
t
I
2O
Figure 14. Test results ]rom an asphalt mastic protective layer.
I route ~
fremee braking , distanc_____ee
time
Figure 15. In[luence o] a dragging anchor on the braking distance o] a ship.
TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
Volume 3, Number 4, 1988
the first tests, the rock fill was not embedded in sand; later, the tests were repeated on rock fill embedded in sand. Protective base. The
structure
on
a bitumen
b i t u m i n o u s protective structure consisted of a horizontal layer of asphalt over the tunnel to be protected. T h e asphalt layer is limited on both sides by penetrated rock fill 10/80 overfilled with asphalt mastic under a gradient of 1:4.
Tests The tests were performed in an underwater basin with dimensions 40 m × 18 m x 4 m (Fig. 12). After the protective construction to be examined had been applied in the middle, the structure was filled to the top with sand. Subsequently, a 1.360-ton Danforth anchor was placed on the sand and connected to two winches. By p u l l i n g the anchor very slowly (hauling speed
Volume 3, Number 4, 1988
0.2-0.7 m/s), the anchor could dig in (digging-in depth of 0.90 to 1.10 m). The anchor was then pulled at a higher speed (6.0 to 8.0 m/s) across the protective structure. During the entire process, the force in the p u l l i n g cable was measured in order to ascertain the gripping force of the anchor. Figure 13 gives the relationship between the measured gripping force and the route traveled by the anchor for a representative test with rock fill and a combination rock-and-sand fill. It is clearly seen that, on reaching the rock fill, the g r i p p i n g force increases slightly, then drops quite abruptly to a very low value and breaks out. The distance between the b e g i n n i n g of the protective structure and the point of break-out varied from 4.00 m to 6.00 m. Figure 14 shows the test results of an asphahmatic protection layer. In this case, the anchor breaks out over a shorter distance.
Consequencesfor Shipping After passing the protective structure, the anchor will dig in again. This means that, across the distance of the protective structure, the ship can derive hardly any braking power from its anchor(s) and, therefore, can only brake by the screw. Figure 15 gives a qualitative respresentation of the effect of breaking out the anchor on the braking distance, showing that the braking distances become longer. This increase in the braking distance will vary from ship to ship. The influence of breaking out the anchor will be no different from the fluctuations in resistance of the natural character of the soil. These tests indicated that the protective structure functions appropriately, and that the influence of a dragging anchor on the braking distance of a ship will be less than 10%. []
TUNNELLING AND UNDERGROUNDSPACETECHNOLOGY 373
Particular
Loads
S. B. de Vries
RCsum&L’auteur discute les impacts de lrois phtnomtmes sur conception de tunnels immersb: (I) la prbence de bateaux en train couler ou sous l’eau; (2) une ancre en tombbe; (3) une ancre draggage. On parle des effets de ces facteurs et des mkthodes conception pour les rkduire.
Abstract-The author deals with the impact of three types of phenomena on the design and immersed tunnels: (I) the occurrence of sinking or sunken ships; (2) a falling anchor; (3) a dragging anchor. The article discusses ways of calculating the effects of these phenomena, and design factors that should be considered to minimize these effects.
la de de de
Introduction
T
he appearance and design of structures in infrastructural transport connections is mainly a matter of optimizing technique and cost. Broadly speaking, such connections include crossings of water and roads. In principle, crossings for traffic connections can be executed as level intersections aboveground or underground. The possibility of a level intersection exists only for similar flows of transport, e.g. road/road or waterway/waterway. When road and waterway intersect, a through connection can be realized only by constructing crossings at different levels. Here, bridge and tunnel come under consideration (the ferry is not considered in this paper). The final choice of the crossing structure will be determined by the local boundary conditions. In a flat country, the structure appears as a bump or dent in the road. The designer always endeavors to keep this effect as small and smooth as possible. Height differences always will be minimized, from the point of view of cost as well as aesthetics. Herein lies an important choice for, with a structure such as a bridge or fly-over, the headroom for navigation or road traffic has been determined; with a tunnel, the bottom profile is permanently fixed. For both road and water crossings, there is always the danger that high loads will collide with the structure, be it fly-over or bridge. This factor always must be taken into account. In principle, this concern also may be valid for tunnels; only the “methodof attack” is different. This danger is dealt with in relation to tunnels. Present address: S. B. de Vries, Head of Building and Civil Structures Department, Rotterdam Public Works, Stadstimmerhuis, Marconiplein, Galvanistraat 15, Postbus 6633,3002 AP Rotterdam, The Netherlands.
Tunncll,ng
and Underground
Printed in Great Britain
SPace Technology.
Figure
1. Longitudinal
section
of a drilled
tunnel.
Backfill --
Figure
2. Longitudinal
section
of a submerged
Owing to the construction method, drilled tunnels pass beneath waterways at a great depth (Fig. 1). The soil cover over the tunnel is 1- 1.5 times the tunnel diameter; in practice, the soil cover amounts to 10 m or more. On the outside of such a tunnel, there can be hardly any load other than that of soil and water. A submerged tunnel “lies high”, usually having a thinner soil cover or no soil cover at all (Fig. 2). Loads on the outside of the tunnel, in addition to those of soil and water, may be of as for example, a specific origin, sinking or sunken ship, falling anchor, or dragging anchor. These three factors, which may have to be considered in the tunnel design, are discussed below.
Sunken Ship-General Concept The tunnel structure acts as a local part of the river or canal bottom, resulting in an increased coefficient of subgrade reaction. Simple calculations
Vol. 3. No. 4, pp. 369-373, 1988.
tunnel.
Figure 3. Protective sheetpile wall.
concrete
slab and
can prove that no special measures are necessary to counteract this effectcertainly, at least, not in practice, where several meters of soil cover the fairway part of the tunnel. A special case is the metro-tunnel in the Coolhaven in Rotterdam. The submerged tunnel in this harbor has a pile foundation and nearly no soil cover (see Fig. 3). The navigation situation at the entrance of the Delfshavense Schie causes an increased risk of collision by maneuvering ships.
0886.7798038 t3.00 + .Oll Pergamon
Press
plr
369
Table 1.
Relationship between the carrying capacity o/ a ship and the anchor weight.
Class number
Carrying capacity class (in tons)
Approx. anchor weight (in kg)
1 2 3 4 5 6
50-199 200-449 750-1149 1150-1549 1550-2549 2550-4999 50OO
350 650 850 1000 1650 2000
The sheetpiles of the b u i l d i n g trench have been burned off under water at the level of the river bottom, and the lower parts of the sheetpiles have not been removed. O n the top of the sheetpiles and the roof of the tunnel, a layer of protective concrete (prefabricated elements) has been applied. The resulting structure has sufficient resistance capacity against sinking or sunken ships and anchor loads. According to their function, ships should float and not sink, so that under normal circumstances the risk of a ship sinking u p o n the tunnel is very small. A falling or dragging anchor is a more probable hazard.
Falling Anchor General Aspects In the event that a tunnel without a covering of soil is struck directly by a falling anchor, the following factors are important: • The weight of the anchor. • The speed at which the tunnel is struck by the anchor. • The resilience of the concrete construction. The weight and speed of the anchor and the "response" of the structure determine the impact on the structure. Weight o/ the anchor. T h e necessary weight of the anchor depends on the dimensions of the ship. For i n l a n d ships, the relationship between the carrying capacity of the ship and the anchor weight is indicated in Table 1. Designers of immersed tunnels need to consider the anticipated size of vessels expected in the waterway in order to determine the calculated m a x i m u m anchor weight. For example, the m a x i m u m anchor weight for a 500,000ton d.w. ocean-going vessel is 29 tons.
The speed o] an anchor Jalling into the water. T o determine this speed, tests were performed with an anchor weighing 5215 kg in 13.60-m-deep water. The distance from the hawse hole to the water level was 8 m. As shown in Figure 4, the speed at which the anchor fell was reduced within approx. 1.00 m to a constant speed of approx. 7.0 m/s. This
370
figure can serve as the base for the calculations.
The resilience structure. When
o/
the
I
2
3
m/see 5
4
6
7
8
9
concrete
considering the resilience of the concrete structure against the anchor impact, a distinction must be made between local response and overall response. The difference between these responses is shown graphically in Fig. 5.
Response Structure The following four regions can be distinguished as a result of an impact load on a concrete structure in relation to the local response (see Fig. 6): (1) A crater region. (2) A crushed aggregate region. (3) A cracking region. (4) A scabbing region. Crater region. This region can be considered the "shooting point" of the impact. The crater depth can be determined by empirical formulas. In the case of a falling anchor, it appears that the crater depth will only be a few millimeters because of the relatively low striking speed of the anchor. Crushed aggregate region. The crushed aggregate region can be considered a transitional region in which the concrete reacts both elastically and plastically. In this region, the concrete disintegrates as a result of the pressure wave, while the reflected tensile waves given rise to further disintegration. A quantitative indication of this p h e n o m e n o n can be obtained by comparing the m a x i m u m compression stresses with the dynamic compressive strength of the concrete, which depends on the speed of loading. This figure increases as the load speed increases. However, the considered rate of fall for a falling anchor is so low that the increase in the compressive strength is only a p p r o x i m a t e l y 2 0 % . Because the compression stresses caused by this type of impact are of the same order of magnitude as the stresses at which crushing occurs, crushing of the concrete must not be ruled out. Cracking region. Cracking in this region is attributable to the transmission of elastic stress waves. Cracking occurs
TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
)
3 2 I
O, -I (m) -2 -3 -4 -5 -6 -7 -8 -9 -10 -II -12 -13 Figure 4. Speed of an anchor as it/alls through the water.
i
b
t Local responr, e
~
Localreil~onle
Figure 5. Difference between local response and overall response to a /ailing anchor.
impact
~ b b i n g
Figure 6. Different regions caused by local response. Volume 3, Number 4, 1988
Bendlnq moment due to normal Load Bending moment, including falling anchor i
c.b.b.a.
Primary rodioL crocks b.c. Secondary ring crocks
Additional, r e i n f o r c e m e n t m l O %
Figure 7. Characteristic cracking, local response, to a falling anchor.
= EJL :/ Figure 9. Additional bending m o m e n t resulting from a [ailing anchor.
i /
/
/
/
/ N
\
\
\
\
1
R
o
c 15m
k
~
-,-
l
o
t
h
30m
~u~
15m
~1
\ b
Figure 10. Protective construction with rock fill.
AsphaltMastic
~.
0.5m
3m
0"2m I
Figure 8. Spalling occurring as a result of a falling anchor. Rock Fill 10 - 80 kg
when the resulting stress exceeds the dynamic tensile strength. Around the point of impact, longitudinal (stresstensile) and transversal (shearing tensile) waves are created (see Fig. 7). The main tensile stresses in the first phase of impact can be estimated mathematically. In the case of a falling anchor, it appears that cracking is very local. Scabbing region. Scabbing occurs when the tensile stress resulting from the initial pressure wave and the reflected tensile wave on the inside of the tunnel roof exceed the dynamic tensile strength of the concrete. As a result, a horizontal crack may occur at some distance from the bottom. O w i n g to the spherical character of the waves, the spall will take the form of a spherical segment (see Fig. 8). In general, the extant reinforcement in the concrete structure will be sufficient to resist damage from cracking and spallings. For the crater and the crushed region, a 100- to 150-ram-thick layer of protective concrete on the roof
Volume 3, Number 4, 1988
I~
~
~|~
20m
-r
~- 1 u~-
30m
20m
--- I
Figure 11. Protective construction with asphalt mastic.
ROCK FILL
control wires 8.75 m
-_ ~ 3.6m..L
~-
-
~'1'1'1 ~S
10 m
"y"
-
; I
d _ 3.~d~ 7
.
.
.
10 m
.
:
i
R_R_R_R_RK_RF_R_IRL_RL_~CC O N S ~
i
I'!'PI' s; 4 o
3 .2
3o
A
B
1
z. 1.
Figure 12. Survey o[ test location.
TUNNELLING
AND UNDERGROUND
SPACE TECHNOLOGY
371
of the tunnel will offer sufficient protection. With regard to the overall response, the load on the structure resulting from a falling anchor increases the forces in the structure. The dynamic effect of the load must be included in determining the quantity of the reinforcement required. The dynamic load factor for the overall response is determined by means of a one-mass spring system with linear spring characteristic. In this case, the designer may want to consider permitting a lower safety coefficient when the required quantity of reinforcement is calculated. In practice, additional reinforcement will be about 10% of the reinforcement due to normal load (see Fig. 9).
GR IPPtI~(~ FORC[
KN6Q0
500
~00
3OO
200
1GO
0 I I
o
CONSTRUCTION
I
i
] 20
Dragging A n c h o r
I i
' ! 15
i
10
I 5
--
, dist~e
in m
Figure 13. Test results from a rock fill layer and a combination rock-sand fill.
General Aspects Theoretically, this effect can be calculated. Apart from n a u t i c a l considerations of the anchor load, it is important that an anchor dragging on the bottom cannot catch behind an obstacle in the bottom, because this will lead to fracture of the anchor chain. A good solution to this problem is to guide a dragging anchor around a structure that lies in or just below the bottom level of the river. Corners of the tunnel roof can be leveled at the topside, and additional special protective construction can be applied (see Figs 10 and 11). In principle, these measures can be similar to those used for sunken structures for pipeline crossings in river bottoms. T h e vulnerability may be even greater in the latter kind of structure, where it is the pipeline, and not the anchor chain, that is fractured, causing a calamity to occur. A test scale of approximately 1:1 will be reported. A requirement imposed on a protective structure is that a dug-in anchor must break out before passing the tunnel or underwater pipe lying in the river bed, and then dig in again after passing it. A n u m b e r of materials may be used as a protective structure: rock fill, asphalt mastic, synthetic material, concrete, or a combination of these materials. After comparing these materials based on the m a n n e r of application, the necessary structure height and cost were taken into consideration, and tests were performed for the following materials: (1) Rock fill, embedded in sand or otherwise/ (2) A combination of asphalt mastic and penetrated rock fill. Rock-]ill protective structure. The rock fill structure consisted of 10-60 kg of rock fill; the bottom of the structure was covered by polypropylene cloth. In
372
~
RIRrl
K~ 5Oo m
m
0 t I i
(ONS'II~U( 1 ION
t
i
i
~
!
i
I
t
I
2O
Figure 14. Test results ]rom an asphalt mastic protective layer.
I route ~
fremee braking , distanc_____ee
time
Figure 15. In[luence o] a dragging anchor on the braking distance o] a ship.
TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY
Volume 3, Number 4, 1988
the first tests, the rock fill was not embedded in sand; later, the tests were repeated on rock fill embedded in sand. Protective base. The
structure
on
a bitumen
b i t u m i n o u s protective structure consisted of a horizontal layer of asphalt over the tunnel to be protected. T h e asphalt layer is limited on both sides by penetrated rock fill 10/80 overfilled with asphalt mastic under a gradient of 1:4.
Tests The tests were performed in an underwater basin with dimensions 40 m × 18 m x 4 m (Fig. 12). After the protective construction to be examined had been applied in the middle, the structure was filled to the top with sand. Subsequently, a 1.360-ton Danforth anchor was placed on the sand and connected to two winches. By p u l l i n g the anchor very slowly (hauling speed
Volume 3, Number 4, 1988
0.2-0.7 m/s), the anchor could dig in (digging-in depth of 0.90 to 1.10 m). The anchor was then pulled at a higher speed (6.0 to 8.0 m/s) across the protective structure. During the entire process, the force in the p u l l i n g cable was measured in order to ascertain the gripping force of the anchor. Figure 13 gives the relationship between the measured gripping force and the route traveled by the anchor for a representative test with rock fill and a combination rock-and-sand fill. It is clearly seen that, on reaching the rock fill, the g r i p p i n g force increases slightly, then drops quite abruptly to a very low value and breaks out. The distance between the b e g i n n i n g of the protective structure and the point of break-out varied from 4.00 m to 6.00 m. Figure 14 shows the test results of an asphahmatic protection layer. In this case, the anchor breaks out over a shorter distance.
Consequencesfor Shipping After passing the protective structure, the anchor will dig in again. This means that, across the distance of the protective structure, the ship can derive hardly any braking power from its anchor(s) and, therefore, can only brake by the screw. Figure 15 gives a qualitative respresentation of the effect of breaking out the anchor on the braking distance, showing that the braking distances become longer. This increase in the braking distance will vary from ship to ship. The influence of breaking out the anchor will be no different from the fluctuations in resistance of the natural character of the soil. These tests indicated that the protective structure functions appropriately, and that the influence of a dragging anchor on the braking distance of a ship will be less than 10%. []
TUNNELLING AND UNDERGROUNDSPACETECHNOLOGY 373