Heavy duty pneumatic tire

1. A heavy duty pneumatic tire comprising:

a tread portion provided with at least three circumferentially and continuously extending main grooves and a plurality of lateral grooves extending between adjacent main grooves or between one main groove and a tread edge to form a block row comprising a plurality of blocks;

the at least three main grooves comprising a pair of crown main grooves disposed on both sides of a tire equator and a pair of middle main grooves disposed axially outward of the crown main grooves;

the middle main grooves comprising shallow bottom parts having a groove depth in a range of from 15% to 70% in relation to a maximum groove depth of the crown main grooves;

the shallow bottom parts comprising first shallow bottom parts and second shallow bottom parts alternately arranged in a circumferential direction of the tire, the second shallow bottom parts having groove depths larger than groove depths of the first shallow bottom parts; and

the block row comprising a plurality of pitch elements each of which consists of circumferentially adjacent one lateral groove and one block, wherein

the tread portion has a land ratio in a range of from 70% to 80%,

the block row comprises a number of pitch elements in a range of from 40 to 50, and

each lateral groove has a groove width in a range of from 5% to 15% in relation to a circumferential length of the block in each pitch element.

2. The tire according to claim 1 ,

wherein the lateral grooves comprise a plurality of crown lateral grooves extending between the crown main grooves to form a crown block row comprising a plurality of crown blocks,

each crown lateral groove comprises a shallow bottom part having a groove depth in a range of from 50% to 60% in relation to a maximum groove depth of the crown main grooves, and

the shallow bottom part is provided with a crown groove-bottom sipe that extends along a longitudinal direction of the crown lateral groove and has a depth in a range of from 25% to 40% in relation to the maximum groove depth of the crown main grooves.

3. The tire according to claim 2 ,

wherein the main grooves further comprise a pair of shoulder main grooves disposed axially outward of the middle main grooves,

the lateral grooves further comprise a plurality of axially outer middle lateral grooves extending between the middle main groove and the shoulder main groove to form an outer middle block row comprising a plurality of axially outer middle blocks on each side of the tire equator,

the middle main grooves and the outer middle lateral grooves comprise respective shallow bottom parts each having a groove depth in a range of from 15% to 70% in relation to the maximum groove depth of the crown main grooves,

the shallow bottom part of the middle main groove is provided with a middle groove-bottom sipe that extends along a longitudinal direction of the middle main groove,

the shallow bottom part of the outer middle lateral groove is provided with an outer middle groove-bottom sipe that extends along a longitudinal direction of the outer middle lateral groove, and

each depth of the middle groove-bottom sipe and the outer middle groove-bottom sipe is in a range of from 20% to 75% in relation to the maximum groove depth of the crown main grooves.

4. The tire according to claim 3 ,

wherein the tread portion further comprises a circumferentially and continuously extending inner middle rib arranged between the crown main groove and the middle main groove,

the inner middle rib is provided with a plurality of axially inner middle lateral grooves extending axially inward from the middle main groove without reaching the crown main groove,

each axially inner middle lateral groove comprises a shallow bottom part having a groove depth in a range of from 15% to 70% in relation to the maximum groove depth of the crown main grooves,

the shallow bottom part of the inner middle lateral groove is provided with an inner middle groove-bottom sipe that extends along a longitudinal direction of the inner middle lateral groove, and

each depth of the inner middle groove-bottom sipe is in a range of from 20% to 75% in relation to the maximum groove depth of the crown main grooves.

5. The tire according to claim 1 , wherein each of the middle main grooves is configured as a zigzag form having a plurality of zigzag corners, the second shallow bottom parts are disposed on each zigzag corner of the middle main groove and each first shallow bottom part is disposed between adjacent second shallow bottom parts.

6. The tire according to claim 1 , wherein the groove depths of the first shallow bottom parts are in a range of from 47% to 58% the groove depths of the second shallow bottom parts.

7. The tire according to claim 1 , wherein

each of the middle main grooves is configured as a zigzag form having a plurality of zigzag corners,

the middle main groove comprises a groove bottom provided with a plurality of middle groove-bottom sipes, and

the middle groove-bottom sipes are configured as inclined elements which are arranged so as to be separated from one another at the respective zigzag corners.

8. The tire according to claim 1 , wherein the block row comprises crown blocks, outer middle blocks and shoulder blocks, and wherein the crown main grooves and the middle main grooves define an inner middle rib therebetween and the first and second shallow parts connect the outer middle blocks with the inner middle rib.

9. The tire according to claim 1 , wherein the first and second shallow bottom parts have a depth of 25% to 60% relative to the maximum groove depth of the crown main grooves.

10. The tire according to claim 1 , wherein the crown main grooves contain protrusions which protrude radially outward from the groove bottom.